37Reaction Mechanisms

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REACTION MECHANISM
▢ Definition: Step-by-step description of an organic reaction is called reaction mechanism.
▢ Basic Idea:
❖ Reaction: \(CH_3Cl + KOH \rightarrow CH_3OH + KCl\)
❖ Meaning: Old covalent bond breaks and a new covalent bond forms during reaction.
▢ Concepts Needed:
  • •
    Electronic displacement in covalent bond
  • •
    Nature of reagent
  • •
    Bond cleavage
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ELECTRONIC DISPLACEMENT IN COVALENT BOND
▢ Definition: Electron displacement means shifting of electrons in a covalent bond due to electronic effects.
▢ Types:
  • •
    Inductive effect
  • •
    Resonance or mesomeric effect
  • •
    Electromeric effect
  • •
    Hyperconjugation effect
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INDUCTIVE EFFECT
▢ Definition: Inductive effect is the partial and permanent displacement of \(\sigma\)-electrons along a carbon chain due to electronegativity difference.
▢ Important Features:
  • •
    It operates through \(\sigma\)-skeleton.
  • •
    It is a permanent effect.
  • •
    It decreases with distance.
  • •
    Theoretically it may continue up to infinite distance, but practically important only up to \(C_3\) or \(C_4\).
  • •
    Polarization of electrons is always in a single direction.
  • •
    Electrons do not leave their original atomic orbital.
Special Point
\(\text{Inductive effect} \propto \dfrac{1}{\text{distance}}\)
▢ Reference Bond: The \(C-H\) bond is taken as standard. Inductive effect of \(C-H\) bond is considered zero.
▢ Types:
❖ Negative Inductive Effect:
◉ Also Called: \(-I\) effect
◉ Definition: If the electronegativity of the group attached to carbon chain is more than hydrogen, the group shows \(-I\) effect.
◉ Direction: Permanent displacement of \(\sigma\)-electrons occurs away from the carbon chain.
◉ Order: \(-NH_3^+ > -NO_2 > -CN > -CHO > -COOH > -F > -Cl > -Br > -I > -OR > -OH > -NH_2 > -C_6H_5 > -H\)
◉ Examples:
  1. \(CH_3-Cl\): \(-I\) effect of \(Cl\)
  2. \(CH_3-NH_2\): \(-I\) effect of \(NH_2\)
❖ Positive Inductive Effect:
◉ Also Called: \(+I\) effect
◉ Definition: If the electronegativity of the group attached to carbon chain is less than hydrogen, the group shows \(+I\) effect.
◉ Direction: Permanent displacement of electrons occurs towards the carbon chain.
◉ Order: \(-O^- > -COO^- > 3^\circ\ alkyl > 2^\circ\ alkyl > 1^\circ\ alkyl > -CH_3 > -H\)
◉ Alkyl Group Order: \(CH_3CH_2CH_2^- > CH_3CH_2^- > CH_3^-\)
◉ Example: \(CH_3\) group shows \(+I\) effect.
▢ Inductive Effect of Multiple Bonds:
❖ Order: \(sp\ carbon > sp^2\ carbon > sp^3\ carbon\)
❖ Meaning: Triple bond gives more \(-I\) effect than double bond, and double bond gives more \(-I\) effect than single bond.
❖ Electronegativity Order: \(sp\ carbon\ [3.25] > sp^2\ carbon\ [2.75] > sp^3\ carbon\ [2.5]\)
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APPLICATIONS OF INDUCTIVE EFFECT
▢ Stability of Carbocation and Free Radical:
❖ Rule:
  • •
    Carbocation and carbon free radical are electron-deficient species.
  • •
    \(+I\) effect increases their stability.
  • •
    \(-I\) effect decreases their stability.
❖ Formula: \(\text{Stability} \propto +I\ effect\), and \(\text{Stability} \propto \dfrac{1}{-I\ effect}\)
❖ Carbocation Stability Order: \(3^\circ > 2^\circ > 1^\circ > methyl\)
❖ Free Radical Stability Order: \(3^\circ > 2^\circ > 1^\circ > methyl\)
❖ Example: \((CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+\)
▢ Stability of Carbanion:
❖ Rule:
  • •
    Carbanion is electron-rich.
  • •
    \(-I\) effect stabilizes carbanion.
  • •
    \(+I\) effect destabilizes carbanion.
❖ Formula: \(\text{Stability of carbanion} \propto -I\ effect\), and \(\text{Stability of carbanion} \propto \dfrac{1}{+I\ effect}\)
❖ Order: \(CH_3^- > CH_3CH_2^- > (CH_3)_2CH^- > (CH_3)_3C^-\)
❖ Important Examples:
  1. \(FCH_2CH_2^- > HOCH_2CH_2^- > NH_2CH_2CH_2^-\)
  2. When \(-I\) group is nearer to negative charge, carbanion is more stable.
  3. For halogens: \(F > Cl > Br > I\) for \(-I\) effect.
▢ Reactivity of Alkyl Halides:
❖ Basic Point: Alkyl halides are more reactive than alkanes because \(C-X\) bond is polar.
❖ Characteristic Reaction: Nucleophilic substitution reaction.
❖ Rule: Reactivity of alkyl halide depends on the stability of the intermediate carbocation.
❖ Formula: \(\text{Reactivity of halide} \propto \dfrac{+I}{-I}\)
❖ Order: \(3^\circ\ halide > 2^\circ\ halide > 1^\circ\ halide\)
❖ Example: \((CH_3)_3CCl > (CH_3)_2CHCl > CH_3CH_2Cl\)
▢ Reactivity Order of Alcohol:
❖ Lucas Test Order: \(3^\circ\ alcohol > 2^\circ\ alcohol > 1^\circ\ alcohol\)
❖ Observation:
◉ 3° Alcohol: Turbidity appears immediately.
◉ 2° Alcohol: Turbidity appears after about 5 minutes.
◉ 1° Alcohol: Turbidity appears late, about 30 minutes.
❖ Example: \((CH_3)_3COH > (CH_3)_2CHOH > CH_3CH_2OH > CH_3OH\)
▢ Reactivity of Carbonyl Compounds:
❖ Characteristic Reaction: Carbonyl compounds mainly undergo nucleophilic addition reaction.
❖ Reason: In carbonyl group, carbon has incomplete octet and partial positive charge, so nucleophile attacks carbon.
❖ Rule: More positive charge on \(sp^2\)-carbon means more reactive carbonyl compound.
❖ Formula: \(\text{Reactivity of carbonyl compound} \propto \dfrac{1}{+I\ effect}\)
❖ General Order: \(HCHO > RCHO > ArCHO > R_2CO > ArCOR > ArCOAr\)
❖ Key Points:
  • •
    Aldehydes are always more reactive than ketones.
  • •
    Aliphatic aldehydes are more reactive than aromatic aldehydes.
  • •
    Aliphatic ketones are more reactive than aromatic ketones.
  • •
    Electron-releasing group decreases reactivity.
  • •
    Electron-withdrawing group increases reactivity.
❖ Example Order: \(HCHO > CH_3CHO > C_2H_5CHO > C_6H_5CHO > CH_3COCH_3 > CH_3COC_2H_5 > C_6H_5COCH_3 > C_6H_5COC_2H_5 > C_6H_5COC_6H_5\)
▢ Dipole Moment:
❖ Rule: As \(-I\) effect increases, dipole moment increases.
❖ Example: \(CH_3I\ [1.64D] < CH_3Br\ [1.79D] < CH_3Cl\ [1.83D]\)
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ACIDIC STRENGTH
▢ Definition: An acid is a species that has tendency to lose proton \((H^+)\) or accept a pair of electrons.
▢ Ionisation: \(HA \rightleftharpoons H^+ + A^-\)
▢ Acid Dissociation Constant: \(K_a = \dfrac{[H^+][A^-]}{[HA]}\)
▢ pKa: \(pK_a = -\log K_a\)
▢ Basic Rule:
  • •
    Greater \(K_a\), stronger acid.
  • •
    Lower \(pK_a\), stronger acid.
  • •
    More stable conjugate base means stronger acid.
  • •
    Less stable conjugate base means weaker acid.
▢ Formula: \(\text{Acid strength} \propto K_a \propto \text{stability of conjugate base} \propto \dfrac{1}{pK_a}\)
▢ Effect of Inductive Effect:
❖ Rule: \(\text{Acid strength} \propto \dfrac{-I\ effect}{+I\ effect}\)
❖ Meaning:
  • •
    As \(-I\) effect increases, acid strength increases.
  • •
    As \(+I\) effect increases, acid strength decreases.
  • •
    \(-I\) group stabilizes conjugate base.
  • •
    \(+I\) group destabilizes conjugate base.
▢ Steps to Find Acidic Strength:
  1. Remove \(H^+\) and make intermediate anion.
  2. Compare stability of anion.
  3. More stable anion means corresponding acid is stronger.
▢ Important Orders:
❖ Halogen Substituted Acids: \(CH_3CH_2COOH < ClCH_2COOH < FCH_2COOH\)
❖ Multiple Halogen Effect: \(CCl_3COOH > CHCl_2COOH > ClCH_2COOH\)
❖ Distance Effect: \(CH_3CH_2CH(F)COOH > CH_3CH(F)CH_2COOH > FCH_2CH_2CH_2COOH\)
❖ Electron Withdrawing vs Electron Releasing Group: \(NO_2CH_2COOH > HCH_2COOH > CH_3CH_2COOH\)
❖ Carbon Chain Effect: \(HCOOH > CH_3COOH > CH_3CH_2COOH\)
❖ Period Trend: \(CH_4 < NH_3 < H_2O < HF\)
❖ Group Trend: \(HI > HBr > HCl > HF\)
❖ Unsaturation Effect: \(CH_3CH_3 < CH_2=CH_2 < HC\equiv CH\)
❖ Alcohol vs Water: \(CH_3OH < H_2O\)
❖ Thiol vs Alcohol: \(CH_3SH > CH_3OH\)
❖ Formic Acid Series: \(HCOOH > CH_3COOH > C_2H_5COOH > C_3H_7COOH\)
▢ Key Points:
  • •
    Negative charge on more electronegative atom is more stable in a period.
  • •
    Positive charge on more electronegative atom is less stable in a period.
  • •
    Negative charge on bigger atom is more stable in a group.
  • •
    Positive charge on bigger atom is less stable in a group.
  • •
    More number of \(-I\) groups gives more acidic strength.
  • •
    \(-I\) effect decreases with distance.
▢ Dicarboxylic Acid Acidic Strength:
❖ Rule: \(\text{Acidic strength} \propto \dfrac{1}{\text{distance between two } -COOH\ groups}\)
❖ Order: Oxalic acid > Malonic acid > Succinic acid > Glutaric acid > Adipic acid > Pimelic acid
❖ Mnemonic: Oh My Son Go And Play
❖ Maleic vs Fumaric Acid: Maleic acid is more acidic than fumaric acid due to shorter distance between two \(-COOH\) groups.
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BASIC STRENGTH
▢ Definition: Base is a species that can donate electron pair or accept \(H^+\).
▢ Formula: \(\text{Basic strength} \propto K_b \propto \dfrac{1}{\text{stability of conjugate acid}} \propto \dfrac{1}{pK_b}\)
▢ Effect of Inductive Effect: \(\text{Basic strength} \propto \dfrac{+I\ effect}{-I\ effect}\)
▢ Aliphatic Amines:
❖ In Gaseous Phase:
◉ Reason: \(+I\) effect mainly controls basicity.
◉ Order: \(R_3N > R_2NH > RNH_2 > NH_3\)
◉ Meaning: \(3^\circ\ amine > 2^\circ\ amine > 1^\circ\ amine > NH_3\)
❖ In Aqueous Medium:
◉ Factors:
  • •
    Solvation or hydration energy
  • •
    Steric hindrance
  • •
    Inductive effect
◉ For Methyl Amines: \(2^\circ\ amine > 1^\circ\ amine > 3^\circ\ amine > NH_3\)
◉ For Ethyl Amines: \(2^\circ > 3^\circ > 1^\circ\)
Default Rule
If medium and alkyl group are not mentioned, take medium as aqueous medium and alkyl group as methyl.
▢ Effect of Substituents on Basicity:
❖ Electron Withdrawing Groups: \(-I\) groups decrease basicity.
❖ Electron Releasing Groups: \(+I\) groups increase basicity.
❖ Example: \(ClCH_2NH_2 > FCH_2NH_2 > NO_2CH_2NH_2 > CNCH_2NH_2\) depends on decreasing \(-I\) withdrawal near amine.
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HYPERCONJUGATION
▢ Definition: Hyperconjugation is delocalization of \(\sigma\)-electrons of \(C-H\) bond of an alkyl group attached to an unsaturated system or electron-deficient carbon.
▢ Also Called: No bond resonance.
▢ Condition:
  • •
    Presence of alpha hydrogen is required.
  • •
    More alpha hydrogen means more hyperconjugation.
  • •
    Number of resonating structures due to hyperconjugation = number of alpha hydrogens + 1.

Table 1: Hyperconjugation and Resonating Structures

Structure
Number of alpha hydrogen
Number of resonating structures
\(CH_3-CH=CH_2\)
3
4
\(CH_3-CH=CH-CH_3\)
6
7
\((CH_3)_3C^+\)
9
10
\(CH_3CH_2^+\)
3
4
\((CH_3)_3C\cdot\)
9
10
▢ Special Points:
  • •
    Stability of alkene, carbocation and carbon free radical can be explained by hyperconjugation.
  • •
    Stability of carbanion cannot be explained by hyperconjugation.
▢ Applications:
❖ Stability of Alkene:
◉ Rule: \(\text{Stability of alkene} \propto \text{number of alpha hydrogens} \propto \dfrac{1}{\text{heat of hydrogenation}}\)
◉ Order: \((CH_3)_2C=C(CH_3)_2 > (CH_3)_2C=CHCH_3 > CH_3CH=CHCH_3 > CH_3CH=CH_2 > CH_2=CH_2\)
◉ Meaning: More substituted alkene is more stable.
❖ Heat of Hydrogenation and Combustion:
◉ Rule: \(\text{Heat of hydrogenation} \propto \dfrac{1}{\text{number of alpha hydrogen}}\)
◉ Important Point: More stable alkene has lower heat of hydrogenation and lower heat of combustion.
❖ Stability of Alkyl Carbocation and Alkyl Free Radical:
◉ Rule: \(\text{Stability} \propto \text{number of alpha hydrogens}\)
◉ Order: \(CH_3^+ < CH_3CH_2^+ < (CH_3)_2CH^+ < (CH_3)_3C^+\)
◉ Free Radical Order: \(CH_3\cdot < CH_3CH_2\cdot < (CH_3)_2CH\cdot < (CH_3)_3C\cdot\)
❖ Bond Length:
◉ Concept: Hyperconjugation gives partial double bond character to adjacent single bond and partial single bond character to double bond.
◉ Example: In propene, \(H_3C-C\) bond length is about \(1.46\ \mathring{A}\), shorter than normal \(C-C\) bond \((1.54\ \mathring{A})\). The \(C=C\) bond length is about \(1.353\ \mathring{A}\), slightly longer due to partial single bond character.
❖ Saytzeff Rule: Hyperconjugation is the basic principle behind Saytzeff rule for determination of major product in dehydrohalogenation of alkyl halide.
❖ Toluene: Toluene shows hyperconjugation effect.
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QUICK EXAM RULES
▢ Carbocation: \(+I\) effect and hyperconjugation increase stability.
▢ Free Radical: \(+I\) effect and hyperconjugation increase stability.
▢ Carbanion: \(-I\) effect increases stability; \(+I\) effect decreases stability.
▢ Acidic Strength: More stable conjugate base means stronger acid.
▢ Basic Strength: More available lone pair means stronger base.
▢ Carbonyl Reactivity: More positive carbonyl carbon means more nucleophilic addition.
▢ Alkyl Halide Reactivity: More stable carbocation intermediate means faster reaction.
▢ Alcohol Lucas Test: \(3^\circ > 2^\circ > 1^\circ\)
▢ Distance Rule: Inductive effect decreases with distance.
▢ Halogen Inductive Order: \(F > Cl > Br > I\)
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Direct or orienting influence of alkyl group in benzene ring
▢ Definition: Alkyl group in benzene is ortho-para directing and activating for electrophilic aromatic substitution due to hyperconjugation.
▢ Reason:
  • •
    Alkyl group donates electrons to benzene ring by hyperconjugation.
  • •
    Electron density increases mainly at ortho and para positions.
  • •
    Therefore electrophile attacks mainly at ortho and para positions.
▢ Electron Donating Power of Alkyl Groups:
❖ Rule: Electron donating power depends on the number of alpha hydrogens and resonating structures due to hyperconjugation.
❖ Order: \(CH_3CH_2CH_2- > (CH_3)_2CH- > (CH_3)_3C-\)
❖ Meaning: More alpha hydrogen means more hyperconjugation and more electron donation.
Important
In hyperconjugation there is \(\sigma-\pi\) orbital conjugation.
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Nature of attacking reagent
▢ Definition: Attacking reagents are the species which attack the substrate during organic reaction.
▢ Main Types:
  • •
    Electrophiles or electrophilic reagents
  • •
    Nucleophiles or nucleophilic reagents
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Electrophiles
▢ Definition: Electrophiles are electron-loving and electron-deficient species.
▢ Main Features:
  • •
    They have tendency to gain electrons.
  • •
    They are attracted towards negative charge density.
  • •
    They may have complete positive charge, partial positive charge or incomplete octet.
  • •
    They behave as Lewis acids.
▢ Electron Condition: Electrophiles are either \(2e^-\) deficient, less than octet, or sometimes octet species with electron deficiency.
▢ Examples: \(H^+, X^+, R^+, NO_2^+, NO^+, SO_3H^+, RCO^+\)
▢ Categories:
❖ Category 1: Positively Charged with Incomplete Valence Shell:
◉ Examples: \(H^+, X^+, R^+, NO_2^+, NO^+, SO_3H^+, RCO^+\)
◉ Nature: All are Lewis acids except pair of electrons.
◉ Special Points:
  • •
    Alkali and alkaline earth metal cations like \(Na^+, K^+, Ca^{2+}, Ba^{2+}\) are not electrophiles because they have stable configuration.
  • •
    \(H_3O^+\) and \(NH_4^+\) do not gain electrons but are considered electrophiles because they are sources of \(H^+\).
◉ Examples of Proton Source:
  1. \(H_3O^+ \rightarrow H^+ + H_2O\)
  2. \(NH_4^+ \rightarrow H^+ + NH_3\)
❖ Category 2: Neutral with Incomplete Valence Shell:
◉ Examples:
  1. \(BX_3\)
  2. \(AlX_3\)
  3. Free radicals
  4. Carbene
  5. Nitrene
◉ Meaning: These species are neutral but electron deficient.
❖ Category 3: Complete Valence Shell but Expandable or Vacant d-Orbital Species:
◉ Examples: \(PCl_3, SbCl_5, SnCl_2, ZnCl_2, SnCl_4, SF_6\)
◉ Meaning: These have complete valence shell but can accept electrons due to vacant orbital or expansion of valence shell.
❖ Category 4: Neutral Complete Valence Shell Non-Expandable Species:
◉ Examples: \(CO_2, SO_2, SO_3\)
◉ Meaning: These create vacant orbital by breaking existing bond.
▢ Strength Order: \(Category\ 1 > Category\ 2 > Category\ 3 > Category\ 4\)
Exam Example
Powerful electrophile order: \(Cl^+ > CH_3^+ > SnCl_2 > SO_2\)
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Nucleophiles
▢ Definition: Nucleophiles are nucleus-loving and electron-donating species.
▢ Main Features:
  • •
    They behave as Lewis bases.
  • •
    They are electron-rich species.
  • •
    They usually have unshared electron pair.
  • •
    They may be negative ions or neutral molecules with free electron pair.
  • •
    They attack electron-deficient or positive centres in substrate.
▢ Types:
❖ Charged Nucleophiles:
◉ Definition: Negatively charged species are charged nucleophiles.
◉ Examples: \(F^-, OH^-, Cl^-, Br^-, I^-, R^-, RS^-, RO^-, NO_2^-, NH_2^-, C_6H_5O^-, CH_3COO^-\)
❖ Neutral Nucleophiles:
◉ Definition: Neutral compounds whose central atom has complete octet and at least one lone pair act as nucleophiles.
◉ Nitrogen Nucleophiles: \(:NH_3, RNH_2, R_2NH, R_3N, NH_2NH_2, pyridine, piperidine\)
◉ Oxygen Nucleophiles: \(H_2O, ROH, ROR, C_6H_5OH, THF\)
◉ Sulphur Nucleophiles: \(H_2S, RSH, RSR\)
◉ Phosphorus Nucleophiles: \(PH_3, RPH_2, R_2PH, R_3P\)
❖ Carbon Containing Nucleophiles:
◉ Examples:
  1. Alkenes \((CH_2=CH_2)\)
  2. Alkynes \((CH\equiv CH)\)
  3. Benzene
◉ Meaning: Carbon multiple bond can also behave as nucleophile.
❖ Organometallic Compounds:
◉ Examples:
  1. \(RMgX\): Grignard reagent
  2. \(RLi\)
  3. \(R_2Cd\)
  4. \((C_2H_5)_4Pb\)
  5. \(R_2CuLi\): Gilman's reagent
◉ Meaning: Organometallic compounds are nucleophilic because carbon bears partial negative character.
❖ Metal Hydrides:
◉ Examples: \(LiAlH_4, NaBH_4\)
◉ Meaning: They are source of hydride ion \((H^-)\), so they act as nucleophiles.
▢ Ambident Nucleophiles:
❖ Definition: Species having two nucleophilic centres are called ambident nucleophiles.
❖ Examples:
  1. \(CN^-\): can attack through \(C\) or \(N\)
  2. \(NO_2^-\): can attack through \(N\) or \(O\)
▢ Ambiphiles:
❖ Definition: Molecules containing multiple bond between carbon and more electronegative atom can act both as electrophiles and nucleophiles.
❖ Examples:
  1. \(C=O\)
  2. \(CH_3-C\equiv N\)
▢ Nucleophilicity:
❖ Definition: The reactivity of nucleophile or tendency to donate electron pair to electron-deficient centre is called nucleophilicity.
❖ Rules:
  • •
    Charged nucleophiles are stronger than their conjugate neutral nucleophiles.
  • •
    Across a period, nucleophilicity decreases from left to right.
  • •
    Down a group, nucleophilicity increases.
  • •
    Bulky group on nucleophilic centre decreases nucleophilicity.
❖ Examples:
  1. \(OH^- > H_2O\)
  2. \(NH_2^- > NH_3\)
  3. \(SH^- > H_2S\)
  4. \(CH_3^- > NH_2^- > OH^- > F^-\)
  5. \(I^- > Br^- > Cl^- > F^-\)
  6. \(SeH^- > SH^- > OH^-\)
  7. \(R_3P > R_3N\)
❖ Steric Order: \(CH_3O^- > (CH_3)_2CHO^- > (CH_3)_3CO^-\)
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Bond cleavage
▢ Definition: When two atoms joined by a covalent bond are separated, the process is called fission or cleavage of bond.
▢ Reaction Terms:
❖ Substrate: The main organic compound converted into new compound by breaking and formation of covalent bonds.
❖ Reagent: Chemical species which causes the change.
❖ Product: New compound formed after reaction.
▢ Example: \(CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-\)
▢ Types:
  • •
    Homolytic bond fission or homolysis
  • •
    Heterolytic bond fission or heterolysis
📖
Homolytic bond fission
▢ Definition: Homolytic bond fission is symmetrical non-polar bond cleavage in which each bonded atom gets one electron.
▢ General Reaction: \(A-B \rightarrow A\cdot + B\cdot\)
▢ Conditions:
  • •
    High temperature \((\geq 500^\circ C)\)
  • •
    Electricity
  • •
    Light or sunlight
  • •
    Peroxide
  • •
    Free radical
  • •
    Non-polar substrate
  • •
    Vapour state of substrate
  • •
    Non-polar solvent like \(CCl_4\) or \(CS_2\)
▢ Product: Neutral species with unpaired electrons are formed, called free radicals.
▢ Special Points:
  • •
    All reactions involving sunlight usually follow free radical mechanism.
  • •
    Homolytic bond fission gives free radicals as reactive intermediates.
  • •
    Reaction mechanism is called free radical or homolytic mechanism.
▢ Example: \(CH_3-H \xrightarrow{h\nu} CH_3\cdot + H\cdot\)
📖
Heterolytic bond fission
▢ Definition: Heterolytic bond fission is unsymmetrical polar bond cleavage in which one species takes both bonding electrons.
▢ General Reaction:
  1. \(A-B \rightarrow A^- + B^+\), when \(A\) is more electronegative.
  2. \(A-B \rightarrow A^+ + B^-\), when \(B\) is more electronegative.
▢ Main Features:
  • •
    It occurs in bond between atoms having significant electronegativity difference.
  • •
    Oppositely charged species are formed.
  • •
    If positive charge is on carbon, it is called carbocation.
  • •
    If negative charge is on carbon, it is called carbanion.
  • •
    Carbocation and carbanion are reaction intermediates.
▢ Conditions:
  • •
    Significant electronegativity difference
  • •
    Polar bond
  • •
    Polar solvent like \(H_2O\), \(HCOOH\) with high dielectric constant
  • •
    Low temperature
Important
  • •
    Reaction mechanism involving heterolytic bond fission is called heterolytic or ionic mechanism.
  • •
    Energy required for heterolytic fission is usually greater than homolytic fission due to electrostatic attraction between ions.
📖
Reaction intermediates
▢ Definition: Reaction intermediates are short-lived, highly reactive species formed during homolytic or heterolytic bond fission.
▢ Features:
  • •
    They live for about \(10^{-6}\) seconds to a few seconds.
  • •
    They disappear after reaction completes.
  • •
    They are highly reactive fragments.
▢ Types:
  • •
    Carbocation
  • •
    Carbanion
  • •
    Free radical
  • •
    Carbene
  • •
    Nitrene
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Carbocation
▢ Definition: Organic species or ion carrying positive charge and six electrons on carbon atom is called carbocation or carbonium ion.
▢ Formation: It is formed by heterolytic bond fission when carbon loses the bonding electron pair to a more electronegative atom or group.
▢ Naming: Carbocations are named by adding the word carbocation to parent alkyl group.
▢ Types:
❖ Alkyl Carbocations:
◉ Definition: If positive charge is present on alkyl carbon, it is called alkyl carbocation.
◉ Types:
  1. \(CH_3^+\): methyl carbocation
  2. \(RCH_2^+\): primary alkyl carbocation
  3. \(R_2CH^+\): secondary alkyl carbocation
  4. \(R_3C^+\): tertiary alkyl carbocation
◉ Stability Factors:
  1. Hyperconjugation
  2. Inductive effect
◉ Stability Order: \(R_3C^+ > R_2CH^+ > RCH_2^+ > CH_3^+\)
❖ Vinyl Carbocation:
◉ Definition: If positive charge is present on vinylic carbon, it is called vinyl carbocation.
◉ Example: \(CH_2=CH^+\)
◉ Stability: It is very unstable because positive charge is present on doubly bonded carbon, which is more electronegative.
❖ Allyl Carbocation:
◉ Definition: If positive charge is present on allylic carbon, it is called allyl carbocation.
◉ Example: \(CH_2=CH-CH_2^+\)
◉ Stability: Allyl carbocation is more stable than alkyl carbocation due to resonance.
◉ Types:
  1. \(CH_2=CH-CH_2^+\): primary allyl carbocation
  2. \(CH_2=CH-CH^+R\): secondary allyl carbocation
  3. \(CH_2=CH-C^+R_2\): tertiary allyl carbocation
◉ Order: Tertiary allyl carbocation > secondary allyl carbocation > primary allyl carbocation
❖ Phenyl Methyl Carbocations:
◉ Definition: If positive charge is present on benzylic carbon, it is called phenyl methyl or benzyl carbocation.
◉ Types:
  1. \(C_6H_5CH_2^+\): benzyl carbocation or phenyl methyl carbocation
  2. \((C_6H_5)_2CH^+\): diphenyl methyl carbocation
  3. \((C_6H_5)_3C^+\): triphenyl methyl carbocation
◉ Stability: Stability is explained by resonance.
◉ Number of Resonating Structures:
◈ Triphenyl Methyl Carbocation: 10
◈ Diphenyl Methyl Carbocation: 7
◈ Benzyl Carbocation: 4
◉ Order: \((C_6H_5)_3C^+ > (C_6H_5)_2CH^+ > C_6H_5CH_2^+\)
❖ Aromatic Carbocations:
◉ Definition: Cations in which positive charge is present on carbon of aromatic system are called aromatic carbocations.
◉ Example: Tropylium cation
◉ Stability: They are highly stable because they obey Huckel rule \((4n+2)\) and positive charge is completely delocalized.
◉ Special Point: Tropylium carbocation is about \(10^{11}\) times more stable than triphenyl methyl carbocation.
▢ Overall Stability Order: \(Tropylium\ cation > (C_6H_5)_3C^+ > (C_6H_5)_2CH^+ > C_6H_5CH_2^+ > CH_2=CH-CH_2^+ > R_3C^+ > R_2CH^+ > RCH_2^+ > CH_3^+ > CH_2=CH^+ > HC\equiv C^+\)
▢ Characteristics:
  • •
    It has three bond pairs and one empty \(p\)-orbital.
  • •
    It has six electrons in valence shell, so octet is incomplete.
  • •
    All six electrons are paired.
  • •
    It is diamagnetic.
  • •
    Carbon atom is \(sp^2\)-hybridized.
  • •
    Shape is trigonal planar.
  • •
    It is formed by heterolytic bond fission.
  • •
    It reacts with nucleophiles.
▢ Reactions Involving Carbocation Intermediate:
  1. Elimination reactions \((E_1)\) to form alkenes from alkyl halides and alcohols.
  2. Electrophilic addition reaction of alkenes and alkynes.
  3. Electrophilic substitution or alkylation reaction of benzene.
  4. \(S_N1\) reactions.
  5. Molecular pinacol-pinacolone rearrangement.
Reactivity Rule
Reactivity is inverse of stability. Greater the stability, lesser the reactivity.
📖
Carbanion
▢ Definition: Organic ion containing negatively charged carbon atom is called carbanion.
▢ Formation: It is formed by heterolytic bond fission when carbon is attached to a less electronegative atom and carbon takes the electron pair.
▢ General Structure: \(R_3C^-\)
▢ Characteristics:
  • •
    Alkyl carbanion has three bond pairs and one lone pair.
  • •
    Hybridization is usually \(sp^3\).
  • •
    Geometry is pyramidal, similar to \(NH_3\).
  • •
    Allyl and benzyl carbanions are almost planar and \(sp^2\) due to resonance.
  • •
    Carbon carrying negative charge contains eight electrons in valence shell.
  • •
    It is diamagnetic because all electrons are paired.
  • •
    It is highly reactive and electron-rich.
  • •
    Carbanions behave as nucleophiles and are attacked by electrophiles.
  • •
    It is formed by heterolytic bond fission.
▢ Stability Factors:
❖ Electronegativity of Carbanionic Carbon:
◉ Rule: \(\text{Stability} \propto \text{electronegativity of carbanionic carbon} \propto \%s\text{-character}\)
◉ Order: \(CH_3CH^-\ (sp^3) < CH_2=CH^-\ (sp^2) < HC\equiv C^-\ (sp)\)
❖ Inductive Effect:
◉ +I Group: \(\text{Stability} \propto \dfrac{1}{+I\ power}\). More \(+I\) group decreases carbanion stability.
◉ -I Group: \(\text{Stability} \propto -I\ power\). More \(-I\) group increases carbanion stability.
◉ Example: \(NO_2CH_2^- > FCH_2^-\)
❖ Resonance or Delocalization:
◉ Rule: Allyl and benzyl carbanions are stabilized by delocalization of negative charge.
◉ Order: \((C_6H_5)_3C^- > (C_6H_5)_2CH^- > C_6H_5CH_2^- > CH_2=CH-CH_2^-\)
❖ Stabilization by Electron Withdrawing Groups:
◉ Groups: \(C=O, NO_2, CN\)
◉ Meaning: These groups stabilize carbanion by resonance effect.
◉ Example: Enolate ion is stabilized because negative charge is delocalized towards oxygen.
❖ Aromatic Carbanions:
◉ Definition: Anions in which negative charge is present on carbon of aromatic system are called aromatic carbanions.
◉ Example: Cyclopentadienyl anion
◉ Stability: Aromatic carbanions are most stable because they obey Huckel rule and negative charge is completely delocalized.
▢ Overall Stability Order: \(Aromatic\ carbanion > Benzyl\ carbanion > Allyl\ carbanion > HC\equiv C^- > H_2C=CH^- > alkyl\ carbanion\)
▢ Relative Stability of Alkyl Carbanions: \(CH_3^- > RCH_2^- > R_2CH^- > R_3C^-\)
▢ Reactions Involving Carbanion Intermediate:
  1. Condensation reaction of carbonyl compounds like aldol condensation and Perkin reaction.
  2. Reformatsky reaction.
  3. Decarboxylation of acids.
  4. Condensation reaction of esters like Claisen condensation.
  5. Wittig reaction.
  6. Reaction of halides by \(Zn/HCl\).
Reactivity Rule
Order of reactivity of carbanions is reverse order of stability.
📖
Carbon free radicals
▢ Definition: Carbon free radicals are odd electron species in which carbon bears one unpaired electron.
▢ Formation: They are formed by homolytic bond fission.
▢ General Reaction: \(R-Z \rightarrow R\cdot + Z\cdot\)
▢ Formation Conditions: Heat, light or catalyst initiate free radical formation.
▢ HELP-R Rule: Chemical reaction in presence of \(H\): heat \((\geq 500^\circ C)\), \(E\): electricity, \(L\): light, \(P\): peroxide, \(R\): radical is called free radical reaction.
▢ Characteristics:
  • •
    Free radicals are generally electrically neutral.
  • •
    Carbon has seven electrons in outermost orbit; six are bonded and one is unpaired.
  • •
    Incomplete octet is present.
  • •
    Due to odd electron, free radical is paramagnetic.
  • •
    They are highly reactive.
  • •
    Radical carbon is usually \(sp^2\)-hybridized and planar.
  • •
    Free radicals are neutral electrophiles.
  • •
    Free radicals generally react with free radicals.
▢ Stability:
❖ Alkyl Free Radicals:
◉ Rule: Stability of alkyl free radicals is explained by hyperconjugation.
◉ Order: \(3^\circ\ free\ radical > 2^\circ\ free\ radical > 1^\circ\ free\ radical > methyl\ free\ radical\)
❖ Allyl and Benzyl Free Radicals:
◉ Rule: Stability is explained by resonance or delocalization.
◉ Order: \((C_6H_5)_3C\cdot > (C_6H_5)_2CH\cdot > C_6H_5CH_2\cdot > CH_2=CH-CH_2\cdot > 3^\circ > 2^\circ > 1^\circ > vinyl\)
❖ Substituted Benzyl Free Radicals:
◉ Rule: Electron donating groups like \(-OCH_3\) and \(-CH_3\) increase free radical stability, while electron withdrawing groups like \(-NO_2\) decrease stability.
▢ Reactions Involving Free Radicals:
  1. Wurtz reaction giving alkanes.
  2. Free radical substitution reactions of alkanes.
  3. Kolbe's electrolytic reaction giving alkane, alkene and alkyne.
  4. Anti-Markovnikov addition or peroxide effect or Kharasch effect.
  5. Side chain halogenation of arenes.
  6. Allylic or benzylic substitution by NBS.
Bond Dissociation Energy
Stability of free radicals can be compared from bond dissociation energy required for homolytic cleavage. Smaller energy required for bond breaking means more stable radical.
📖
Carbenes
▢ Definition: Carbenes are neutral, divalent, highly reactive carbon intermediates represented by \(:CX_2\).
▢ Electron Count: Carbon in carbene has six electrons in outer shell: four bonded and two non-bonded electrons.
▢ Formation:
  • •
    They are formed by homolytic cleavage.
  • •
    They are related to carbanions through alpha-elimination reaction.
▢ Nature:
  • •
    Carbenes are electron deficient.
  • •
    They accept electron pair to complete octet.
  • •
    Therefore they behave as electrophiles.
▢ Simplest Carbene: \(:CH_2\), called methylene.
▢ Examples:
  1. \(C_6H_5CH:\): phenyl carbene
  2. \(R_2C:\): dialkyl carbene
  3. \(:CCl_2\): dichlorocarbene
▢ Hybridization: Divalent carbon of carbene may be \(sp^2\) or \(sp\).
▢ Types:
❖ Singlet Carbene:
◉ Definition: Carbene in which two non-bonded electrons are present in one orbital with opposite spin is singlet carbene.
◉ Features:
  • •
    Unshared electrons are paired in one orbital.
  • •
    It has bent or angular structure.
  • •
    Hybridization is \(sp^2\).
  • •
    It is electron deficient and acts as electrophile.
  • •
    It is diamagnetic.
  • •
    It is less stable and highly reactive.
  • •
    It is also called hot methylene.
  • •
    Singlet state has single spin multiplicity.
❖ Triplet Carbene:
◉ Definition: Carbene in which two non-bonded electrons are present in different orbitals with same spin is triplet carbene.
◉ Features:
  • •
    Unshared electrons are not paired.
  • •
    It has permanent magnetic moment in magnetic field.
  • •
    Hybridization is \(sp\).
  • •
    Shape is linear.
  • •
    It is paramagnetic.
  • •
    It is electron deficient and acts as electrophile.
  • •
    It is more stable than singlet carbene.
  • •
    It is also called cold methylene.
▢ Special Points:
  • •
    Triplet carbene is more stable than singlet carbene due to less repulsion between two non-bonded electrons in different orbitals.
  • •
    Stability of singlet carbenes: \(:CH_2 > :CF_2 > :CCl_2 > :CBr_2\)
  • •
    \(:CH_2\) can exist in singlet and triplet states, but \(:CX_2\) exists only in singlet state.
  • •
    Dichlorocarbene acts as electrophile in carbylamine reaction and Reimer-Tiemann reaction.
▢ Main Reactions:
  • •
    Addition with alkenes
  • •
    Insertion reaction between \(C-H\) bond
📖
Nitrenes or imidogens
▢ Definition: Nitrenes are neutral, monovalent nitrogen intermediates with one bond and two non-bonded electron pairs.
▢ Similarity: They are similar to carbenes.
▢ General Structure: \(:N-R\)
▢ Electron Count: Nitrogen has six electrons in outermost shell, so it is electron deficient.
▢ Nature:
  • •
    Nitrenes are highly reactive.
  • •
    They act as strong electrophiles.
  • •
    They need one electron pair to complete octet.
▢ Parent Species: \(H-N:\), called nitrene, imidogen, azene or imine.
▢ Examples:
  1. \(RCON:\): acyl nitrene
  2. \(C_6H_5N:\): phenyl nitrene
  3. \(C_2H_5N:\): ethyl nitrene
▢ Reaction Importance:
  • •
    Intermediate acyl nitrene is formed in Hofmann bromamide reaction.
  • •
    Nitrene intermediate is also present in Schmidt reaction and Curtius reaction.
📖
Resonance or mesomeric effect
▢ Definition: Resonance is the delocalization of \(\pi\)-electrons, lone pair, odd electron, positive charge or negative charge in a conjugated system.
▢ Term: The term mesomeric effect was given by Ingold.
▢ Resonating Structures: When one structure is not enough to explain all properties of a compound, many possible structures are drawn on paper. These are called resonating structures, canonical structures, valence bond structures or contributing structures.
▢ Resonance Hybrid: The actual structure of the compound is resonance hybrid formed by all contributing structures.
▢ Cause: Resonating structures arise due to alternate overlapping of \(p\)-orbitals.
▢ Example: Benzene has two Kekule structures, and real benzene is resonance hybrid.
▢ Conjugation:
❖ Definition: An atom or group is in conjugation with an unsaturated system if it is directly linked to one atom of multiple bond through a single bond, or if it has \(\pi\)-bond, positive charge, negative charge, odd electron or lone pair.
❖ Examples:
  1. \(CH_2=CH-CH=CH_2\): conjugation between \(C=C\) and \(C=C\)
  2. \(CH_2=CH-C\equiv N\): conjugation between \(C=C\) and \(C\equiv N\)
  3. \(CH_2=CH-CH_2^+\): positive charge conjugated with \(C=C\)
  4. \(CH_2=CH-NH_2\): lone pair conjugated with \(C=C\)
▢ Conjugate Positions:
❖ Definition: Alternate positions of molecule having \(\pi\)-bond, positive charge, negative charge, odd electron or lone pair are called conjugate positions.
❖ Rule: Compound having at least two conjugate positions is called conjugated compound.
❖ Formula: \(\text{Number of resonating structures} = \text{Number of conjugate positions}\)
▢ Situations Where Resonance is Possible:
  1. When \(\pi\)-bond makes conjugation with another \(\pi\)-bond.
  2. When lone pair makes conjugation with double bond.
  3. When positive charge makes conjugation with double bond.
  4. When negative charge makes conjugation with double bond.
  5. When odd electron makes conjugation with double bond.
Important Rules
  1. If any conjugate position has more than one \(\pi\)-bond, only one \(\pi\)-bond takes part in conjugation.
  2. If any conjugate position has more than one lone pair, only one lone pair takes part in resonance.
  3. If any conjugate position has \(\pi\)-bond and also positive charge, negative charge, odd electron or lone pair, then only \(\pi\)-bond takes part in conjugation.
  4. In aromatic compounds, key atom attached with benzene is in conjugation if it has \(\pi\)-bond, positive charge, negative charge, odd electron or lone pair.
  5. Electron negative charge or lone pair behaves as two \(\pi\)-electrons in conjugation.
📖
Properties of conjugated compounds
▢ Properties:
  1. Conjugated electrons migrate from one conjugate position to another.
  2. Conjugated electrons are delocalized electrons.
  3. Conjugated compounds are delocalized compounds.
  4. They can be represented by two or more possible structures called resonating structures.
  5. Number of resonating structures of conjugated compounds equals number of conjugate positions.
  6. This formula is not valid for benzene and fused benzene systems like naphthalene and anthracene.
  7. Resonating structures are not real structures.
  8. Real structure is hybrid of all resonating structures.
▢ Benzene Derivative Rule: Number of resonating structures of benzene derivative in which key atom is in conjugation with benzene ring = three resonating structures per benzene ring + one resonating structure corresponding to key atom.
▢ Examples:
❖ Benzyl Carbocation: \(C_6H_5CH_2^+\): 4 resonating structures
❖ Diphenyl Methyl Carbocation: \((C_6H_5)_2CH^+\): 7 resonating structures
❖ Triphenyl Methyl Carbocation: \((C_6H_5)_3C^+\): 10 resonating structures
📖
Types of resonance effect
▢ Classification: A group in conjugation with conjugated system can be classified as \(+R/+M\) group or \(-R/-M\) group.
▢ +R or +M Group:
❖ Definition: Groups which give electrons to the conjugated system are called \(+R\) or \(+M\) groups.
❖ Effects:
  • •
    They increase electron density on benzene ring.
  • •
    They activate benzene ring.
  • •
    They are ortho-para directing groups.
  • •
    They increase reactivity of benzene ring towards electrophilic substitution.
❖ Identification: If the atom directly attached to benzene has negative charge or at least one lone pair, then the group is usually \(+R\).
❖ Examples: \(-O^-, -NH_2, -OH, -OR, -NHCOR\)
❖ Power Order: \(-O^- > -NH_2 > -NHR > -OH > -OR > -NHCOR\)
Important
All \(+R\) groups are ring activators except halogens, which are ring deactivators due to strong \(-I\) effect.
▢ -R or -M Group:
❖ Definition: Groups which withdraw electrons from the conjugated system are called \(-R\) or \(-M\) groups.
❖ Effects:
  • •
    They decrease electron density on benzene ring.
  • •
    They deactivate benzene ring.
  • •
    They are meta-directing groups.
  • •
    They decrease reactivity of benzene ring towards electrophilic substitution.
❖ Identification: If the atom directly attached to benzene has positive charge or is bonded with electronegative atom by multiple bond, then the group is \(-R/-M\).
❖ Examples: \(-NO_2, -CN, -SO_3H, -CHO, -COR, -COOH, -COOR, -COCl\)
❖ Power Order: \(-NO_2 > -CN > -SO_3H > -CHO > -COR\)
Position Rule
  • •
    Resonance effect is always seen at ortho and para positions of benzene ring.
  • •
    There is no resonance effect at meta position.
  • •
    \(+R\) group gives electrons to ortho and para positions.
  • •
    \(-R\) group withdraws electrons from ortho and para positions.
📖
Applications of resonance
▢ Effect on Bond Length:
❖ Concept:
  • •
    Due to resonance, bond length becomes intermediate between single and double bond.
  • •
    Resonance decreases bond length of single bond.
  • •
    Resonance increases bond length of double bond.
  • •
    Single bond gets partial double bond character and double bond gets partial single bond character.
❖ Benzene Example:
  1. Normal \(C-C\) single bond length = \(1.54\ \mathring{A}\)
  2. Normal \(C=C\) double bond length = \(1.33\ \mathring{A}\)
  3. Experimental benzene \(C-C\) bond length = \(1.40\ \mathring{A}\)
▢ Effect on Bond Length of Functional Group:
❖ Rule: \(\text{Bond length of functional group} \propto \dfrac{1}{\text{Number of resonating structures}}\)
❖ Examples:
  1. \(CH_3CH_2Cl\): no resonance, longest \(C-Cl\) bond
  2. \(CH_2=CHCl\): 2 resonating structures
  3. \(C_6H_5Cl\): 4 resonating structures, shortest \(C-Cl\) bond
❖ Order: \(CH_3CH_2Cl > CH_2=CHCl > C_6H_5Cl\) for \(C-Cl\) bond length
▢ Effect on Bond Strength and Reactivity:
❖ Bond Strength: Bond strength is directly proportional to number of resonating structures.
❖ Reactivity: \(\text{Reactivity due to bond breaking} \propto \dfrac{1}{\text{Number of resonating structures}}\)
❖ Example: \(CH_3CH_2Cl > CH_2=CHCl > C_6H_5Cl\)
❖ Reason: Chlorobenzene is least reactive because \(C-Cl\) bond gets partial double bond character due to resonance.
▢ Stability of Conjugated Species:
❖ Conjugated Compound: Conjugated compound is more stable than corresponding non-conjugated compound.
❖ Aromatic Compound: Aromatic compound is more stable than non-aromatic conjugated compound.
❖ Overall Stability: Aromatic compound > conjugated non-aromatic compound > non-conjugated compound > antiaromatic compound
▢ Stability of Carbocation, Carbanion and Free Radical:
❖ By Resonance: \((C_6H_5)_3C^+ > (C_6H_5)_2CH^+ > C_6H_5CH_2^+ > CH_2=CH-CH_2^+\)
❖ By +I Effect: \(3^\circ > 2^\circ > 1^\circ > methyl\)
❖ By Electronegativity: \(CH_2=CH^+ > HC\equiv C^+\) is less stable because positive charge on more electronegative carbon is unstable.
❖ Benzyl vs Allyl: Benzyl carbocation is more stable than allyl carbocation because benzyl has more resonating structures.
📖
Stability of substituted benzyl carbocations
▢ Case I: Group is -R and -I:
❖ Examples: \(-NO_2, -CHO, -COOH\)
❖ Rule:
  • •
    \(-I\) group withdraws electrons and increases positive charge, so stability decreases.
  • •
    \(-R\) group at ortho and para withdraws electrons and further decreases stability.
  • •
    At meta position, resonance effect is zero.
❖ Order: meta > para > ortho
▢ Case II: Group is +R and +I:
❖ Example: \(-CH_3\)
❖ Rule:
  • •
    \(+I\) increases stability.
  • •
    \(+R\) at ortho and para increases stability.
  • •
    At meta position, resonance effect is zero.
❖ Order: para > ortho > meta
▢ Case III: Group is +R and -I:
❖ Example: \(-OCH_3\)
❖ Rule:
  • •
    \(+R\) stabilizes carbocation at ortho and para.
  • •
    \(-I\) destabilizes carbocation.
  • •
    Resonance effect is usually stronger than inductive effect.
❖ Order: para > ortho > meta
Important
  • •
    Resonance effect at meta position is zero.
  • •
    Resonance effect does not depend on distance.
  • •
    Inductive effect depends on distance and is inversely proportional to distance.
📖
Acidic nature of phenol and derivatives
▢ Phenol Ionization: \(C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+\)
▢ Main Rule: Acidity of substituted phenols depends on stability of phenoxide ion.
▢ Electron Withdrawing Groups:
❖ Examples: \(-NO_2, -CN, -SO_3H\)
❖ Effect: They increase acidic character of phenol because they stabilize phenoxide ion by \(-I\), \(-R\) or both effects.
❖ Order Example: p-nitrophenol > o-nitrophenol > m-nitrophenol > phenol
❖ Special Point: Ortho nitrophenol is expected to be more acidic than para due to stronger \(-I\), but intramolecular hydrogen bonding reduces its tendency to lose \(H^+\).
▢ Electron Donating Groups:
❖ Examples: \(-CH_3, -OH, -NH_2, -OCH_3\)
❖ Effect: They suppress acidic character because they destabilize phenoxide ion.
❖ Reason: Electron releasing groups destabilize phenoxide ion by \(+I\), \(+R\) or hyperconjugation.
▢ Case II: Group is +H and +I:
❖ Example: \(-CH_3\)
❖ Order: phenol > m-cresol > p-cresol > o-cresol
❖ Reason: At meta position hyperconjugation effect is zero; ortho is least acidic due to stronger donating effect.
▢ Case III: Group is +M and -I:
❖ Examples: \(-OCH_3, -OH, -NH_2\)
❖ Rule: Mesomeric donation destabilizes phenoxide ion more strongly than inductive withdrawal stabilizes it.
❖ Order Example: m-methoxyphenol > phenol > o-methoxyphenol > p-methoxyphenol
▢ Case IV: Acidity of Halo Phenols:
❖ Rule: Halogens donate by resonance but withdraw by inductive effect. Inductive effect predominates, so halophenols are more acidic than phenol.
❖ Position Order: ortho-halophenol > meta-halophenol > para-halophenol
❖ Halogen Order: o-chlorophenol > o-bromophenol > o-iodophenol > o-fluorophenol
❖ Special Point: In p-fluorophenol, \(+R\) and \(-I\) effects of fluorine balance each other, so acidity is almost like phenol.
📖
Acidic strength of benzoic acid and derivatives
▢ Basic Comparison: Benzoic acid is stronger than phenol and generally stronger than aliphatic carboxylic acid.
▢ Exception: \(HCOOH > C_6H_5COOH\)
▢ Ionization: \(C_6H_5COOH \rightleftharpoons C_6H_5COO^- + H^+\)
▢ Main Rule: Acidity of carboxylic acid depends on stability of carboxylate ion.
▢ Electron Withdrawing Groups: Electron withdrawing groups stabilize carboxylate ion and increase acidity.
▢ Electron Donating Groups: Electron donating groups destabilize carboxylate ion and decrease acidity.
Ortho Effect
Due to ortho effect, ortho derivative of benzoic acid is usually stronger than meta and para derivatives whether the group is electron withdrawing or donating.
▢ Case I: Group is -I and -M:
❖ Example: \(-NO_2\)
❖ Order: o-nitrobenzoic acid > p-nitrobenzoic acid > m-nitrobenzoic acid > benzoic acid
❖ Reason:
  • •
    Ortho and para are stabilized by both inductive and resonance effects.
  • •
    Meta is stabilized only by inductive effect.
  • •
    Ortho derivative is strongest due to ortho effect.
▢ Case II: Group is +H and +I:
❖ Example: \(-CH_3\)
❖ Order: o-toluic acid > benzoic acid > m-toluic acid > p-toluic acid
❖ Reason:
  • •
    Methyl group destabilizes carboxylate ion by \(+I\) and hyperconjugation.
  • •
    Ortho derivative is still highly acidic due to ortho effect.
▢ Case III: Group is -I and +M:
❖ Example: \(-OCH_3\)
❖ Order: o-methoxybenzoic acid > benzoic acid > m-methoxybenzoic acid > p-methoxybenzoic acid
❖ Reason:
  • •
    At ortho: high acidity due to ortho effect.
  • •
    At meta: \(+M\) is zero and \(-I\) stabilizes.
  • •
    At para: \(+M\) destabilizes more strongly.
📖
Basicity of aromatic amines
▢ Ionization: \(B + H^+ \rightarrow BH^+\)
▢ Basicity Constant: \(K_b = \dfrac{[BH^+]}{[B][H^+]}\)
▢ Formula: \(\text{Basic strength of base} \propto K_b \propto \text{stability of conjugate acid} \propto \dfrac{1}{pK_b}\)
▢ Main Rule:
  • •
    Basicity of aromatic amines depends on electron density on nitrogen.
  • •
    More electron density on nitrogen means more basicity.
  • •
    In aromatic amines, lone pair on nitrogen is delocalized into benzene ring.
  • •
    Due to resonance, electron density on nitrogen decreases and basicity decreases.
▢ Resonance Rule: \(\text{Basicity} \propto \dfrac{1}{\text{Number of resonating structures}}\)
▢ Example Order: \(C_6H_5NH_2 > C_6H_5NHC_6H_5 > C_6H_5N(C_6H_5)_2\)
▢ Basicity of Substituted Aniline:
Important
Ortho derivative of aniline usually has least basic character due to ortho effect, whatever the nature of group may be.
❖ Case I: Group is -I and -M:
◉ Example: \(-NO_2\)
◉ Order: aniline > m-nitroaniline > p-nitroaniline > o-nitroaniline
◉ Reason:
  • •
    Nitro group decreases basicity.
  • •
    At meta, \(-M\) effect is zero.
  • •
    Ortho is least basic due to ortho effect.
❖ Case II: Group is +I and +H:
◉ Example: \(-CH_3\)
◉ Order: p-toluidine > m-toluidine > aniline > o-toluidine
◉ Reason:
  • •
    Methyl group increases basicity by \(+I\) and hyperconjugation.
  • •
    Ortho derivative is least basic due to ortho effect.
❖ Case III: Group is +M and -I:
◉ Example: \(-OCH_3\)
◉ Order: p-anisidine > aniline > m-anisidine > o-anisidine
◉ Reason:
  • •
    At para, \(+M\) increases electron density and increases basicity.
  • •
    At meta, \(+M\) effect is zero and \(-I\) decreases basicity.
  • •
    At ortho, ortho effect decreases basicity.
📖
Directive influence and reactivity of benzene ring towards electrophilic substitution
▢ Types of Directing Groups:
  • •
    Ortho and para directing groups
  • •
    Meta directing groups
▢ Ortho and Para Directive Groups:
❖ General Order: \(O^- > NH_2 > OH > OR > NHCOR > OCOR > R > X\)
❖ Nature:
  • •
    Most ortho-para directing groups increase electron density on benzene ring.
  • •
    They activate ring towards electrophilic substitution.
  • •
    Halogens are exception: they are ortho-para directing but deactivating.
▢ Meta Directing Groups:
❖ Examples: \(-NO_2, -CN, -SO_3H, -CHO, -COR, -COOH, -COOR\)
❖ Nature:
  • •
    They withdraw electrons from benzene ring.
  • •
    They decrease electron density.
  • •
    They deactivate benzene ring.
  • •
    They direct incoming electrophile to meta position.
Rate Rule
Rate of electrophilic substitution is proportional to activating power of the group, which increases electron density of benzene ring, and inversely proportional to deactivating power of the group.
Q1.
NH4CNO → NH2CONH2. This reaction is an example of
📅BPKIHS 2011
Q2.
Resonance (mesomeric effect) occurs due to
📅BPKIHS 2002
Q3.
Which of the following groups exert +I effect?
📅IOM 2008
Q4.
An alkyl halide can be converted into alcohol by
📅IOM 2008•IOM 2004
Q5.
Nitration in benzene is
📅MOE 2008
Q6.
Which of the following is the strongest acid?
📅MOE - Curriculum
Q7.
The temporary effect in which bond pair of electrons and double bonds are shifted completely is called
📅KU 2008
Q8.
Delocalization of electrons in a π bond is
📅IOM 2008
Q9.
Nitrobenzene forms aniline by
📅IOM 2002
Q10.
Covalent bond undergoes homolysis resulting in formation of
📅MOE
Q11.
The shortest bond length is in
📅I.E.
Q12.
Which of the following is electrophile?
📅MOE 2053