43Aldehyde and Ketones

📚
INTRODUCTION
▢ Carbonyl Compounds: Organic compounds containing carbonyl/oxo group \(>C=O\); mainly aldehydes and ketones.
▢ General Formula: \(C_nH*{2n}O\)
▢ Functional Isomerism: Aldehydes and ketones are functional isomers.

Table 1: Aldehydes vs Ketones

Point
Aldehydes
Ketones
General formula
\(RCHO\), \(ArCHO\)
\(RCOR\), \(RCOAr\), \(ArCOAr\)
Functional group
\(-CHO\) / formyl group
\(-CO-\) / keto group
Valency of group
Monovalent
Bivalent
Position
Always terminal
Never terminal
Oxidation source
1° alcohol → aldehyde
2° alcohol → ketone

Table 2: Carbonyl compounds vs acid derivatives

Point
Aldehydes / Ketones
Acids / Acid derivatives
Carbonyl group
Free characteristic \(>C=O\)
\(>C=O\) involved in resonance
Reaction of carbonyl group
Gives \(>C=O\) reactions
Does not give usual \(>C=O\) reactions
Characteristic reaction
Nucleophilic addition reaction / NAR
Nucleophilic substitution reaction / NSR
▢ Ketone Classification:
❖ Simple / Symmetrical Ketone: Two similar alkyl/aryl groups attached to carbonyl carbon.
❖ Mixed / Unsymmetrical Ketone: Two different alkyl/aryl groups attached to carbonyl carbon.
❖ Example: Acetophenone = mixed ketone; acetone, diethyl ketone and benzophenone = symmetrical ketones.
▢ Carbonyl Carbon:
  • •
    Hybridisation: \(sp^2\)
  • •
    Bond angle: nearly \(120^\circ\)
  • •
    Carbonyl group is polar due to electronegativity difference between C and O.
  • •
    Carbonyl carbon has partial positive charge.
  • •
    Partial positive charge on C → high reactivity towards nucleophilic addition.
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PREPARATION OF ALDEHYDES AND KETONES
▢ For Both Aldehydes and Ketones:
❖ From Alkenes: Wacker Process:
  • •
    Oxidation of alkenes by \(O_2\) / air.
  • •
    Catalyst: \(PdCl_2 + CuCl_2\) in water.
  • •
    Double bond not cleaved.
  • •
    Same number of carbon atoms retained.
  • •
    Formaldehyde cannot be prepared.
  • •
    \(CH_2=CH_2 + \frac{1}{2}O_2 \xrightarrow{PdCl_2/CuCl_2,H_2O} CH_3CHO\)
  • •
    \(CH_3CH=CH_2 + \frac{1}{2}O_2 \xrightarrow{PdCl_2/CuCl_2,H_2O} CH_3COCH_3\)
❖ From Alkynes: Kucherov Reaction:
  • •
    Hydration of alkyne.
  • •
    Reagents: dilute \(H_2SO_4\), 1% \(HgSO_4\).
  • •
    Temperature: nearly \(60^\circ C\).
  • •
    Enol formed first, then tautomerises to carbonyl compound.
  • •
    Formaldehyde cannot be prepared.
  • •
    \(HC\equiv CH + H_2O \rightarrow CH_2=CHOH \rightarrow CH_3CHO\)
  • •
    \(CH_3C\equiv CH + H_2O \rightarrow CH_3C(OH)=CH_2 \rightarrow CH_3COCH_3\)
❖ Kucherov Product Table:

Table 1: Alkyne hydration products

Alkyne
Product
Acetylene
Acetaldehyde
Propyne
Acetone
1-Butyne
Butanone
2-Butyne
Butanone
2-Pentyne
2-Pentanone
3-Pentyne
2-Pentanone
❖ From Haloalkanes:
◉ Oxidation with DMSO:
  1. \(RCH_2X \xrightarrow{DMSO} RCHO\)
  2. \(R_2CHX \xrightarrow{DMSO} R_2CO\)
◉ Alkaline Hydrolysis of Gem-Dihalides:
  • •
    Terminal gem-dihalide → aldehyde.
  • •
    Non-terminal gem-dihalide → ketone.
  • •
    \(RCHCl_2 \xrightarrow{aq.Ba(OH)_2} RCH(OH)_2 \rightarrow RCHO + H_2O\)
  • •
    \(R_2CCl_2 \xrightarrow{aq.Ba(OH)_2} R_2C(OH)_2 \rightarrow R_2CO + H_2O\)
❖ From Grignard Reagent and Cyanides:
  • •
    HCN + Grignard reagent → aldehyde after hydrolysis.
  • •
    Alkyl/aryl nitrile + Grignard reagent → ketone after hydrolysis.
  • •
    \(RCN + R'MgX \rightarrow\) imine magnesium salt \(\xrightarrow{H_3O^+} RCOR'\)
❖ From Monohydric Alcohols:
◉ Controlled Oxidation:
  • •
    1° alcohol → aldehyde.
  • •
    2° alcohol → ketone.
  • •
    Oxidizing agents: acidified \(KMnO_4\), acidified \(K_2Cr_2O_7\), \(CrO_3\).
  • •
    Aldehydes are easily further oxidized to acids.
  • •
    PCC gives aldehydes from 1° alcohols in good yield.
  • •
    PCC does not oxidise \(C=C\).
  • •
    Collin's reagent: \(CrO_3 + 2\) pyridine.
  • •
    Sarett's reagent: \(CrO_3 + 2\) pyridine in dichloromethane.
◉ Catalytic Dehydrogenation:
  • •
    Reagent/condition: Cu, \(300^\circ C\).
  • •
    1° alcohol → aldehyde.
  • •
    2° alcohol → ketone.
  • •
    3° alcohol → alkene by dehydration.
  • •
    Better method because aldehyde is not further oxidized.
❖ From Vicinal Diols:
  • •
    Vicinal diols oxidized by periodic acid \((HIO_4)\) or lead tetraacetate \((CH_3COO)_4Pb\).
  • •
    C-C bond between two \(-OH\)-bearing carbons is cleaved.
  • •
    Carbonyl compounds formed.
❖ From Monocarboxylic Acids:
◉ Heating with MnO:
  • •
    Condition: manganous oxide, \(300^\circ C\).
  • •
    Decarboxylation + dehydration.
  • •
    Formic acid alone → formaldehyde.
  • •
    Acids other than formic acid → ketone.
  • •
    Formic acid + other acid → aldehyde other than formaldehyde.
◉ Dry Distillation of Calcium Salts:
  • •
    Calcium formate alone → formaldehyde.
  • •
    Calcium salt + calcium formate → aldehyde.
  • •
    Calcium salt of carboxylic acid other than formic acid → ketone.
  • •
    Poor yield for aldehydes due to side reactions.
  • •
    Unsymmetrical ketones not prepared well because mixture of ketones forms.
  • •
    Calcium salts of dicarboxylic acids on heating → cyclic ketones.
▢ For Aldehydes Only:
❖ Oxo Process:
  • •
    Industrial method.
  • •
    Alkene + water gas \((CO+H_2)\) → aldehyde.
  • •
    Catalyst: octacarbonyl dicobalt \([Co(CO)_4]_2\).
  • •
    Condition: high temperature and pressure.
  • •
    \(RCH=CH_2 + CO + H_2 \rightarrow RCH_2CH_2CHO\)
  • •
    Further reduction with \(LiAlH_4\) → 1° alcohol.
❖ Rosenmund Reduction:
  • •
    Acid chloride → aldehyde.
  • •
    \(RCOCl + H_2 \xrightarrow{Pd/BaSO_4} RCHO + HCl\)
  • •
    Solvent: boiling xylene.
  • •
    Catalyst: Pd supported on \(BaSO_4\), poisoned by S or quinoline.
  • •
    Poisoned catalyst prevents aldehyde → alcohol reduction.
  • •
    Formaldehyde cannot be prepared because formyl chloride \((HCOCl)\) is unstable.
❖ Stephen Reduction:
  • •
    Alkyl cyanide reduced by \(SnCl_2/HCl\) in ether.
  • •
    Iminochloride formed first.
  • •
    Hydrolysis gives aldehyde.
  • •
    \(RCN \xrightarrow{SnCl_2/HCl} \) iminochloride \(\xrightarrow{H_2O/\Delta} RCHO\)
❖ Atomic Hydrogen Method: \(CO + 2H \rightarrow HCHO\)
❖ Aromatic Aldehydes:
◉ Etard Reaction:
  • •
    Toluene partially oxidized by chromyl chloride \((CrO_2Cl_2)\).
  • •
    Hydrolysis gives benzaldehyde.
  • •
    Side chain higher than \(-CH_3\): terminal carbon of side chain oxidized to \(-CHO\).
◉ With \(CrO_3\) in Acetic Anhydride:
  • •
    Toluene → benzaldehyde.
  • •
    Aldehyde trapped as gem-diacetate.
  • •
    Further oxidation to benzoic acid prevented.
◉ With \(V_2O_5\):
  • •
    Toluene oxidized by air/O2.
  • •
    Catalyst: \(V_2O_5\).
  • •
    Temperature: \(350^\circ C\).
  • •
    Product: benzaldehyde.
◉ Side Chain Chlorination + Hydrolysis:
  • •
    Industrial method for benzaldehyde.
  • •
    Toluene → benzal chloride → benzaldehyde.
◉ Gattermann-Koch Reaction:
  • •
    Arene + \(CO+HCl\) in presence of \(AlCl_3/CuCl\) → aromatic aldehyde.
  • •
    Since formyl chloride is unstable, \(CO+HCl\) acts as formylating system.
  • •
    Electrophile: formyl cation \(HCO^+\).
  • •
    \(CO + HCl + AlCl_3 \rightarrow HCO^+ + AlCl_4^-\)
◉ Gattermann Aldehyde Synthesis: Aromatic compound formylation using \(HCN/HCl\) in presence of Lewis acid.
◉ Vilsmeier Reaction: Aromatic compound → aldehyde using secondary amine/formamide derivative and acid chloride-type reagent.
▢ For Ketones Only:
❖ With Dialkyl Cadmium:
  • •
    Acid chloride + dialkyl cadmium → ketone.
  • •
    \(RCOCl + R'_2Cd \rightarrow RCOR'\)
  • •
    Dialkyl cadmium prepared from \(CdCl_2\) and Grignard reagent.
  • •
    Grignard reagent not used directly because it further reacts with ketone to give 3° alcohol.
  • •
    Dialkyl cadmium is organometallic compound.
❖ Oppenauer Oxidation:
  • •
    2° alcohol → ketone.
  • •
    Reagent: aluminium tert-butoxide / aluminium isopropoxide.
  • •
    No further oxidation to acid.
  • •
    Does not attack \(C=C\).
  • •
    Reverse of Meerwein-Ponndorf-Verley reduction.
❖ Friedel-Crafts Acylation:
  • •
    Benzene + acyl chloride / acid anhydride in anhydrous \(AlCl_3\) → aromatic ketone.
  • •
    Benzene + phosgene / \(AlCl_3\) → benzophenone.
  • •
    Electrophilic substitution reaction.
❖ Fries Rearrangement:
  • •
    Phenyl ester \(\xrightarrow{AlCl_3}\) o- and p-hydroxyaryl ketones.
  • •
    Low temperature \((\le 330K)\) favours p-isomer.
  • •
    High temperature \((\ge 430K)\) favours o-isomer.
❖ Special Notes:
  • •
    Acetophenone used in perfumery and medicine as hypnotic under name hypnone.
  • •
    Benzaldehyde occurs as amygdalin in bitter almonds; called oil of bitter almonds.
  • •
    Acetaldehyde used as antiseptic inhalant in nose trouble.
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PHYSICAL PROPERTIES
▢ State:
  • •
    Formaldehyde = gas at room temperature.
  • •
    Other members = colourless liquids or solids depending on molecular size.
▢ Smell:
  • •
    Lower aliphatic aldehydes → unpleasant smell.
  • •
    Benzaldehyde → bitter almond smell.
  • •
    Higher aldehydes and ketones → mostly pleasant smell.
▢ Solubility:
  • •
    Lower members up to C4 are water soluble due to H-bonding with water.
  • •
    Solubility decreases with increase in alkyl/aryl size.
  • •
    Less soluble than corresponding alcohols.
  • •
    Aromatic aldehydes and ketones are generally insoluble in water.
  • •
    Soluble in organic solvents: alcohol, ether, benzene.
  • •
    Solubility \(\propto \frac{1}{molecular\ weight}\)
▢ Boiling Points:
  • •
    Higher than hydrocarbons of comparable molecular mass due to dipole-dipole attraction.
  • •
    Lower than alcohols due to absence of self intermolecular H-bonding.
  • •
    Ketones have slightly higher boiling points than isomeric aldehydes.
  • •
    Order: alcohol > ketone > aldehyde > alkane.
  • •
    Order example: \(HCHO < CH_3CHO < CH_3CH_2CHO < CH_3COCH_3\).
  • •
    Boiling point \(\propto \frac{molecular\ weight}{number\ of\ side\ chains}\)
▢ Density: Density of carbonyl compounds is lower than water.
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CHEMICAL PROPERTIES
▢ General Reactivity:
  • •
    Aldehydes and ketones are highly reactive due to polar carbonyl group.
  • •
    Aldehydes are more reactive than ketones because of less steric hindrance and presence of H on carbonyl carbon.
  • •
    Nucleophilic addition reaction is characteristic reaction.
▢ Reactivity Towards NAR:
  • •
    Reactivity \(\propto\) positive charge on \(sp^2\) carbon.
  • •
    Reactivity \(\propto \frac{1}{steric\ hindrance}\).
  • •
    \(CCl_3CHO > HCHO > CH_3CHO\)
  • •
    \(CH_3CHO > CH_3CH_2CHO > CH_3COCH_3\)
  • •
    \(RCHO > ArCHO > ArCOR > ArCOAr\)
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NUCLEOPHILIC ADDITION REACTIONS
▢ Addition of Water:
❖ Product: Gem-diol / hydrate / 1,1-diol.
❖ Nature: Reversible reaction; hydrates usually unstable.
❖ Order: \(HCHO > CH_3CHO > (CH_3)_3CCHO > CH_3COCH_3\)
❖ Key Point: More reactive carbonyl compound forms more stable hydrate.
❖ Stable Exception: Chloral hydrate; reaction with water is not easily reversible.
❖ Most Stable Hydrate: Formaldehyde gives maximum stable hydrate among common carbonyl compounds.
▢ Addition of Alcohols:
❖ Aldehydes:
  • •
    Aldehyde + 1 equivalent alcohol / dry HCl → hemiacetal.
  • •
    Hemiacetal + alcohol / dry HCl → acetal.
  • •
    Hemiacetal = alcohol + ether functional character.
  • •
    Acetal = ether-type product.
  • •
    Acetal can be hydrolysed by dilute mineral acid to regenerate aldehyde.
  • •
    Dihydric alcohol such as ethylene glycol gives cyclic acetal.
❖ Ketones:
  • •
    Ketones generally do not react with monohydric alcohols easily.
  • •
    Ketones react with dihydric alcohols to give cyclic ketals.
▢ Addition of Hydrogen Cyanide:
❖ Reaction: Aldehyde/ketone + HCN → cyanohydrin.
❖ Products: α-hydroxy nitriles.
❖ Important:
  • •
    All aldehydes form cyanohydrins.
  • •
    Among ketones, common reactive examples: acetone, butanone, pentan-2-one.
  • •
    Reaction increases carbon chain by one carbon after hydrolysis of nitrile.
▢ Addition of Sodium Bisulphite:
❖ Reaction: Carbonyl compound + \(NaHSO_3\) → bisulphite addition compound.
❖ Positive Compounds:
  1. All aldehydes
  2. Aliphatic methyl ketones \(RCOCH_3\)
  3. Benzaldehyde
  4. Acetone
❖ Negative Example: Acetophenone does not react with \(NaHSO_3\).
❖ Product: White crystalline sodium bisulphite addition compound.
❖ Uses:
  • •
    Identification of carbonyl compounds.
  • •
    Separation and purification of aldehydes/ketones from non-carbonyl compounds.
  • •
    Separation of acetaldehyde from acetophenone.
❖ Regeneration: Adduct + dilute mineral acid/alkali on heating → original carbonyl compound.
▢ Derivatives of Ammonia:
❖ Nature:
  • •
    Condensation / addition-elimination reactions.
  • •
    Proceed best in weakly acidic medium.
  • •
    Products are crystalline solids with sharp melting points.
  • •
    Used for identification, characterization and purification of aldehydes and ketones.

Table 1: Ammonia derivatives and products

Reagent
Product
Example name
\(RNH_2\) / primary amine
Imine / Schiff base
Benzaldehyde + aniline → benzalaniline
\(NH_2OH\) / hydroxylamine
Oxime
Acetaldehyde → acetaldoxime
\(NH_2NH_2\) / hydrazine
Hydrazone
Acetaldehyde hydrazone
\(C_6H_5NHNH_2\) / phenylhydrazine
Phenylhydrazone
Acetaldehyde phenylhydrazone
2,4-DNP / 2,4-dinitrophenylhydrazine
2,4-dinitrophenylhydrazone
Yellow/orange/red precipitate
Semicarbazide
Semicarbazone
Acetaldehyde semicarbazone
❖ 2,4-DNP Test: Aldehydes and ketones give yellow/orange/red precipitate with 2,4-DNP.
▢ Reaction with Grignard Reagent:
❖ General: Carbonyl compound + \(RMgX\) followed by hydrolysis → alcohol.

Table 1: Grignard reagent products

Carbonyl compound
Product after hydrolysis
Formaldehyde
1° alcohol
Aldehyde except formaldehyde
2° alcohol
Ketone
3° alcohol
❖ Reaction Type: Nucleophilic addition reaction.
📚
REDUCTION REACTIONS
▢ Reduction to Methylene Group:
❖ Transformation: \(>C=O \rightarrow >CH_2\)
❖ Reagents:
  • •
    Red P/HI at \(150^\circ C\)
  • •
    Zn-Hg / conc. HCl = Clemmensen reduction
  • •
    \(NH_2NH_2/KOH\) = Wolff-Kishner reduction
▢ Reduction to Alcohols:
❖ Transformation: \(>C=O \rightarrow >CHOH\)
❖ Reagents:
  • •
    \(H_2/Ni\), \(H_2/Pd\), \(H_2/Pt\)
  • •
    \(LiAlH_4\)
  • •
    \(NaBH_4\)
  • •
    Na/\(C_2H_5OH\)
  • •
    NaH/benzene
❖ Product Rule:
  • •
    Aldehyde → 1° alcohol.
  • •
    Ketone → 2° alcohol.
❖ LiAlH4 vs NaBH4:
  • •
    \(LiAlH_4\) is stronger reducing agent.
  • •
    \(LiAlH_4\) reduces aldehydes, ketones, acids, acid chlorides, esters, amides, nitriles, oximes, alkyl halides and nitro compounds.
  • •
    \(NaBH_4\) mainly reduces aldehydes and ketones; also acid chlorides and alkyl halides in given notes.
  • •
    Neither \(LiAlH_4\) nor \(NaBH_4\) reduces isolated \(C=C\).
  • •
    Catalytic hydrogenation can reduce both \(C=C\) and \(C=O\).
▢ Meerwein-Ponndorf-Verley Reduction:
  • •
    Ketone → 2° alcohol.
  • •
    Reagent: aluminium isopropoxide in isopropyl alcohol.
  • •
    Hydride transfer from isopropyl alcohol to ketone.
  • •
    Reverse of Oppenauer oxidation.
▢ Pinacol Formation:
  • •
    Ketones on bimolecular reduction give pinacols.
  • •
    Reagent: magnesium amalgam + water.
📚
OXIDATION REACTIONS
▢ Oxidation of Aldehydes:
❖ General: Aldehydes are easily oxidized to carboxylic acids with same number of carbon atoms.
❖ Oxidizing Agents:
  • •
    Strong: \(HNO_3\), \(KMnO_4\), \(K_2Cr_2O_7\).
  • •
    Weak: bromine water, \(Cu^{2+}\), \(Ag^+\).
❖ Reducing Nature: Aldehydes act as reducing agents.
❖ Fehling Solution:
  • •
    Alkaline \(CuSO_4\) solution complexed with Rochelle salt.
  • •
    Aldehyde reduces \(Cu^{2+}\) to red precipitate of \(Cu_2O\).
  • •
    \(RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O\downarrow + 3H_2O\)
❖ Benedict Solution:
  • •
    Alkaline \(Cu^{2+}\) solution complexed with citrate ions.
  • •
    Gives red precipitate of \(Cu_2O\).
  • •
    Standard test for glucose/sugar in urine.
❖ Tollens Reagent:
  • •
    Ammoniacal \(AgNO_3\), contains \([Ag(NH_3)_2]^+OH^-\).
  • •
    Aldehyde reduces Tollens reagent to metallic Ag.
  • •
    Silver mirror test.
  • •
    Both aliphatic and aromatic aldehydes reduce Tollens reagent.
  • •
    Ketones do not reduce Tollens reagent.
❖ Important Distinction:
  • •
    Fehling and Benedict: mainly aliphatic aldehydes.
  • •
    Benzaldehyde reduces Tollens reagent but does not reduce Fehling/Benedict.
  • •
    Glucose and fructose reduce Tollens, Fehling and Benedict reagents.
▢ Oxidation of Ketones:
❖ General:
  • •
    Ketones do not have H attached to carbonyl carbon.
  • •
    Do not oxidize with weak oxidizing agents such as Tollens/Fehling.
  • •
    Strong oxidizing agents cleave C-C bonds adjacent to carbonyl.
  • •
    Products: mixture of carboxylic acids with fewer carbon atoms.
❖ Oxidizing Agents: \(K_2Cr_2O_7/H_2SO_4\), \(KMnO_4/H_2SO_4\)
❖ Popoff Rule: In unsymmetrical ketones, keto group preferentially remains with smaller alkyl group during oxidative cleavage.
❖ Symmetrical Ketones: Only one type of cleavage product.
📚
MISCELLANEOUS REACTIONS
▢ Aldol Reaction and Aldol Condensation:
❖ Condition: Aldehyde/ketone having at least one α-H.
❖ Reagent: Dilute NaOH or dilute KOH.
❖ Intermediate: Carbanion/enolate.
❖ Aldol Reaction: Formation of β-hydroxy aldehyde or β-hydroxy ketone.
❖ Aldol Condensation: Dehydration of aldol on heating/acid → α,β-unsaturated carbonyl compound.
❖ Aldol Meaning: Aldehyde + alcohol groups present in product.
❖ Examples:
  1. 2CH3CHO \(\xrightarrow{dil.NaOH}\) CH3CH(OH)CH2CHO
  2. 2CH3COCH3 \(\xrightarrow{dil.NaOH}\) diacetone alcohol
❖ Important:
  • •
    HCHO does not give aldol because no α-H.
  • •
    Acetaldehyde gives aldol but not Cannizzaro.
  • •
    If conc. NaOH used, continuous condensation may form resin polymer.
▢ Crossed / Mixed Aldol:
❖ Definition: Aldol condensation between two different aldehydes/ketones or aldehyde + ketone.
❖ Condition: At least one reactant should have α-H.
❖ Product Rule:
  • •
    If both carbonyl compounds have α-H → 4 products possible: 2 self + 2 cross.
  • •
    If only one has α-H → 2 products possible: 1 self + 1 cross.
❖ Claisen-Schmidt Condensation: Base-catalysed crossed aldol between aromatic aldehyde and aliphatic aldehyde/ketone.
▢ Cannizzaro Reaction:
❖ Condition:
  • •
    Only aldehydes having no α-H.
  • •
    Strong/concentrated alkali: conc. NaOH or KOH.
  • •
    Ketones do not give Cannizzaro.
❖ Nature: Redox reaction; one molecule oxidized to carboxylate, another reduced to alcohol.
❖ Examples:
  1. \(2HCHO \xrightarrow{conc.NaOH} HCOONa + CH_3OH\)
  2. \(2PhCHO \xrightarrow{conc.NaOH} PhCOONa + PhCH_2OH\)
❖ Special Points:
  • •
    Formaldehyde and benzaldehyde give Cannizzaro but not aldol.
  • •
    Acetaldehyde gives aldol but not Cannizzaro.
  • •
    Simple Cannizzaro = disproportionation / auto-redox.
  • •
    Cross Cannizzaro = redox but not disproportionation.
❖ Cross Cannizzaro:
  • •
    Two different aldehydes without α-H.
  • •
    More active aldehyde/aliphatic aldehyde is oxidized.
  • •
    Less active aldehyde is reduced.
  • •
    \(PhCHO + HCHO \xrightarrow{conc.NaOH} HCOONa + PhCH_2OH\)
❖ Exceptions:
  • •
    Chloral \((CCl_3CHO)\) has no α-H but does not give Cannizzaro.
  • •
    2-methylpropanal has one α-H but may show Cannizzaro in given notes.
▢ Tischenko Reaction:
❖ Definition: Modified Cannizzaro reaction.
❖ Condition: Aluminium ethoxide.
❖ Given By: All aldehydes, with or without α-H.
❖ Product: Ester.
❖ Concept: Alcohol and acid formed in Cannizzaro-type process combine to form ester.
▢ Halogenation:
❖ Condition: Aldehydes/ketones having α-H.
❖ Acidic Medium:
  • •
    Can stop at monohalogenation using 1 mole halogen.
  • •
    \(CH_3CHO + Cl_2 \xrightarrow{CH_3COOH} ClCH_2CHO + HCl\)
  • •
    \(CH_3COCH_3 + Br_2 \xrightarrow{CH_3COOH} BrCH_2COCH_3 + HCl\)
❖ Basic Medium: Polyhalogenation occurs; haloform reaction possible.
❖ Formaldehyde: Does not undergo α-halogenation because no α-H.
▢ Reaction with Ammonia:
❖ Aldehydes Except Formaldehyde: Give aldehyde-ammonia adducts; on heating lose water to form aldimines.
❖ Formaldehyde: \(6HCHO + 4NH_3 \rightarrow (CH_2)_6N_4 + 6H_2O\)
❖ Urotropine: Hexamethylenetetramine; urinary antiseptic.
❖ RDX: Controlled nitration of urotropine gives RDX.
❖ Benzaldehyde: Forms hydrobenzamide with ammonia.
❖ Acetone: Reacts with ammonia to give diacetonamine.
❖ Aliphatic Ketones: Give complex condensation products with ammonia.
▢ Benzoin Condensation:
❖ Reaction: 2 aromatic aldehyde molecules \(\xrightarrow{alc.KCN}\) benzoin.
❖ Catalyst: Alcoholic KCN.
❖ Example: \(2PhCHO \xrightarrow{alc.KCN} PhCOCH(OH)Ph\)
❖ Oxidation: Benzoin → benzil.
▢ Reaction with Chloroform:
❖ Given By: Ketones only; not aldehydes.
❖ Condition: Chloroform + alkali.
❖ Example: Acetone + chloroform → chloretone.
❖ Use: Chloretone used as hypnotic.
▢ Reaction with PCl5: Aldehydes/ketones + \(PCl_5\) → gem-dichlorides; carbonyl oxygen replaced by two Cl atoms.
▢ Schiff Test:
❖ Reagent: Schiff reagent = decolourised rosaniline hydrochloride solution using \(SO_2\).
❖ Positive: Aldehydes restore pink colour.
❖ Negative: Ketones do not give Schiff test.
❖ Use: Test for aldehydes.
📚
SPECIAL REACTIONS OF AROMATIC ALDEHYDES AND KETONES
▢ Perkin Reaction:
  • •
    Aromatic aldehyde + acid anhydride + sodium salt of acid → α,β-unsaturated acid after hydrolysis.
  • •
    Benzaldehyde + acetic anhydride + sodium acetate → cinnamic acid.
  • •
    \(PhCHO + (CH_3CO)_2O \xrightarrow{CH_3COONa,H_3O^+} PhCH=CHCOOH\)
▢ Phenacyl Halide Formation:
  • •
    Acetophenone + halogen may give phenacyl halide.
  • •
    Phenacyl chloride = powerful lachrymator / tear gas.
  • •
    Used to disperse mobs.
▢ Reformatsky Reaction:
  • •
    Aldehyde/ketone + α-bromoester + Zn/ether → β-hydroxy ester.
  • •
    Organometallic zinc intermediate involved.
▢ Polymerisation:
❖ Formaldehyde:
  • •
    60% HCHO + conc. \(H_2SO_4\) → trioxane / metaformaldehyde / cyclic trimer.
  • •
    40% HCHO / heat → paraformaldehyde \((CH_2O)_n·H_2O\), n = 6 to 100, white crystalline linear polymer.
  • •
    Paraformaldehyde on heating regenerates formaldehyde.
  • •
    6HCHO / \(Ba(OH)_2\) → formose sugar.
  • •
    Formalin = 40% HCHO + 60% water.
❖ Acetaldehyde:
  • •
    Forms paraldehyde.
  • •
    Forms metaldehyde.
  • •
    Paraldehyde = sweet-smelling liquid; hypnotic.
  • •
    Metaldehyde = used as fuel in spirit lamps.
❖ Acetone:
  • •
    Forms condensation/polymerisation products under suitable conditions.
  • •
    With conc. \(H_2SO_4\) → mesitylene.
▢ Ring Substitution:
❖ General:
  • •
    \(-CHO\) and \(-COR\) groups are electron-withdrawing.
  • •
    Both are deactivating and meta-directing.
  • •
    Electrophilic substitution occurs at meta position.
❖ Halogenation:
  • •
    Nuclear halogenation difficult because side-chain halogenation is faster.
  • •
    Benzaldehyde + cold \(Cl_2\) → benzoyl chloride.
  • •
    Acetophenone + \(Br_2\)/ether/0°C/catalytic \(AlCl_3\) → phenacyl bromide.
  • •
    Acetophenone + \(Br_2\)/excess anhydrous \(AlCl_3\) → m-bromoacetophenone.
  • •
    Halogens oxidise benzaldehyde to benzoic acid, so nuclear halo derivatives prepared indirectly.
❖ Nitration:
  • •
    Benzaldehyde nitration → m-nitrobenzaldehyde, low yield because some benzaldehyde oxidized to benzoic acid.
  • •
    Acetophenone nitration → m-nitroacetophenone.
❖ Sulphonation: Aromatic aldehydes and ketones give meta sulphonic acid derivatives.
📚
TESTS AND DISTINCTIONS
▢ Aldehyde vs Ketone:

Table 1: Distinction between aldehydes and ketones

Test / Reagent
Aldehydes
Ketones
Tollens reagent
Silver mirror
No action
Fehling solution
Red ppt. of \(Cu_2O\); mainly aliphatic aldehydes
No action
Benedict solution
Red ppt.; mainly aliphatic aldehydes
No action
Schiff reagent
Pink colour restored
No action
Alcohol + dry HCl
Acetals easily formed
Ketals not formed easily with monohydric alcohol
Ammonia
Aldehyde-ammonia adducts; HCHO gives urotropine
Complex condensation products
Reduction with \(LiAlH_4\)
1° alcohol
2° alcohol
▢ Comparison of Some Carbonyl Compounds:

Table 1: Reaction comparison

Reagent / Test
HCHO
CH3CHO
C6H5CHO
CH3COCH3
C6H5COCH3
2,4-DNP
Yellow/orange ppt.
Yellow/orange ppt.
Yellow/orange ppt.
Yellow/orange ppt.
Yellow/orange ppt.
Schiff reagent
Pink
Pink
Pink
No action
No action
Tollens reagent
Positive
Positive
Positive
Negative
Negative
Fehling/Benedict
Positive
Positive
Negative
Negative
Negative
I2/NaOH
Negative
Positive
Negative
Positive
Positive
NaHSO3
Addition compound
Addition compound
Addition compound
Addition compound
Negative
NH2OH
Oxime
Oxime
Oxime
Oxime
Oxime
NH3
Urotropine
Aldehyde-ammonia adduct
Hydrobenzamide
Complex product
Complex product
Conc. alkali
Cannizzaro
Aldol/resin
Cannizzaro
Diacetone alcohol
Negative
PCl5
Usually not applicable
Gem-dichloride
Gem-dichloride
Gem-dichloride
Gem-dichloride
▢ High-Yield Test Notes:
  • •
    All aldehydes and ketones give 2,4-DNP test.
  • •
    Only aldehydes give Schiff test.
  • •
    Tollens test: aliphatic + aromatic aldehydes.
  • •
    Fehling/Benedict: mainly aliphatic aldehydes.
  • •
    Haloform test positive for \(CH_3CHO\), acetone and acetophenone.
  • •
    HCHO gives white colour with pyrogallol in given notes.
📚
USES
▢ Formaldehyde:
  • •
    40% aqueous HCHO = formalin.
  • •
    Formalin used for preservation of biological/anatomical specimens.
  • •
    Formalin used as disinfectant and germicide.
  • •
    Used in manufacture of Bakelite.
▢ Acetaldehyde:
  • •
    Used as antiseptic inhalant.
▢ Paraldehyde:
  • •
    Trimer of acetaldehyde.
  • •
    Pleasant smell.
  • •
    Used as hypnotic.
▢ Metaldehyde:
  • •
    Tetramer of acetaldehyde.
  • •
    Used as fuel in spirit lamp.
▢ Benzaldehyde:
  • •
    Oil of bitter almonds.
  • •
    Used as flavouring agent.
  • •
    Used in perfume industry.
▢ Acetone and Methyl Ketones:
  • •
    Widely used as industrial solvents.
📚
INDIVIDUAL MEMBERS
▢ Formaldehyde: Methanal, \(HCHO\).
▢ Acetaldehyde: Ethanal, \(CH_3CHO\).
▢ Acrolein: Acraldehyde, \(CH_2=CHCHO\).
▢ Acetone: Dimethyl ketone, propanone, \(CH_3COCH_3\).
▢ Benzaldehyde: Oil of bitter almonds, \(C_6H_5CHO\).
▢ Acetophenone: Acetylbenzene / methyl phenyl ketone, \(C_6H_5COCH_3\).
▢ Quinones: Cyclic conjugated diketone-type aromatic oxidation products.
📚
HIGH-YIELD POINTS
▢ Must Remember:
  • •
    Aldehydes and ketones show nucleophilic addition reaction.
  • •
    Acids and acid derivatives show nucleophilic substitution reaction.
  • •
    Aldehydes are more reactive than ketones toward NAR.
  • •
    PCC is best for converting 1° alcohol to aldehyde.
  • •
    Cu/300°C dehydrogenation avoids further oxidation of aldehyde.
  • •
    Formaldehyde cannot be prepared by Wacker, Kucherov or Rosenmund reactions.
  • •
    Rosenmund reduction requires poisoned Pd/BaSO4 catalyst.
  • •
    Stephen reduction converts nitriles to aldehydes.
  • •
    Dialkyl cadmium converts acid chlorides to ketones without further addition.
  • •
    Benzaldehyde gives Tollens test but not Fehling/Benedict tests.
  • •
    Ketones do not give Tollens, Fehling, Benedict or Schiff tests.
  • •
    Benzaldehyde and formaldehyde give Cannizzaro reaction.
  • •
    Acetaldehyde gives aldol condensation.
  • •
    2,4-DNP test detects carbonyl group.
  • •
    NaHSO3 test useful for aldehydes and methyl ketones.
  • •
    Acetophenone does not give NaHSO3 addition product.
  • •
    Perkin reaction gives cinnamic acid from benzaldehyde.
  • •
    \(-CHO\) and \(-COR\) are meta-directing and deactivating groups in electrophilic aromatic substitution.
Q1.
In the following reaction product formed is: CH₃COCl —(Pd/H₂, BaSO₄)→ ?
📅IOM 2007
Q2.
In Cannizzaro's reaction
📅IOM 2006
Q3.
Which of the following reacts with KCN to form Benzoin?
📅IOM 2006
Q4.
Atomic hydrogen produces formaldehyde when it reacts with
📅IOM 2004
Q5.
Which of the following can't reduce Fehling's solution?
📅IOM•KU
Q6.
Reduction of aldehyde gives
📅IOM 2000
Q7.
Which of the following compound is obtained by the reaction of formaldehyde and ammonia?
📅MOE
Q8.
Dry heating of calcium acetate gives
📅MOE 2061
Q9.
Acetone is prepared in lab by dry distillation of
📅IOE
Q10.
The ozonolysis of ethylene followed by reduction with Zn dust and water gives
📅Bangladesh Embassy 2008
Q11.
Aldehyde and Ketone are separated by
📅Bangladesh Embassy 2008
Q12.
Which one shows Cannizzaro reaction?
📅KU 2008
Q13.
The compound which will give only acetaldehyde on ozonolysis is
📅MOE 2065
Q14.
Which of the following organic compound respond to both Iodoform test and Fehling's test?
📅MOE 2065
Q15.
When ethyl alcohol is treated with acidified potassium dichromate it forms acetaldehyde. It is an example of
📅MOE 2064
Q16.
Aldehyde and Ketones are distinguished by
📅MOE 2063
Q17.
Formaldehyde reacts with ammonia to give
📅MOE
Q18.
Which of the following is produced when acetaldehyde reacts with dilute alkali?
📅MOE 2056
Q19.
Formaldehyde and acetaldehyde can be distinguished by
📅KU
Q20.
Which of the following undergoes haloform reaction?
📅BPKIHS
Q21.
The Cannizzaro's reaction is not given by
📅MOE 2062
Q22.
Iodoform test is given by
📅MOE 2060
Q23.
Which of the following alcohol gives ketone on oxidation?
📅MOE 2060
Q24.
In the reaction CH3COCl treated with H2/Pd-BaSO4, the product formed is
📅IOM 2007
Q25.
In Cannizzaro reaction
📅IOM 2006
Q26.
Which of the following reacts with KCN to form benzoin?
📅IOM 2006
Q27.
Atomic hydrogen produces formaldehyde when it reacts with
📅IOM 2004
Q28.
Which of the following cannot reduce Fehling's solution?
📅IOM•KU
Q29.
Reduction of aldehyde gives
📅IOM 2000
Q30.
Which compound is obtained by the reaction of formaldehyde and ammonia?
📅MOE
Q31.
Dry heating of calcium acetate gives
📅MOE 2061
Q32.
Acetone is prepared in laboratory by dry distillation of
📅IOE
Q33.
Ozonolysis of ethylene followed by reduction with Zn dust and water gives
📅Bangladesh Embassy 2008
Q34.
Aldehyde and ketone are separated by
📅Bangladesh Embassy 2008
Q35.
Which one shows Cannizzaro reaction?
📅KU 2008
Q36.
The compound which will give only acetaldehyde on ozonolysis is
📅MOE 2065
Q37.
Which organic compound responds to both iodoform test and Fehling's test?
📅MOE 2065
Q38.
When ethyl alcohol is treated with acidified potassium dichromate, it forms acetaldehyde. It is an example of
📅MOE 2064
Q39.
Aldehydes and ketones are distinguished by
📅MOE 2063
Q40.
Formaldehyde reacts with ammonia to give
📅MOE
Q41.
Which product is formed when acetaldehyde reacts with dilute alkali?
📅MOE 2056
Q42.
Formaldehyde and acetaldehyde can be distinguished by
📅KU
Q43.
Which of the following undergoes haloform reaction?
📅BPKIHS
Q44.
Cannizzaro reaction is not given by
📅MOE 2062
Q45.
Iodoform test is given by
📅MOE 2060
Q46.
Which alcohol gives ketone on oxidation?
📅MOE 2060
Q47.
Which homologous series is represented by the general formula CnH2nO?
Q48.
Which is a mixed ketone?
Q49.
Ozone reacts with H2C=CH2 to form ozonide which on hydrolysis forms
Q50.
Isopropanol on treatment with copper at 300°C forms
Q51.
Calcium acetate on heating yields
Q52.
On heating calcium acetate and calcium formate, the product formed is
Q53.
When propyne is treated with dilute H2SO4 in presence of HgSO4, the major product is
Q54.
Which compound gives a ketone with a Grignard reagent?
Q55.
During reduction of carbonyl compounds by hydrazine and KOH, the first intermediate formed is
Q56.
Reduction of C=O to CH2 can be carried out with
Q57.
C2H5COCl treated with H2/Pd-BaSO4 gives X + Y. Compounds X and Y are
Q58.
In Stephen reduction, the reducing agent is
Q59.
Toluene on treatment with CrO2Cl2 gives
Q60.
Acetaldehyde reacts with
Q61.
Formation of cyanohydrin from a ketone is an example of
Q62.
The correct order of reactivity of CH3CHO, CH3COCH3 and C6H5COCH3 is
Q63.
In which reaction are aldehydes and ketones distinguished?
Q64.
Schiff's reagent is
Q65.
Acetaldehyde reduces Fehling's solution to form
Q66.
Cu2O is
Q67.
The most reactive compound towards formation of cyanohydrin is
Q68.
Fehling's test is positive for
Q69.
Which does not react with Fehling's solution?
Q70.
Which of the following gives silver mirror with Tollens' reagent?
Q71.
C6H5CHO and (CH3)2CO can be distinguished by testing with
Q72.
Aldehydes may be distinguished from ketones by
Q73.
From which compound is tertiary butyl alcohol obtained by the action of methyl magnesium iodide?
Q74.
Acetone is easily oxidised with
Q75.
Aldehydes and ketones can be reduced to corresponding alkane by
Q76.
Clemmensen reduction of ketone is carried out in presence of
Q77.
An organic compound A on treatment with acidified K2Cr2O7 gives compound B which reacts with I2 and sodium carbonate to form triiodomethane. The compound A is
Q78.
If formaldehyde and KOH are treated together, we get
Q79.
A mixture of benzaldehyde and formaldehyde on treating with aqueous NaOH gives
Q80.
Which forces explain the boiling point of aldehydes and ketones?
Q81.
Benzaldehyde can be prepared by oxidation of toluene by
Q82.
Benzophenone can be converted into benzene using