34Purification and analysis of organic compounds

📚
PURIFICATION AND CHARACTERIZATION OF ORGANIC COMPOUNDS
▢ Main Methods:
  • •
    Purification
  • •
    Qualitative analysis
  • •
    Quantitative analysis
  • •
    Molecular mass determination
  • •
    Calculation of empirical formula and molecular formula
▢ Purification:
❖ Meaning: Removal of undesirable impurities from an organic compound to obtain it in pure state
❖ Criteria of Purity:
  • •
    Constant melting point for solids
  • •
    Constant boiling point for liquids
❖ Effect of Impurity: Impurity decreases melting point of solids but increases boiling point of liquids
▢ Methods for Purification:

Table 1: Purification Methods

Method
Used For
Crystallization
Purification of solids
Sublimation
Sublimable solids
Distillation
Purification of liquids
Differential extraction
Extraction using immiscible solvent
Chromatography
Separation / purification of complex mixtures
Atmolysis
Separation of gases / vapours by diffusion rate
▢ Crystallization:
❖ Principle: Compound and impurities must differ considerably in solubility in a particular solvent
❖ Process: Dissolve impure solid in hot solvent → filter hot → cool solution → crystals separate → filter and dry
❖ Suitable Solvent:
  • •
    Dissolves solute at high temperature
  • •
    Dissolves little solute at low temperature
  • •
    Does not react chemically with solute
  • •
    Impurities should either remain insoluble or highly soluble
❖ Seeding: Addition of small crystal to initiate crystallization
❖ Simple Crystallization: Used when impurity is either insoluble or highly soluble
❖ Fractional Crystallization: Used to separate two soluble solids having different solubility in same solvent
❖ Examples:
  • •
    Sugar + common salt separated by ethanol
  • •
    Benzoic acid + naphthalene separated by hot water
  • •
    KClO3 and KCl separated by fractional crystallization
  • •
    Na2SO4 and Na2Cr2O7 separated by fractional crystallization
▢ Sublimation:
❖ Definition: Direct conversion of solid into vapour by heating and vapour into solid on cooling without passing through liquid state
❖ Use: Purification of substances that sublime and are non-volatile impurities free
❖ Examples:
  • •
    Camphor
  • •
    Iodine
  • •
    Benzoic acid
  • •
    Naphthalene
  • •
    Anthracene
  • •
    NH4Cl
  • •
    Indigo
  • •
    Phthalic anhydride
  • •
    Salicylic acid
  • •
    HgCl2
❖ Special Point: Mixture of camphor and benzoic acid can be separated by chemical method, not by sublimation
▢ Distillation:
❖ Definition: Purification of liquids by vaporization followed by condensation
❖ Basic Steps:
  1. Vaporization
  2. Condensation
❖ Simple Distillation:
◉ Use: Volatile liquids boiling without decomposition
◉ Condition: Boiling point difference about 30–50 K
◉ Examples:
  • •
    Hexane + toluene
  • •
    Benzene + aniline
  • •
    Ether + toluene
❖ Steam Distillation:
◉ Definition: Co-distillation with water
◉ Use: Purification of steam-volatile, water-insoluble substances having high vapour pressure at boiling point of water
◉ Principle: Total vapour pressure of two immiscible liquids = sum of their individual vapour pressures
◉ Examples:
  • •
    Essential oils from flowers
  • •
    Aniline
  • •
    Nitrobenzene
  • •
    o-Hydroxy acetophenone
  • •
    Sandalwood oil
  • •
    Bromobenzene
  • •
    o- and p-nitrophenol mixture
❖ Fractional Distillation:
◉ Use: Separation of liquids with close boiling points
◉ Condition: Boiling point difference about 10–15 K
◉ Example: Acetone and methyl alcohol
◉ Important Point: Petroleum is separated by fractional distillation
❖ Azeotropic Distillation:
◉ Azeotrope: Mixture having constant boiling temperature and constant composition
◉ Point: Azeotropic mixtures cannot be separated by ordinary fractional distillation
◉ Examples:
  • •
    HCl + H2O
  • •
    Ethanol + water
❖ Vacuum Distillation:
◉ Use: Purification of liquids that decompose below or near normal boiling point
◉ Principle: Reduced pressure lowers boiling point
◉ Example: Glycerin
▢ Atmolysis:
❖ Definition: Separation of mixture of gases or vapours based on difference in rates of diffusion
❖ Example: Methane and ethane
▢ Chromatography:
❖ Definition: Technique used for separation and purification of complex mixtures
❖ Inventor: Tswett, 1906
❖ Principle: Based on selective adsorption / partition between stationary and mobile phases
❖ Uses:
  • •
    Purification of organic compounds
  • •
    Separation of proteins, amino acids, vitamins, hormones and plant pigments
  • •
    Removal of moisture by desiccation
❖ Types:

Table 1: Chromatography Types

Type
Principle / Feature
Column chromatography
Adsorption chromatography; simplest method
Paper chromatography
Partition chromatography; stationary phase is water in paper
❖ Adsorbents:
  • •
    Alumina
  • •
    Silica gel
  • •
    MgO
  • •
    Fuller's earth
❖ Special Points:
  • •
    Chromatography is based on selective adsorption
  • •
    It is the fastest technique for purification of small amount of organic compound
  • •
    Stationary phase can be solid or liquid
  • •
    Mobile phase can be liquid or gas
▢ Differential Extraction:
❖ Definition: Separation of organic compound from aqueous solution by shaking with suitable immiscible organic solvent
❖ Principle: Organic compound should be more soluble in organic solvent than in water
❖ Apparatus: Separating funnel
❖ Example: Benzoic acid can be extracted from aqueous solution using benzene
❖ Special Point: Mixture of two immiscible liquids like oil and water is separated by separating funnel
▢ Desiccation:
❖ Meaning: Removal of moisture
❖ Desiccants:
  • •
    Anhydrous CaCl2
  • •
    Silica gel
  • •
    Alumina
  • •
    MgO
❖ Special Point: Anhydrous CaCl2 absorbs water and is used as drying agent
▢ Qualitative Analysis:
❖ Meaning: Detection of elements present in an organic compound
❖ Common Elements:

Table 1: Elements in Organic Compounds

Element
Frequency
Carbon
Always present
Hydrogen
Nearly always present
Oxygen
Generally present
Nitrogen, halogens, sulphur
Less commonly present
Phosphorus and metals
Rarely present
▢ Detection of Carbon and Hydrogen:
❖ Method: Dry organic compound heated with dry CuO
❖ Principle: Carbon and hydrogen are oxidized to CO2 and H2O
❖ Detection:

Table 1: Carbon and Hydrogen Tests

Product
Test
CO2
Turns lime water milky due to CaCO3
H2O
Turns anhydrous CuSO4 blue
▢ Lassaigne Test:
❖ Use: Detection of N, S and halogens
❖ Principle: Organic elements are converted into ionic sodium salts by fusion with sodium
❖ Sodium Fusion Extract: Prepared by fusing organic compound with sodium, then boiling fused mass with water and filtering
❖ Conversions:

Table 1: Lassaigne Conversion

Element
Converted Into
Nitrogen
NaCN
Sulphur
Na2S
Halogen
NaX
Nitrogen + Sulphur
NaSCN
▢ Detection of Nitrogen:
❖ Soda Lime Test: Organic compound heated with soda lime gives ammonia
❖ Lassaigne Test: Sodium cyanide forms sodium ferrocyanide with FeSO4 and finally Prussian blue ferric ferrocyanide with FeCl3
❖ Positive Result: Prussian blue / greenish blue colour
❖ Compound Responsible: Ferric ferrocyanide, Fe4[Fe(CN)6]3
❖ Special Points:
  • •
    Hydrazine does not give this test due to absence of carbon
  • •
    Urea and its derivatives may need cane sugar to increase carbon content
▢ Detection of Sulphur:
❖ Sodium Nitroprusside Test: Sodium extract + sodium nitroprusside → violet or purple colour
❖ Lead Acetate Test: Sodium extract + lead acetate → black precipitate of PbS
❖ Positive Results:
  • •
    Violet colour with sodium nitroprusside
  • •
    Black ppt. with lead acetate
▢ Detection of Nitrogen and Sulphur Together:
❖ Compound Formed: Sodium thiocyanate, NaSCN
❖ Positive Test: Blood red colour with FeCl3 due to ferric thiocyanate
▢ Detection of Halogens:
❖ Beilstein Test:
◉ Also Called: Copper wire test
◉ Positive Result: Green or bluish-green flame
◉ Limitation: Not sure test; compounds like urea and thiourea may give positive flame due to volatile copper halide formation
❖ Lassaigne Test:
◉ Preparation: Sodium extract is boiled with HNO3 to remove NaCN and Na2S before adding AgNO3

Table 1: Halogen Detection

Halogen
AgNO3 Result
NH4OH Solubility
Chlorine
White ppt. AgCl
Soluble
Bromine
Pale yellow ppt. AgBr
Sparingly soluble
Iodine
Yellow ppt. AgI
Insoluble
❖ Special Test for Bromine and Iodine: Chlorine water test using CS2 or CCl4 layer; orange colour indicates bromine and violet colour indicates iodine
▢ Quantitative Analysis:

Table 1: Element Estimation Methods

Element
Method
Carbon and hydrogen
Liebig's combustion method
Nitrogen
Dumas method / Kjeldahl method
Halogens
Carius method / Robertson method / Asboth method
Sulphur
Carius method / Messenger method
Phosphorus
Carius method
Oxygen
By difference: 100 − sum of percentages of other elements
▢ Empirical and Molecular Formula:
❖ Empirical Formula: Simplest formula showing simplest whole number ratio of atoms of constituent elements
❖ Molecular Formula: Actual number of atoms of different elements present in one molecule
❖ Relation: Molecular formula = n × empirical formula
❖ n Formula: n = molecular mass / empirical formula mass
❖ Example: Acetic acid empirical formula = CH2O; molecular formula = C2H4O2
▢ High-Yield Read and Digest:
  • •
    Sodium test detects nitrogen
  • •
    Sodium extract is heated with conc. HNO3 before halogen test to decompose Na2S and NaCN
  • •
    Positive Beilstein test suggests halogen may be present but is not confirmatory
  • •
    Cu-wire test is also called Beilstein test
  • •
    In Dumas method nitrogen is estimated as N2
  • •
    In Kjeldahl method nitrogen is estimated as NH3
  • •
    Simple distillation is based on Dalton's law of partial pressure
  • •
    Steam distillation is best for separating o- and p-nitrophenol
  • •
    Atmolysis separates gases or vapours by rate of diffusion
  • •
    Moisture is removed by desiccation
  • •
    King of elements is carbon
  • •
    Standard fixed phase adsorbents include alumina, silica gel and MgO
  • •
    Most abundant organic compound is cellulose
  • •
    Impurity decreases melting point of solids but increases boiling point of liquids
Q1.
The process used to remove undesirable impurities from an organic compound is called
Q2.
The purity of a solid organic compound is generally checked by its
Q3.
The purity of a liquid organic compound is generally checked by its
Q4.
Presence of impurity generally
Q5.
Presence of impurity in a liquid generally
Q6.
Crystallization is mainly used for purification of
Q7.
For crystallization, the compound and impurities should differ considerably in
Q8.
Addition of a small crystal to initiate crystallization is called
Q9.
Fractional crystallization is used when two solids have different
Q10.
Sublimation involves direct conversion of
Q11.
Which of the following is purified by sublimation?
Q12.
A mixture of camphor and benzoic acid can be separated easily by
📅BPKIHS
Q13.
Distillation is mainly used for purification of
Q14.
The two main steps of distillation are
Q15.
Simple distillation is suitable when boiling point difference is about
Q16.
Fractional distillation is used when boiling point difference is about
Q17.
Petroleum is mainly separated by
Q18.
Steam distillation is useful for substances which are
Q19.
Steam distillation works on the principle that total vapour pressure of immiscible liquids is
Q20.
Which pair can be separated by steam distillation?
Q21.
A mixture having constant boiling temperature is called
Q22.
Azeotropic mixtures are separated by
Q23.
Vacuum distillation is used for liquids which
Q24.
Glycerin is purified by
📅BPKIHS
Q25.
Atmolysis separates gases or vapours on the basis of
Q26.
Chromatography was discovered by
Q27.
Chromatography is mainly based on
Q28.
Column chromatography is mainly a type of
Q29.
Paper chromatography is mainly based on
Q30.
Which of the following is a common adsorbent in chromatography?
Q31.
Differential extraction uses
Q32.
Benzoic acid can be extracted from aqueous solution using
Q33.
Oil and water are best separated by
Q34.
Qualitative analysis of organic compounds means
Q35.
Carbon and hydrogen are detected by heating organic compound with
Q36.
Carbon is detected by passing evolved CO2 through
Q37.
Hydrogen in an organic compound is detected by
Q38.
Lassaigne test is used for detection of
Q39.
In Lassaigne test, nitrogen is converted into
Q40.
In Lassaigne test, sulphur is converted into
Q41.
When both nitrogen and sulphur are present, sodium fusion may form
Q42.
Prussian blue colour in nitrogen test is due to
📅MOE
Q43.
Hydrazine does not give Lassaigne nitrogen test because it lacks
Q44.
Sulphur gives violet or purple colour with
Q45.
Sulphur gives black precipitate with lead acetate due to formation of
Q46.
Blood red colour with FeCl3 in sodium extract indicates presence of
📅MOE 2063
Q47.
Beilstein test is also called
Q48.
Positive Beilstein test gives
Q49.
Before adding AgNO3 in Lassaigne halogen test, sodium extract is boiled with HNO3 to remove
Q50.
Chlorine in Lassaigne extract gives with AgNO3
Q51.
Bromine gives with AgNO3
Q52.
Iodine gives with AgNO3
Q53.
Chlorine water test with CS2 layer gives orange colour for
Q54.
Chlorine water test with CS2 layer gives violet colour for
Q55.
Liebig's combustion method is used to estimate
Q56.
Dumas method estimates nitrogen as
Q57.
Kjeldahl method estimates nitrogen as
Q58.
Carius method is used for estimation of
Q59.
Percentage of oxygen in an organic compound is usually found by
Q60.
Empirical formula represents
Q61.
Molecular formula is related to empirical formula by
Q62.
The value of n in molecular formula calculation is