Unit-7

Carboxylic acid

The aromatic compound having at least one carboxyl group is called “Carboxylic acid”.

Carboxylic acid is not included with carbonyl compound because the bond betⁿ carbon and oxygen is partial double bond.

Some example of carboxylic acid

Isomers

1) Chain Isomers

2) Functional isomers

General preparation of carboxylic acid

1) By the rxn of sod. alkoxide with carbonmonoxide

Sodium alkoxide when heated with carbon monoxide gives sodium salt of carboxylic acid which on hydrolysis by dil HCl gives carboxylic acid.

2) By heating dicarboxylic acid

3) From trihaloalkane

a) From 1° alcohol (by oxidation of alcohol)

Primary alcohol on oxidation gives aldehyde which further oxidized into carboxylic acid.

b) From Grignard reagent

Grignard reagent react with CO₂ gives addition product which on acidic hydrolysis gives carboxylic acid.

c) From cyanide (Kolbe nitrile rxn)

Cyanide can be converted into carboxylic acid, amide and primary amine.

Properties

• lower member (C₁−C₃) of carboxylic acids are colourless liquids with unpleasant odour and (C₄−C₆) are colourless with rancid butery odour and higher are waxy solid.
• Carboxylic acid are highly soluble in water due to the formation inter-molecular hydrogen bond.
• Boiling point of carboxylic acid is much higher than alcohol having comparable molecular mass because carboxylic acid produce double intermolecular hydrogen bond and exist in dimer form.

1) Acidic character

Carboxylic acid when dissolve in water produce hydrogen ion (H⁺).

On increasing alkyl group acidic character of carboxylic acid decreases.

Acidic character

On increasing electronegativity atom of same carboxylic acid increases the acidic character.

Acidic character of carboxylic acid is decreased on decreasing electronegativity of the atom.

On increasing distance betⁿ electronegative atom and oxygen atom decreases the acidic character.

Carboxylic acid react with highly reactive metal gives hydrogen gas.

Carboxylic acid react with base produce salt and water

Carboxylic acid react with metallic carbonate and bicarbonate

But phenol doesn’t react with metallic carbonate and bicarbonate this shows that carboxylic acid are highly reactive than phenol.

2) Rxn due to hydroxylation (−OH)

Carboxylic acid like react with phosphorus halide, thionyl chloride in presence of pyridine gives corresponding acid halide.

3) Rxn due to carbonyl group

Carboxylic acid undergoes reduction in presence of catalyst gives alcohol.

4) Rxn due to alkyl group

a) Hell−Volhard Zelinsky (HVZ-rxn)

Carboxylic acid having at least one α-hydrogen react with red P and halogen except fluorine (F₂) and iodine (I₂) gives α-halo carboxylic acid.

Rxn with Ammonia

Rxn with alcohol “Esterification”

Preparation of benzoic acid

Properties

Electrophilic Substitution rxn

Carboxylic acid is ring deactivating group it decreases the reactivity of benzene ring toward electrophilic substitution reaction.

According to resonating structure ortho-para position contain positive charge it means meta-position is electron rich centre therefore incoming electrophile attack on meta-position. Hence benzoic acid is meta-director toward electrophilic substitution reaction.

Uses of benzoic acid

• Benzoic acid or sod. benzoate is used in the preservation of food stuff as pickle, tomato ketchup, fruit juice etc.
• It is used in the manufacture of dyes and the antiseptic perfume etc.

Formic acid (Methanoic) (lab preparation)

Principle

Abnormal behaviour of methanoic acid

Methanoic acid can act as both aldehyde and a carboxylic acid. This behaviour of methanoic acid is called Abnormal behaviour.

a) formic acid as an acid

i) Rxn with base

ii) Rxn with alcohol

iii) Rxn with ammonia

b) formic acid as an aldehyde

iv) Tollens test

v) Fehling test

vi) Action with acidified KMnO₄ soln

Uses of methanoic acid

1. It is used for dehydration of hides in leather industry.
2. It is used as antiseptic and preservative for fruits.
3. It is used as a medicine in the treatment of [Unreadable in source].
4. It is used in the lab preparation of carbonmonoxide gas.

Derivative of Carboxylic acid

The organic compound which are derived from the carboxylic acid are called derivative of carboxylic acid.

They are four types.

Acid halide: (R−COX)

Preparation: From carboxylic acid

ii) From salt of carboxylic acid

Preparation

1) From alkane

2) From haloalkane

Properties

i) Catalytic redⁿ gives amine

ii) Redⁿ with LiAlH₄ gives amine

iii) Acidic redⁿ [Zn/HCl / SnCl / Fe/HCl] gives amine

Redⁿ in neutral medium [Zn/NH₄Cl / Zn/H₂O]

Gives hydroxyl amine.

Aromatic Nitro compound

Preparation

Principle: Nitrobenzene is prepared by heating mixture [conc HNO₃ and H₂SO₄] with benzene below 60°C.

Properties

• It is pale yellow liquid with bitter almond odour.
• It is carcinogenic bitter almond odour.
• It is steam volatile in nature.

Properties: Reduction of nitrobenzene

a) Catalytic reduction

b) Reduction by LiAlH₄

c) Redⁿ in acidic medium [Sn/HCl] [Fe/HCl] [Zn/HCl] (lab preparation of aniline)

d) redⁿ in alkaline medium

e) Redⁿ in neutral medium [Zn/NH₄Cl]

f) electrolytic redⁿ

Electrophilic Substitution rxn

According to resonating structure ortho-para position bears positive charge that behaves meta position is electron rich centre. Incoming electrophile are electron deficient particle they always attack on electron rich centre therefore nitrobenzene is meta director toward electrophilic substitution rxn.

Uses of nitrobenzene

• It is used as an oxidizing agent.
• It is used in manufacture of soap polish, floor polish.
• It is use to prepare aniline.

3) Tertiary (3°) amine

Isomerism of amine

i) Chain isomerism

ii) Position isomerism

iii) Functional isomers

iv) Metamerism

The isomers in which the no of alkyl group is different on either side of functional group are called metamers.

and the phenomenon is called metamerism.

Preparation of amine

1) From alkyl halide (Hoffmann’s ammonolysis)

When a alkyl halide is heated with alcoholic soln of amine in a sealed tube gives the mixture of 1°, 2°, 3° as well as ammonium salt.

2) Reduction of Nitroalkane

i) Catalytic redⁿ gives amine

ii) redⁿ with LiAlH₄

iii) Acidic redⁿ [Zn/HCl / Sn/HCl / Fe/HCl]

3) Reduction of alkanenitrile (cyanide)

Separation of 1°, 2° and 3° amine (Hoffmann’s method)

The mixture of 1°, 2° and 3° amine can be separated by the action of diethyl oxalate by following process.

Primary amine produce solid form of di-alkyl oxamide.

2° amine produce liquid form of di-alkyl oxamic ester.

3° amine is unaffected due to the lack of replaceable hydrogen atom.

The mixture is subjected for filtration then the obtained mixture is subjected for filtration and solid mass of dialkyl oxamide is separated. Thus, obtained solid mass is treated with alkali to obtain primary amine.

Remaining mixture is followed by fractional distillation then 3° amine is easily separated.

Remaining mixture again followed by fractional distillation to separate 2° amine which further react with alkali gives secondary amine.

All impurities are removed by fractional distillation. Hence in this way 1° and 2° and 3° amine are separated.

Rxn with nitrous acid (Distinction of 1°, 2° and 3° amine) (Nitrous acid test)

1°, 2° and 3° amine can be distinguished by nitrous acid test.

Primary amine react with nitrous acid gives alcohol and nitrogen gas

Secondary amine react with nitrous acid produce yellow oily layer of nitrosoamine

Basicity of amine

Amine contain electron donating alkyl group which donate electron toward nitrogen atom therefore conc of available of electron in nitrogen increases hence amine are strong base then ammonia.

Basic character

Lewis Concept:

• electron pair donor → base
• electron pair acceptor → acid

Comparative study of 1°, 2° and 3° amine: Basic nature of 1°, 2° and 3° amine is which can be discussed by following facts.

1) Effect of alkyl group (+ve inductive effect)

Alkyl group are electron donating group they donate electron toward nitrogen atom. 3° amine contain 3 alkyl group due to which conc of available of electron is maximum in 3° amine. Hence basicity of amine is:

b) Steric effect

Crowding of alkyl group around the nitrogen atom is called steric effect.

In 3° amine there are three alkyl group around the nitrogen therefore steric effect is very high than 2° and 1° amine. Hence basicity of amine is

c) Solvation effect

protonated amine produce intermolecular hydrogen bond by releasing OH⁻ ion. 1° amine produce 3 intermolecular hydrogen bond hence it is highly soluble than other. Therefore basicity of amine:

By combining all these effect the basicity of amine is

Reactions of amine

i) Carbylamine amine rxn

ii) rxn with HCl

iii) Rxn with alkyl halide (alkylation rxn)

iv) Acylation

v) Benzoylation