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Saturday, 25 February 2017

Aldehydes, Ketones and Carboxylic acid - V


Carboxylic Acids:
Carboxylic acids are the compounds containing the carboxylfunctional group (-COOH).
Preparation of carboxylic acid:
(i) From alcohols: Primary alcohols are readily oxidised to carboxylic acids with
common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic or alkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media.

(ii) From aldehydes: Oxidation of aldehydes in presence of mild oxidizing agents like Tollen’s reagent (ammoniacal solution of AgNO3) or Fehling reagent (Fehling solution A (aqueous solution of CuSO4) + Fehling solution B (aqueous solution of sodium potassium tartarate)) forms carboxylic acids.

(iii) From alkyl benzenes: Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate.

(iv) From alkenes: Suitably substituted alkenes are oxidised to carboxylic acids on oxidation with acidic potassium permanganate or acidic potassium dichromate.

(v) From Nitriles: Nitriles on hydrolysis in presence of dilute acids or bases forms amide which on further hydrolysis gives carboxylic acid.
(vi) From Grignard reagent: Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which on hydrolysis forms carboxylic acids.
(vii) From acyl halides and anhydrides: Acid chlorides when hydrolysed with water give carboxylic acids .On basic hydrolysis carboxylate ions are formed which on further acidification forms corresponding carboxylic acids. Anhydrides on hydrolysis forms corresponding acid(s)

(viii) From esters: Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.
Physical properties of carboxylic acids:
(i) Solubility: As the size of alky group increases solubility of carboxylic acid decreases because non-polar part of the acid increases

(ii) Boiling points: Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to extensive association of carboxylic acid molecules through intermolecular hydrogen bonding.
Acidity of carboxylic acids:
Carboxylic acids are more acidic than phenols. The strength of acid depends on extent of ionization which in turn depends on stability of anion formed.

(i) Effect of electron donating substituents on the acidity of carboxylic acids: Electron donating substituent decreases stability of carboxylate ion by intensifying the negative charge and hence decreases acidity of carboxylic acids.

(ii) Effect of electron withdrawing substituent on the acidity of carboxylic acids: Electron withdrawing group increases the stability of carboxylate ion by delocalizing negative charge and hence, increases acidity of carboxylic acid. The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3

(a) Effect of number of electron withdrawing groups: As the number of electron withdrawing groups increases –I effect increases, increasing the acid strength

(b) Effect of position of electron withdrawing group: As the distance between electron withdrawing group and carboxylic group increases, electron withdrawing influence decreases.

Reaction of carboxylic acids:

Reactions involving cleavage of C-OH bond:
Carboxylic acids on heating with mineral acids such as H2SO4 or with P2O5 give corresponding anhydride.

(i) Anhydride formation:

(ii) Esterification: Carboxylic acids are esterified with alcohols in the presence of a
mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.

(iii) Carboxylic acids react with PCl5, PCl3 and SOCl2 to form acyl chlorides.

(iv) Reaction with ammonia (NH3): Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature gives amides.

Reactions involving COOH group:

(i) Reduction: Carboxylic acids are reduced to alcohols in presence of LiAlH4 or B2H6.

(ii) Decarboxylation : Sodium or potassium salts of carboxylic acids on heating with soda lime (NaOH + CaO in ratio of 3:1) gives hydrocarbons which contain one carbon less than the parent acid.

(c) Reactions involving substitution reaction in hydrocarbon part:
(i) Hell-Volhard-Zelinsky reaction: Carboxylic acids having an a -hydrogen are halogenated at the a -position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a -halocarboxylic acids)

(ii) Ring substitution in aromatic acids: Aromatic carboxylic acids undergo electrophilic substitution reactions. Carboxyl group in benzoic acid is electron withdrawing group and is meta directing.


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