This article is on the Aldehydes Ketones and Carboxylic Acids Notes Class 12 of Chemistry. The notes on Aldehydes Ketones and Carboxylic Acids of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the Aldehydes Ketones and Carboxylic Acids chapter of Class 12 for the students so that they can give a quick glance of the chapter.
The chapter has been divided into two articles. This article (Part 2) is on Carboxylic acids. The second article (Part 1) is on Aldehydes and Ketones.
You can check Part 1 of the chapter here: ALDEHYDES AND KETONES
CARBOXYLIC ACIDS (Part 2)
Compounds containing the carboxyl group are distinctly acidic and are called carboxylic acids. They have general formula CnH2nO2. Carboxylic acid derivatives are compounds with functional groups that can be converted to carboxylic acids by a simple acidic or basic hydrolysis.
NOMENCLATURE OF CARBOXYLIC ACIDS
The IUPAC system of nomenclature assigns a characteristic suffix to these classes. The ending “e” is removed from the name of the parent chain and is replaced –”oic” acid.
PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS AND ACID DERIVATIVES
(a) Physical appearance and odour
C1 – C3 = Liquid = Colourless & pungent smelling
C4 – C6 = Liquid (oily) = Colourless & unpleasant smell
When C > C7 = waxy solids= Colourless
(b) Boiling points: Refer the following Flow-chart for a better understanding.
(c) Melting Points:
(i) Carboxylic acid with more than 8 carbon atom and a double bond have low melting point due to the inability to form a stable lattice.
(ii) Structures with even number of C atoms have a higher melting point as compared to structures having odd number of C-atoms.
(iii) The high M.P of primary and secondary amides are due to the strong hydrogen bonding and the presence of electron-withdrawing and electron-donating group.
(i) Lower carboxylic acid (C1 – C2) are miscible with water acids
(ii) Instead of dimerization, they form H-bonds with water.
(iii) As the length of chain increases, the solubility decreases.
(iv) The derivatives like acid chlorides and anhydrides react with solvents like H2O & alcohol.
METHODS OF PREPARATION OF CARBOXYLIC ACIDS
The general reaction of preparation of carboxylic acid:
(a) Synthesis of carboxylic acids by the carboxylation of Grignard reagents
(b) Synthesis of Carboxylic acids by the hydrolysis of nitriles Mechanism:
Hydrolysis of cyanides (Acid catalyzed):
CHEMICAL REACTIONS OF CARBOXYLIC ACIDS
Acidity of carboxylic acids :
Tips and Tricks: Acidity of acids is compared by comparing the stability of the conjugate base
Reactions Involving Removal of Proton From–OH Group
A stronger acid displaces a weaker acid from the salt of the weaker acid.
CH3COOH (Stronger acid) + CH3ONa → CH3COONa CH3-OH
Example: CH3COOH (Stronger acid) + NaHCO3 → CH3COONa + H2CO3 (Weaker acid) → H2O +CO2↑ (lab. test of carboxylic acid).
Reactions Involving Replacement of –OH Group
(a) Formation of acid chlorides
(b) Fisher Esterification: Carboxylic acid reacts with alcohol to form esters through a condensation reaction known as esterification.
Mechanism : (Acid-catalyzed esterification)
(i) A reactive electrophile is generated by addition of a proton or a Lewis acid.
(ii) A tetrahedral intermediate containing two equivalent hydroxyl groups is obtained by the nucleophilic attack of the alcohol.
(iii) Elimination of these hydroxyl groups after a proton shift (tautomerism) occurs leading to the formation of water and the ester.
The forward reactions give acid catalyzed esterification of an acid while the reverse account for the acid-catalyzed hydrolysis of an ester
Acid-catalyzed ester hydrolysis.
For esterification of an acid we can use an excess of the alcohol and removal of water to prevent the reverse reaction, Hydrolysis of an ester uses a large excess of water and reﬂuxing the ester with dilute aqueous HCl or dilute aqueous H2SO4 occurs.
(c) Formation of amides:
In fact, amides cannot be prepared from carboxylic acids and amines unless the ammonium salt is heated strongly to dehydrate it. This is not usually a good method of preparing amides.
(d) Formation of acid anhydride:
(a) Soda-lime decarboxylation:
(i) The stability of carbanion intermediate decides the rate of reaction.
(ii) The rate of decarboxylation increases with the presence of an electron withdrawing group at R-COOH.
(iii) Presence of some functional groups on aliphatic acids enables the decarboxylation.
Aliphatic acids that do undergo successful decarboxylation have certain functional groups or double or triple bonds in the α or β positions.
(b) Kolbe’s electrolysis:
HVZ Reaction (Halogenation of Aliphatic Acids and Substituted Acids)
Converts a carboxylic acid possessing an α-hydrogen to an α-halocarboxylic acid when treated with phosphorus and halogen. It is called as Hell-Volhard-Zelinsky reaction where regioselectivity allows alpha-halogenation only.
(a)Carbonyl oxygen reacts with phosphorus trihalide to form a P-O bond giving the release of a halide anion.
(b) Attack of halide forms an intermediate to release a rearranged acyl halide, an acid and a phosphine oxide.
(c) Enol tautomer of acyl halide attacks the halogen molecule to form α-halo acyl halide.
(d) On hydrolysis, α-halocarboxylic acid is formed.