Important revision notes for CBSE Class 12 Chemistry: Chapter 12 - Aldehydes, Ketones and Carboxylic Acids are available in this article. This article is a continuation of the revision notes on Class 12 Chemistry, Chapter 12 - Aldehydes, Ketones and Carboxylic Acids, Part-I & Part-II. In previous parts, we have studied about Aldehydes and Ketones along with their preparation, properties, uses etc. In this part or part-III, we will know about the Carboxylic acids, its preparation, properties and uses.
These quick notes are prepared as per the latest CBSE syllabus of Class 12th Chemistry.
The main topics covered in this part are:
o Carboxylic acids
• Structure of Carboxyl Group
• Physical Properties
• Chemical reactions
The key notes of the chapter are as follows:
Compounds containing carboxyl group (−COOH) are called carboxylic acids. The word carboxyl is a combination of two words carbonyl (>C=O) and hydroxyl (−OH).
Structure of Carboxyl Group
The carboxyl carbon is sp2 hybridised and is trigonal planar in shape just like aldehydes and ketones. All bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less positive than carbonyl carbon because of the possible resonance structure shown below:
Nomenclature of Carboxylic Acids
• IUPAC names of carboxylic acids have been derived from the corresponding alkanes by replacing the letter ‘e’ of the parent hydrocarbon by ‘oic’ and adding suffix 'acid' at the end.
• For compounds containing more than one carboxyl group, the number of carboxyl groups are indicated by adding the multiplicative prefix, di, tri, etc. to the term oic.
HOOC – CH2 – CH2 – CH2 –COOH
Preparation of Carboxylic Acids
(a) By oxidation of primary alcohols and aldehydes:
(b) By hydrolysis of nitriles:
(c) From alkylbenzenes:
(d) From Grignard's reagent:
(e) By hydrolysis of acid derivatives [like amides (RCONH2), acid halides (RCOCl), esters (RCOOR’), acid anhydrides (RCO-O-COR)]:
Physical Properties of Carboxylic Acids
• Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax-like solids.
• Carboxyl carbon is less positive due to resonance. Hence, the carboxylic acid group does not give nucleophilic substitution reactions.
• Lower carboxylic acids are completely miscible in water due to their ability to form hydrogen bonds. Higher member of carboxylic acids are solids like waxes and are insoluble in water but soluble in oil.
• Carboxylic acids have a higher boiling point than aldehydes, ketones or even alcohols of comparable molecular masses. This is due to an extensive network of intermolecular hydrogen bonding.
• Melting points of aliphatic monocarboxylic acids shows alternation effect, i.e., the melting point of an acid with an even number of carbon atoms is higher than the next lower and next higher homologue containing an odd number of carbon atoms. This is because, in the case of acids with the even number of carbon atoms, the terminal -CH3 and -COOH groups lie on the opposite sides of the zig-zag chain. As a result, they get closely packed in the crystal lattice.
Chemical Properties of Carboxylic Acids
Various chemical reactions of carboxylic acids are described below:
(a) Reactions Involving Cleavage of O–H Bond
• Acidity: Carboxylic acids dissociate in water to give resonance stabilised carboxylate anions and hydronium ion.
The presence of electron withdrawing group (EWG) increases the acidic strength and presence of electron denoting group (EDG) decreases the acidic strength.
• Reactions with metals and alkalies:
(b) Nucleophilic substitution reactions
These reactions involve the replacement of the −OH group of –COOH group by attacking electrophile.
• Formation of acid chlorides:
• Formation of Esters (Esterification):
• Formation of Amides:
• Formation of Anhydrides:
(c) Reactions involving –COOH group as a whole
(d) Substitution Reactions Involving Hydrocarbon Part
α - halogenation of carboxylic acid is known as Hell-Volhard-Zelinsky (HVZ) reaction.
• Ring substitution
–COOH group of aromatic carboxylic acids is a meta–directing group and is the deactivating group.
Uses of Carboxylic Acids
• Formic acid is used in rubber, textile, dyeing, leather and electroplating industries
• It is used as a reducing agent.
• Ethanoic acid is used as a solvent and as vinegar in the food industry.
• Benzoic acid and its salts are used as urinary antiseptics.
• Benzoic acid is also used in the preparation of esters and in perfumery
• Higher fatty acids are used for the manufacture of soaps and detergents