Title 21. Carboxylic Acids and their Derivatives.pdf ✅

Carboxylic Acids and their Derivatives 363 21 Carboxylic Acids and their Derivatives QUICK LOOK Carboxylic acids are the compounds containing the carboxyl functional group C — C — OH || O       The carboxyl group is made up of carbonyl ( C O) > == and hydroxyl (—OH) group. Carbonyl Hydroxyl Carboxyl O O || || —C — —C — H —C — O — H + ≡≡ Carboxylic acids may be aliphatic or aromatic Comparison of Resonating Structures of Carboxylic Group and Carbonyl Group: Carbonyl group has two resonance structures (I and II) However, for a carboxyl group, three resonance structures (A, B and C) can be written. In both structures (A) and (C), the C–atom and the two O– atoms have eight electrons in their respective valence shells while in structure (B), C–atom has only six electrons. Therefore, structure (B) is less stable than structure (C), in other words the two important resonance structures of carboxyl group are structures (A) and (C). In both these structures, carboxyl carbon is electrically neutral. However in case of aldehydes and ketones, only one structure i.e. I is electrically neutral. As a result, the carboxyl carbon of the resonance hybrid is less positive and hence less electrophilic than the carbonyl carbon of aldehydes and ketones. However, it may be noted that like carbonyl group, carboxyl group is also polar due to resonance structures (B) and (C). Nomenclature: The aliphatic carboxylic acids are commonly known by their initial names, which have been derived from the source of the particular acid. Examples HCOOH Formic acid [Latin: Fermica = ant] CH COOH 3 Acetic acid [Latin: acetum = Vinegar] CH — CH — COOH 3 2 Propionic acid [Greek: Proton = First; Pion = Fat] CH (CH ) COOH 3 2 2 Butyric acid [Latin: Butyrum = Butter] CH (CH ) COOH 3 2 3 Valeric acid CH (CH ) COOH 3 2 14 Palmitic acid CH (CH ) COOH 3 2 16 Stearic acid Alternative system of nomenclature is naming the acids as the derivatives of acetic acids. The only exception being formic acid. Example CH — CH — COOH 3 2 Methyl acetic acid 3 3 (CH ) C — COOH Trimethyl acetic acid According to the IUPAC system of nomenclature, the suffix of the monocarboxylic acid is ‘oic acid’, which is added to the name of the alkane corresponding to the longest carbon chain containing the carboxyl group, e.g. HCOOHmethanoic acid CH — CH — CH — COOH 3 2 2 butanoic acid. The positions of side-chains (or substituents) are indicated by numbers, the numbering to be started from the side of the carboxyl group. 5 4 3 2 1 3 2 3 3 CH — CH— CH CH — COOH | | CH CH 3,4-dimethylpentanoic acid Naming of Acyl Groups, Acid Chlorides and Anhydrides: The group obtained from a carboxylic acid by the removal of the hydroxyl portion is known as an acyl group. The name of an acyl group is created by changing the - ic acid at the end of the name of the carboxylic acid to –yl, examples: Formic acid Formyl group O O || || H — C — O — H H — C — → O: ɺɺ O H ɺɺ— ɺɺ —C (A) O: − ɺɺ ɺɺ C—H + ɺɺ —C O H ɺɺ— ɺɺ —C + O: − ɺɺ ←→ ɺɺ ←→ (B) (C) ←→ (I) (II) C O: + − == ɺɺ ɺɺ C O == : ɺɺ Aromatic carboxylic acid (Where Ar is any aryl group) O O — H Ar — C Aliphatic carboxylic acid (Where R = H or any alkyl group) O O — H R — C


366 Quick Revision NCERT - CHEMISTRY 3 2 (i)O CH C CCH 2CH COOH 3 3 3 (ii) H O ≡≡ → 4 (i)Alkaline KMnO (ii) H / RCH CHR 2RCOOH + ∆ == → KMnO / / H 4 + → ∆ 2 2 CH COOH | CH COOH Carboxylic Acid Derivatives: There are four carboxylic acid derivatives. These are generally represented as O || R — C — Z, where Z is halogen (usually Cl), OCOR , OR' ' or NH2 (or NHR' or 2 NR )' . When Z is halogen (usually Cl), the derivatives are called as acid chlorides. O || R — C — Cl When Z is OR′, the derivatives are called as esters. When Z is O || —O — C — R', the derivatives are called carboxylic anhydrides. Where Z is  NH2, the derivatives are called amides. When Z is NHR′ or NR2′ they are called N–substituted amides. Synthesis of Acid Derivatives: Carboxylic acid derivatives are exclusively prepared from carboxylic acids. The preparation methods of carboxylic acid derivatives are already discussed under the chemical reactions of carboxylic acids. Chemical Reactions of Acid Derivatives Acyl chloride: We have already seen that acyl chlorides are the most reactive of all acid derivatives. As a result, acyl chlorides are often selected as the starting material for the preparation any other acid derivative. Let us see how this is done. O O O O || || || || R —C —Cl R —C —O Na R —C —O —C—R' − + + → O O || || R — C — Cl R — OH R — C — O — R + → ' 3 2 O O || || R — C — Cl NH R — C — NH + → 2 O O || || R — C — Cl R NH R — C — NHR + → ' ' O O || || R — C — Cl R R NH R — C — NR R + → ′ ′′ ′ ′′ 3 2 6 5 6 5 3 O O || || H C — C — Cl H NC H C H NHC — CH HCl + → + Reaction of Acetyl Chloride with Olefins: Acetyl chlorides add on to the double bond of an olefin in the presence of a catalyst (AlCl3 or ZnCl2) to form a chloro ketone which on heating, eliminates a molecule of hydrogen chloride to form an unsaturated ketone. ZnCl2 H C — CH CH CH COCl H C — CH — CH — C — CH 3 2 3 3 2 3 == + → 3 2 3 3 2 3 2 O || H C — CH CH CH COCl H C — CH — CH — C — CH | Cl == + → 3 3 O || H C — CH CH — C — CH HCl → == + ∆ Conversion into acids: Hydrolysis. 2 An acid O O || || R — C — Cl H O R — C — OH HCl + → + Carboxylic acid anhydrides: Carboxylic acid anhydrides can be used to prepare esters and amides. R'OH O O O || || || R — C — O — C — R R — C — OR → ' NH3 Excess O || + → R — C — OH 2 4 O O || || R — C — NH R — C — O NH − + + Anhydrides can be hydrolysed to get back acids. H O2 O O O || || || R — C — O — C — R 2R — C — OH → Esters Ester Hydrolysis: Acid catalysed esterification is an essentially reversible reaction. If you follow the backward course of reactions of esterification it gives you the mechanism for ester hydrolysis. O || R — C — OR' H O2 H →+ O || R — C — OH R OH + ' Base promoted Hydrolysis of Esters Saponification: Esters undergo base promoted hydrolysis also. This reaction is known as saponification, because it is the way most of the soaps are manufactured. Refluxing an ester with Acetyl salicylic acid (aspirin) COOH OH 3 O || O — C — CH COOH + → CH COCl 3