(i) 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.
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.
Conversion into acids: Hydrolysis.
Illustration 1. Acetyl chloride reacts with water more readily than methyl chloride. Explain.
Solution: Alkyl halides are much less reactive than acyl halides in nucleophilic substitution because nucleophilic attack on the tetrahedral carbon of RX involves a hindered transition state. Also, to permit the attachment of the nucleophile a bond must be partly broken. In CH3COCl, the nucleophile, attack on > C = O involves a relatively unhindered transition of acyl halides occurs in two steps. The first step is similar to addition to carbonyl compound and the second involves the loss of chlorine in this case.
Illustration 2. Hydrogenation of benzoyl chloride in the presence of Pd and BaSO4 gives
(A) benzyl alcohol (B) benzaldehyde
(C) benzoic acid (D) phenol
(ii) Carboxylic acid anhydrides
Carboxylic acid anhydrides can be used to prepare esters and amides.
Anhydrides can be hydrolysed to get back acids.
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.
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 aqueous NaOH produces an alcohol and the sodium salt of the acid.
This reaction is essentially irreversible because carboxylate ion is inert towards nucleophilic substitution.
If an ester is hydrolysed in a known amount of base (taken in excess), the amount of base used up can be measured and used to calculate the saponification equivalent; the equivalent weight of the ester, which is similar to the neutralization equivalent of an acid.
Esters can also be prepared by transesterification (an alcohol displacing another from an ester)
The mechanism of this reactions is quite similar to esterification.
Transesterification is an equilibrium reaction. To shift the equilibrium to right, it is necessary to use a large excess of the alcohol whose ester we wish to make or else to remove one of the products from the reaction mixture.
Reduction of esters
(i) Catalytic hydrogenation
(ii) Chemical reduction
Reaction of esters with Grignard reagents
The reaction of carboxylic esters with Grignard’s reagent is a good method for the preparation of 3° alcohols.
Initially ketones are formed. However, as we know, ketones themselves readily react with Grignard reagent to yield teriary alcohols.
When ethyl acetate reacts with sodium ethoxide, it undergoes a condensation reaction. After acidification, the product is a b – keto ester, ethyl aceto acetate (commonly known as – aceto acetic ester)
Condensation of this type is known as Claisen Condensation. For esters, it is the exact counterpart of the Aldol Condensation. Like the Aldol Condensation, the Claisen Condensation involves nucleophilic attack by a carbanion on an electron – deficient carbonyl compound. In the Aldol Condensation, nuclephilic attack leads to addition (the typical reaction of aldehydes and ketones). In the Claisen Condensation, nucleophilic attack leads to substitution (the typical reaction of acyl compounds)
This step is highly favourable and draws the overall equilibrium toward the product formation
When planning a claisen condensation with an ester it is important to use alkoxide ion that has the same alkyl group as the alkoxyl group of the ester. This is to avoid the possibility of transesterification.
An intramolecular claisen condensation is called Dieckmann condensation.
In general, the Dieckmann condensation is useful only for the preparation of five and six membered rings.
Preparation: In the laboratory amides are prepared by the reaction of ammonia with acid chlorides or, acid anhydrides.
Basic Character of Amides:
Amides are very feebly basic and form unstable salts with strong inorganic acids. e.g. RCONH2HCl. The structure of these salts may be I or II
Acidic Character of Amides:
Amides are also feebly acidic e.g. they dissolve mercuric oxide to form covalent mercury compound in which the mercury is probably linked to the nitrogen.
2RCONH2 + HgO -> (RCONH)2Hg + H2O
(a) Hydrolysis of amides
Amides undergo hydrolysis when they are heated with aqueous acid or aqueous base.
(b) Reduction of amides
Amides are reduced by Na/C2H5OH or by LiAlH4 to a primary amine.
(c) Reaction with P2O5
When heated with P2O5 amides are dehydrated to cyanides.
Amides may also be converted to cyanides by PCl5.
(d) Hofmann rearrangement
Amides with no substitutents on the nitrogen react with solutions of Br2 or Cl2 in NaOH to yield amines through a reaction known as Hofmann rearrangement.
Illustration 1. Acetamide reacts with NaOBr in alkaline medium to form
(A) NH3 (B) CH3NH2
(C) CH3CN (D) C2H5NH2
Keeping in mind the fact that B, C, D and E are all isomers of molecular formula (C8H9NO), identify A to G.
Illustration 3. Formic acid and acetic acid may be distinguished by reaction with
(A) sodium (B) dilute acidic permangnate
(C) 2, 4 dinitrophenylhydrazine (D) sodium ethoxide
|« Click Here for Previous Topic||Click Here for Next Topic »|