Acid chloride, alcohols from reaction with carboxylate ions

Esters are usually prepared from carboxylic acids by the methods already discussed. Thus, carboxylic acids are converted directly into esters by SK2 reaction of a carboxyfate ion with a primary alkyl halide or by Fischer esterification of a carboxylic acid with an alcohol in the presence of a mineral acid catalyst. In addition, acid chlorides are converted into esters by treatment with an alcohol in the presence of base (Section 21.4). 

This chapter will revisit the lUPAC nomenclature system for aldehydes, ketones, and carboxylic acids, as well as introduce nomenclature for the four main acid derivatives acid chlorides, anhydrides, esters, and amides. The chapter will show the similarity of a carbonyl and an alkene in that both react with a Br0nsted-Lowry acid or a Lewis acid. The reaction of a carbonyl compound with an acid will generate a resonance stabilized oxocarbenium ion. Ketones and aldehydes react with nucleophiles by what is known as acyl addition to give an alkoxide product, which is converted to an alcohol in a second chemical step. Acid derivatives differ from aldehydes or ketones in that a leaving group is attached to the carbonyl carbon. Acid derivatives react with nucleophiles by what is known as acyl substitution, via a tetrahedral intermediate. 

Loss of ethanol from 33 gives oxocarbenium ion 34, and loss of the acidic proton from that species gives butanoic acid (7). The two products of the acid hydrolysis are the carboxylic acid (butanoic acid) and the alcohol (ethanol). As with the hydrolysis of acid chlorides and acid anhydrides before, this mechanism is a series of acid-base reactions. 

Esters are commonly synthesized from carboxylic acids by reaction of the acid with an excess of alcohol containing a catalytic amount of a mineral acid. In cases where practical considerations dictate it, the acid can be converted to an acyl halide (usually the chloride) and then condensed with the appropriate alcohol. A less commonly used procedure involves direct alkylation of the carboxylate ion with an alkyl halide. Even when this latter procedure (Eq. 6.1) involves a silver carboxylate and alkyl chloride, the reaction is of marginal practical value. 

Polymeric amines can be proton acceptors, acyl transfer agents, or ligands for metal ions. The 2- and 4-isomers of poly(vinylpyridine) (11) and (12) and the weakly basic ion exchange resins, p-dimethylaminomethylated PS (2) and poly(2-dimethylaminoethyl acrylate), are commercial. The ion exchange resins are catalysts for aldol condensations, Knoevenagel condensations, Perkin reactions, cyanohydrin formation and redistributions of chlorosilanes. " The poly(vinylpyridine)s have been used in stoichiometric amounts for preparation of esters from acid chlorides and alcohols, and for preparation of trimethylsilyl ethers and trimethylsilylamines from chlorotrimethylsilane and alcohols or amines. Polymer-suppored DBU (l,8-diazabicyclo[5.4.0]undec-7-ene) (52) in stoichiometric amounts promotes dehydrohalogenation of alkyl bromides and esterification of carboxylic acids with alkyl halides. The protonated tertiary amine resins are converted to free base form by treatment with aqueous sodium hydroxide. 

The reaction occurs by a nucleophilic acyl substitution pathway in which the carboxylic acid is first converted into a chlorosulfite intermediate, thereby replacing the -OH of the acid with a much better leaving group. The chloro-sulfitc then reacts with a nucleophilic chloride ion. You might recall from Section 17.6 Hint an analogous chlorosulfite is involved in reaction of an alcohol with SOCb to yield an alkyl chloride. 

The tendency to undergo the hydride transfer reaction (and therefore the yield of carboxylic acids) falls as we pass from lower to higher homologs of isoparaffins. Of course, yield largely depends on the structure of the hydride ion acceptor. The following may be used as hydride ion acceptors olefins, alcohols or alkyl chlorides. Most efficient are tert.- and 2-butanol [640] as well as isobutanol [641] and triisobutylene [641]. Apart from H2SO4, HF may be used as catalyst in the carboxylation of isoparaffins. Yields however, are lower [638, 639]. Carboxylic acids with a tertiary hydrogen atom react in the same manner as dicarboxylic acids. 

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