Esters reaction with carbocations

Organoaluminum compounds, reaction with imino carbocations, 66, 189 Orthoester Claisen rearrangement, 66, 22 Orthoformic acid, triethyl ester, 65, 146 Oxalic acid, diethyl ester, 65, 146... 

A naive look at the product suggests an oxidation to a ketone followed by a Baeyer-Villiger like reaction. The product is best explained by a fragmentation from an intermediate chromate ester, resulting on an aldehyde and a stabilized tertiary carbocation that is transformed into a tertiary alcohol by reaction with water. The hydroxyaldehyde so obtained may evolve to the final lactone either via a lactol or a hydroxyacid. 

Butyl esters can be cleaved by reaction with dilute acid under milder conditions than those required to hydrolyze a methyl ester. The reaction follows an SNI mechanism, rather than the reverse of the Fischer esterification mechanism, because of the stability of the /-butyl carbocation ... 

Most of the Fischer esterification mechanism is identical with the mechanism of acetal formation. The difference is in the final step, where a carbocation loses a proton to give the ester. Write mechanisms for the following reactions, with the comparable steps directly above and below each other. Explain why the final step of the esterification (proton loss) cannot occur in acetal formation, and show what happens instead. 

As seen in 10-6, departure of the leaving group from an acetal gives a particularly stable carbocation. It is also possible to convert a dimethylketal directly to a dithiane by reaction with butane 1,4-dithiol on clay. ° These are equilibrium reactions, and most often the equilibrium is shifted by removing the lower-boiling alcohol by distillation. Enol ethers can be prepared by treating an alcohol with an enol ester or a different enol ether, with mercuric acetate as a catalyst, example. 

In an undivided cell the yields are reduced substantially. Methyl hexanoate gave (co — 1) and also some (co — 2) products. The selectivity of the reaction is reminiscent of the ester chlorination with N-chloro amines . In the anodic oxidation, however, carbocations seem to be involved, being formed most remote from the ester function. In suitable cases carbocation rearrangements are observed. 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

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