Isobutene carboxylic acid addition

Figure 3.53 shows an addition of a carboxybc acid to isobutene, which takes place via the tert-butyl cation. This reaction is a method for forming ferf-butyl esters. Because the acid shown in Figure 3.53 is a /i-hydroxycarboxylic acid whose alcohol group adds to an additional isobutene molecule, this also shows an addition of a primary alcohol to isobutene, which takes place via the ferf-butyl cation. Because neither an ordinary carboxylic acid nor, of course, an alcohol is sufficiently acidic to protonate the alkene to give a carbenium ion, catalytic amounts of a mineral or sulfonic acid are also required here. 

The classical method for making tert-butyl esters involves mineral acid-catalysed addition of the carboxylic acid to isobutene but it is a rather harsh procedure for use in any but the most insensitive of substrates [Scheme 6.33].80-82 Moreover, the method is hazardous because a sealed apparatus is needed to prevent evaporation of the volatile isobutene. A simpler procedure [Scheme 6.34] involves use of tert-butyl alcohol in the presence of a heterogeneous acid catalyst — concentrated sulfuric acid dispersed on powdered anhydrous magnesium sulfate. 3 No interna] pressure is developed during the reaction and the method is successful for various aromatic, aliphatic, olefinic, heteroaromatic, and protected amino acids. Also primary and secondary alcohols can be converted into the corresponding /erf-butyl ethers using essentially the same procedure (with the exception of alcohols particularly prone to carbonium ion formation (e.g. p-... 

Addition of the carboxyl group to isobutene, in the presence of strong acids, is used for the preparation of tert.butyl esters ... 

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