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Dolbier HW Solutions 469

Dolbier HW Solutions 469 - the bonds are staggered in a...

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( b ) Crowding is increased as the transition state for nucleophilic addition to the carbonyl group is approached. The carbonyl carbon undergoes a change in hybridization from sp 2 to sp 3 . The tert -butyl group of ethyl 2,2-dimethylpropanoate causes more crowding than the methyl group of ethyl acetate; the rate-determining intermediate is less stable and is formed more slowly. ( c ) We see here another example of a steric effect of a tert -butyl group. The intermediate formed when hydroxide ion adds to the carbonyl group of tert -butyl acetate is more crowded and less stable than the corresponding intermediate formed from methyl acetate. ( d ) Here, as in part ( a ), we have an electron-withdrawing substituent increasing the rate of ester saponi f cation. It does so by stabilizing the negatively charged intermediate formed in the rate-determining step. ( e ) Addition of hydroxide to 4-butanolide introduces torsional strain in the intermediate because of eclipsed bonds. The corresponding intermediate from 5-butanolide is more stable because
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Unformatted text preview: the bonds are staggered in a six-membered ring. Eclipsed bonds H H OH O O 2 Less stable; formed more slowly Staggered bonds H H OH O 2 O More stable; formed faster Rate-determining intermediate from methyl m-nitrobenzoate COCH 3 O 2 OH N 1 more stable than Rate-determining intermediate from methyl benzoate COCH 3 O 2 OH O 2 O HO 2 1 CH 3 COCH 3 O Methyl acetate Hydroxide ion CH 3 COCH 3 HO O 2 Rate-determining intermediate; less crowded than intermediate from tert-butyl acetate 1 CH 3 COCCH 3 CH 3 O CH 3 tert-Butyl acetate CH 3 COCCH 3 CH 3 O 2 HO CH 3 Rate-determining intermediate; crowded HO 2 Hydroxide ion 1 2 OH Hydroxide ion Ethyl 2,2-dimethylpropanoate CH 3 C COCH 2 CH 3 CH 3 O CH 3 sp 2 Rate-determining intermediate; crowded CH 3 C CH 3 CH 3 COCH 2 CH 3 O 2 OH sp 3 CARBOXYLIC ACID DERIVATIVES: NUCLEOPHILIC ACYL SUBSTITUTION 563...
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