Enolate chemistry A hindered base such as LDA would result into transition state
ID: 700715 • Letter: E
Question
Enolate chemistry
A hindered base such as LDA would result into transition state which would be sterically crowded and thus of higher energy, thus it forms enolate which is least substtituted (product A).
On the other hand, a less hindered base as triethylamine would form transition state which is of lower energy and thus more substituted enolate (product B) is generated in this case.
(a) Kinetically formed enolates are generally formed at a faster rate. Since the formation of less sterically hindered enolate would be formed faster here (lower energy requirement), product B would be kinetically formed enolate.
(b) In case of triethylamine, the lower energy product is product B which is the major product formed in this case. The higher energy transition state would be for the more substituted enolate (minor product A) in this case.
(c) In case of LDA as a base, running the reaction at lower temperature would yield the kinetic enolate product B. Whereas, with triethylamine the enolate formed is product B at room temperature. The increase in reaction time in case when LDA is employed as base, increases the chances of formation of thermodynamic enolate product A. Therefore, increasing time in this case would give us product A.
(d) At lower time and temperature, the reaction entropy is also lower, which results into higher yield of product B. Also with less energy supplied for the reaction, the conversion of product B to product A equilibrium is minimal and thus we can get higher yield of product B with lower energy for the reaction.
Explanation / Answer
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onan 2301 , selectivity is an important issune in chestry and can be roughls ifferent categories, Le, thermolynamic and kinetic control. Let'suse reaction of 2-methylcycohexanone into two enolate ions, product A a two ionic products depends on the hase used, for example a stromg and BThe distribution of isopropylamine(L.DA) and a weak base like t triethylamine, as shown below LOA sy Triethylamine 22% 78% a)Which enolate is the kinetic product? Explain your answer b)For the reaction pathway related to trimethylamine, which product is with y the symbols lower energy Which reaction route has the transition state with higher energy? (c)In order to obtain high yield of product B, should we increase reaction perature or lower it? How about the reaction time, extend or shorten? (d)If one wants to enhance the yield of product B other than reaction temperature and reaction time, there are still some different approaches in chemistry to achieve this task. According to Arrhenius rate constant, it can divided it into two factors, entropy and energy. Please give one method related to entropic approach and the other one related to energetic approach. (0) LDA:, ft,A-11 ; B--,Related Questions
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