Fundamentals of Organic ChemistryChemistryAldehydes and Ketones
When 2-methylcyclohexanone reacts with a base and is then treated with an electrophile such as methyl iodide, the regioselectivity of the product depends on the reaction conditions, as shown below. At high temperature, the major product is the geminal dimethyl compound (2,2-dimethylcyclohexanone). This is because:
Options
- ALithium diisopropylamide (LDA) is a much stronger base than Et ₃ N .
- BLithium diisopropylamide (LDA) promotes an anti-Markovnikov type addition.
- CThe enolate formed at high temperature is the thermodynamically more stable enolate.
- DAlkyl groups are weak electron donors due to inductive effects and hyperconjugation.
Correct answer
C. The enolate formed at high temperature is the thermodynamically more stable enolate.
Step-by-step solution
The regioselectivity of enolate formation from an unsymmetrical ketone depends on the reaction conditions. At lower temperatures with a strong, bulky base like LDA, the reaction is under kinetic control. The base abstracts the less sterically hindered proton, forming the less substituted, kinetically favored enolate, which upon alkylation yields 2,6-dimethylcyclohexanone. At higher temperatures with a weaker base like Et ₃ N , the deprotonation is reversible, and the reaction is under thermodynamic control. This al