Organic Chemistry MCAT Practice Exam

What reagent is used to select the kinetic enolate at low temperatures?

1. MeOH

2. LDA

3. NaOH

4. LiAlH4

The correct answer is: LDA

The selection of the kinetic enolate at low temperatures is best achieved using LDA (Lithium diisopropylamide). This reagent is a strong, non-nucleophilic base that effectively deprotonates carbonyl compounds, favoring the formation of the more thermodynamically unstable, but more reactive, kinetic enolate. In the context of enolate formation, temperature plays a crucial role. At low temperatures, the reaction is subjected to less kinetic energy; therefore, the base must be able to selectively deprotonate the least hindered carbon to form the kinetic enolate. LDA's strong basicity allows it to perform this action effectively, particularly in less sterically hindered systems, leading to a predominance of the kinetic product. Using other bases like NaOH does not work for this purpose because they are mild bases compared to LDA and typically favor the formation of the thermodynamic enolate at higher temperatures. Similarly, MeOH is more suited as a solvent or a co-solvent for reactions rather than as a base for enolate formation. LiAlH4 is a strong reducing agent primarily used for reducing carbonyl compounds but is not appropriate for enolate formation. Thus, LDA is the reagent