Which conditions favor the formation of enolates with LDA versus NaH or OH-?

• A. LDA forms kinetic enolate at high temperature
• B. LDA, a bulky, non-nucleophilic base, forms the kinetic enolate at low temperature
• C. NaH forms only thermodynamic enolate
• D. OH- forms kinetic enolate at room temperature

Answer: (B)

The key idea is how base size and temperature steer whether the less-substituted, kinetic enolate or the more substituted, thermodynamic enolate forms. A bulky, non-nucleophilic base like LDA favors deprotonation at the less hindered alpha carbon, because it can’t easily approach the more substituted site. Doing this at low temperature slows down any subsequent equilibration, so the kinetic enolate is formed preferentially. If you heat the mixture, the enolate can equilibrate to the more stable, thermodynamic product, so a bulky base would no longer reliably give the kinetic enolate.

That’s why the statement describing LDA as forming the kinetic enolate at low temperature is the best choice. In contrast, claiming LDA forms the kinetic enolate at high temperature goes against the idea that higher temperature allows equilibration to the thermodynamic product. Saying NaH forms only the thermodynamic enolate isn’t accurate because NaH isn’t bulky, so it doesn’t strongly bias toward the kinetic product. And hydroxide, being a small base, doesn’t reliably enforce a kinetic preference at room temperature; its outcomes are less selective and depend more on the specific substrate and conditions.