Question

Identify which statement among the following is true in case of E2 and SN2: 

• A. Secondary substitution undergoes slow elimination and fast substitution
• B. Primary substitution undergoes slow elimination and fast substitution
• C. Tertiary substitution undergoes slow elimination and fast substitution
• D. Primary substitution does not undergo elimination and substitution reactions

Answer 1

The Correct Option is B

The question examines the behavior of substitution and elimination reactions in organic chemistry, specifically focusing on the E2 and SN2 mechanisms. Let's analyze each option to identify which statement about these reactions is true:

1. Secondary substitution undergoes slow elimination and fast substitution:
   • In secondary substrates, both SN2 and E2 reactions are feasible. However, the reaction conditions such as the strength of the nucleophile or base, and the solvent play significant roles.
   • Often, secondary substrates can undergo both reactions at comparable rates, depending on these conditions.
2. Primary substitution undergoes slow elimination and fast substitution:
   • Primary substrates are well-suited for SN2 reactions because steric hindrance is minimized, allowing nucleophiles easy access to the reaction center.
   • Elimination (E2) in primary substrates is typically slower because the primary carbocation is less stable. Thus, primary substrates favor substitution over elimination.
   This statement is correct.
3. Tertiary substitution undergoes slow elimination and fast substitution:
   • Tertiary substrates are poor candidates for SN2 reactions due to steric hindrance, leading to slow substitution.
   • Instead, they often undergo faster elimination (E2) as bulky bases can abstract a proton readily to form alkenes.
4. Primary substitution does not undergo elimination and substitution reactions:
   • This statement is false because primary substrates are very likely to undergo SN2 reactions effectively, albeit less so for elimination (E2) under normal conditions.

Therefore, the correct statement is: Primary substitution undergoes slow elimination and fast substitution, which aligns with the characteristic behaviors observed for primary substrates in E2 and SN2 reactions.