# Comparing between SN2 and E2

Question I am confused between the questions B. And C. The earlier is SN2 and later is E2. But what makes the difference?

• Look at the transition state in SN2. How likely (energy barrier?) is an inversion in C)? – imalipusram Apr 22 '20 at 5:45
• It is pretty clear that you can't perform any kind of elimination in B due to valency constraints, it has to be SN2. For C,you can look at the chair form and decide – Yusuf Hasan Apr 22 '20 at 6:45
• With methoxide and benzyl chloride, the substitution is more likely to be bimolecular. Regular solvolysis of benzyl halides is on the border between the two mechanisms. – Zhe Apr 22 '20 at 13:46
• I didn't. Solvolysis would be in methanol, without methoxide. Since methoxide is better at SN2, I think we're breaking the tie. – Zhe Apr 22 '20 at 19:33
• For B you get a benzylic cation in a protic solvent. Surely SN1 would be favoured. – Josh Mitchell Apr 23 '20 at 14:01

## 1 Answer

The most important factor in these reactions are nucleophiles. Here, the use of strong base (Sodium methoxide) in both of the reactions favors the bi-molecular mechanisms ($$\ce{S_N2}$$ and $$\ce{E2}$$).

Now, the second factor would be degree of halogen-linked carbon. Lower degree carbons ($$\ce{\text{deg.} < 2}$$) favor $$\ce{S_N2}$$, while higher degree favors $$\ce{E2, E1, S_N1}$$. So, $$\ce{S_N2}$$ is favored in first and $$\ce{E2}$$ in second.

Temperature conditions and nature of solvent are least important factors, and should be looked up when above two factors are similar.