Elimination of a Benzyl Halide under a Strong Base

Question

 Predict the product and propose a mechanism when (S)-(1-chloropropyl)benzene reacts with sodium ethoxide and added heat.

Answer 1

This would be the E2 elimination. In this mechanism, the ethoxide O atom attacks an H atom attached to C-2 (the middle one) of the chloropropyl group. Simultaneously, an arrow is drawn from the C-H bond (C-2, that is) to form another bond between C-1 and C-2. Simultaneously, an arrow is drawn from the C-Cl bond (C-1, that is) on to the Cl. The product then shows CH3OH, Cl- , and an alkene (double bond between C-1 and C-2 of the attached group). The alkene product is 1-phenyl-2-butene.

 

Now the stereochemistry: the elimination is anti, meaning the H on C-2 is anti-coplanar with the Cl on C-1. Since there are 2 H atoms on C-2, there are 2 anti-coplanar possibilities, one leading to the cis-alkene, and the other to the trans-alkene. Trans alkenes are more stable than cis, so the major product would be trans-1-phenyl-2-butene.