Question

The relative reactivity towards 1,3-cyclopentadiene of each of the following is

• A. iii > ii > i
• B. ii>iii>i
• C. i>iii > ii
• D. iii>i>ii

Answer 1

Answer: (A)

iii > ii > i

Answer 2

The Diels-Alder reaction is a [4+2] cycloaddition between a conjugated diene and a dienophile. The reactivity of the dienophile is generally increased by the presence of electron-withdrawing groups, especially when reacting with an electron-rich diene like 1,3-cyclopentadiene.

The given dienophiles are:

(i) Cyclohexene: A simple alkene with no significant electron-withdrawing groups.

(ii) Methyl acrylate: An $\alpha, \beta$-unsaturated ester, with one electron-withdrawing ester group conjugated to the double bond.

(iii) Maleic anhydride: A cyclic anhydride with two strong electron-withdrawing carbonyl groups conjugated to the double bond.

The electron-withdrawing strength of the groups attached to the double bond determines the reactivity of the dienophile. Two carbonyl groups (as in maleic anhydride) are stronger electron-withdrawing groups than one ester group (as in methyl acrylate). Simple alkenes without activating groups are the least reactive dienophiles.

Therefore, the order of reactivity of the dienophiles towards 1,3-cyclopentadiene is:

Maleic anhydride (iii) > Methyl acrylate (ii) > Cyclohexene (i)

This corresponds to the order iii > ii > i.