Question

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

(i)

(ii) OMe

(iii)

• 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 towards an electron-rich diene like 1,3-cyclopentadiene is enhanced by the presence of electron-withdrawing groups on the double bond. We need to compare the electron-withdrawing ability of the substituents in the given dienophiles.

(i) Cyclohexene is a simple alkene. There are no significant electron-withdrawing or electron-donating groups directly attached to the double bond. Its reactivity as a dienophile is relatively low.

(ii) Methyl acrylate ($\text{CH}_2=\text{CH-COOMe}$) is an $\alpha,\beta$-unsaturated ester. The ester group ($\text{-COOMe}$) is an electron-withdrawing group due to resonance and inductive effects. This makes the double bond electron-deficient, increasing its reactivity as a dienophile.

(iii) Maleic anhydride is a cyclic anhydride containing a double bond conjugated with two carbonyl groups. Carbonyl groups are strongly electron-withdrawing. The presence of two strongly electron-withdrawing carbonyl groups directly attached to the double bond makes maleic anhydride a very reactive dienophile. Anhydrides are generally more activating than esters in Diels-Alder reactions.

Comparing the electron-withdrawing strength of the substituents on the double bond:

• In (i), there are no significant electron-withdrawing groups.
• In (ii), there is one ester group, which is electron-withdrawing.
• In (iii), there is a cyclic anhydride group with two carbonyls, which is strongly electron-withdrawing.

The order of electron-withdrawing strength of the substituents on the double bond is (iii) > (ii) > (i).

Since 1,3-cyclopentadiene is an electron-rich diene, the reactivity of the dienophile towards it increases with the electron deficiency of the double bond. Therefore, the order of reactivity is directly proportional to the electron-withdrawing strength of the substituents.

Order of reactivity: (iii) > (ii) > (i).