Q.42. - Organic Chemistry

Question

Question: Explain why  $\mathit{\alpha }$-pyrone reacts with $\mathit{B}{\mathit{r}}_{\mathbf{2}}$ to yield a substitution product (like benzene does),rather than an addition product to one of its $\mathit{C}\mathbf{=}\mathit{C}$ bonds.

Step-by-Step Solution

Answer

Answer

 

The aromaticity of  $\alpha$-pyrone is lost during addition reactions. Hence  $\alpha$-pyrone reacts with $B{r}_2$  to give rise to a substitution product rather than an addition product. 

Step 1: Substitution reaction

Compounds comprising multiple bonds get subjected to addition reactions to generate compounds containing a single bond. Numerous kinds of addition reactions are found, and one among them is the electrophilic addition reaction.

Step 3: Reactivity of α - p y r o n e

$\alpha$-pyrone reacts with benzene as it is aromatic because of the electron delocalization in the compound. The second resonance structure of benzene can be shown as:

Second resonance structure of  $\alpha$-pyrone

 

In addition reactions, the aromaticity of the compound is lost, whereas it is retained in substitution reactions. Therefore aromatic compounds get subjected to substitution reactions rather than addition reactions. Hence, $\alpha$-pyrone reacts with bromine to give a substitution product rather than an addition product to one of its $C=C$ bonds.