Question

Question: Explain the following observation. Ethyl 3-phenylpropanoate $\left({\mathbf{C}}_6{\mathbf{H}}_5{\mathbf{CH}}_2{\mathbf{CH}}_2{\mathbf{CO}}_2{\mathbf{CH}}_2{\mathbf{CH}}_3\right)$ reacts with electrophiles to afford ortho- and para-disubstituted arenes, but ethyl 3-phenylprop-2-enoate $({\mathbf{C}}_6{\mathbf{H}}_5\mathbf{=}{\mathbf{CHCO}}_2{\mathbf{CH}}_2{\mathbf{CH}}_3)$ reacts with electrophiles to afford meta-disubstituted arenes.

Short answer

Ethyl 3-phenylpropanoate with an electrophile yields ortho-para disubstituted arenes as it contains the electron-donating alkyl group as a substituent.

However, ethyl 3-phenylprop-2-enoate yields meta-disubstituted arene as it has an electron-deficient substituent on the ring.

Step-by-step solution

Effect of substituents on aromatic electrophilic substitution reactions

Electron-rich species undergo a faster electrophilic substitution reaction.

Therefore, the benzene ring that contains an electron-donating group as a substituent undergoes electrophilic reactions faster than those containing electron-withdrawing groups.

Reaction of ethyl 3-phenylpropanoate with an electrophile

Ethyl 3-phenylpropanoate

Ethyl 3-phenylpropanoate consists of an alkyl substituent that increases the electron density on the ring (alkyl groups increase the electron density on the ortho and para position of the benzene ring).

Therefore, the reaction of ethyl 3-phenylpropanoate with an electrophile yields ortho-para disubstituted arenes.

Reaction of ethyl 3-phenylprop-2-enoate with an electrophile

Ethyl 3-phenylprop-2-enoate

The carbocation generated due to conjugation of the double bond with the carbonyl group decreases the electron density of the ring.

The carbocation pulls the electron density from the ring, causing electron deficiency on the ortho and para carbons.

Therefore, the electrophile attacks the electron-rich meta center, yielding a meta-disubstituted arene.