Q60E - Organic Chemistry (Mcmurry)

Question

Electrophilic substitution on 3-phenylpropanenitrile occurs at the ortho and para positions, but reaction with 3-phenylpropenenitrile occurs at the meta position. Explain, using resonance structures of the intermediates.

Step-by-Step Solution

Answer

In 3-phenylpropanenitrile, the saturated side does not allow resonance interactions of –CN with aromatic ring, and the group is too far from the ring to its inductive effect to be strongly felt.

 Hence the side chain acts as an alkyl substituent and ortho-para substitution is observed.

In 3-phenylpropenenitrile, the –CN group interacts with the ring through the pi-electrons on the side chain. Resonance forms show that –CN deactivates ring toward electrophilic substitution, and substitution occurs at the meta position.

Step 1: Ortho para and meta directing groups

If an electron withdrawing groups also known as deactivating group present on the aromatic ring then the electrophilic substitution takes place at meta position. 

Example of meta directing groups are cyano group (CN), aldehyde group (CHO), nitro group (NO₂) and any unsaturated group attached to the aromatic ring with conjugation.

If an electron donating groups present on the aromatic ring then the electrophilic substitution takes place at ortho and para positions. Example for ortho-para directing groups are alkyl group, hydroxyl group (OH).

Step 2: Electrophilic substitution reaction

Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not aromatic substitution reactions are characteristics of aromatic compounds and the most common way of introducing functional groups into benzene ring.

Step 3: Identify the position on which substitution occurs

When directly bonded to a ring, the –CN group is meta directing deactivator for both inductive and resonance reasons.

In 3-phenylpropanenitrile, the saturated side does not allow resonance interactions of –CN with aromatic ring, and the group is too far from the ring to its inductive effect to be strongly felt. Hence the side chain acts as an alkyl substituent and ortho-para substitution is observed.

 

In 3-phenylpropenenitrile, the –CN group interacts with the ring through the pi-electrons on the side chain. Resonance forms show that –CN deactivates ring toward electrophilic substitution, and substitution occurs at the meta position as shown in the figure given below:

Resonating structures of 3-phenylpropenenitrile

Attack occur at the meta position of the ring because the positively charged intermediate can be stabilized by resonance contributions.