Q.48. - Organic Chemistry

Question

Question: Considering both 5-methylcyclopenta-1,3-diene (A) and 7-methylcyclohepta-1,3,5-triene (B), which labeled H atom is most acidic? Which labeled H atom is least acidic? Explain your choices.

Step-by-Step Solution

Answer

Answer

 

$H_b$  is more acidic than $H_a$ as the conjugate base formed by the removal of $H_b$ is aromatic. $H_c$ is less acidic than $H_d$ as the conjugate base formed by the removal of $H_c$ is anti-aromatic.

Step 1: Acidic hydrogen

Acidic hydrogens are hydrogens that generate positive ions when subjected to dissociation. The hydrogen atom situated on the carboxylate group(-COOH) in methanoic acid is acidic.

Step 2: Predicting the acidity of hydrogens

Hydrogens situated near the electronegative atom like nitrogen and oxygen are regarded to be acidic. Those linked to the positively charged nitrogen, oxygen, and sulfur are also acidic.

Step 3: Identifying the most acidic hydrogen atom

The conjugate base of compound A by removal of  $H_b$ can be given as:

The conjugate base of compound A by removal of  $H_b$

 

$H_b$ is more acidic than $H_a$ as its conjugate base has  $6\mathrm{\pi }$electrons and is aromatic. The conjugate base formed by the removal of $H_a$  proton, the negative charge, and the lone pair are present on the alkene carbon. They are neither aromatic nor antiaromatic.  

 

The conjugate base of compound B by removal of  $H_c$ can be given as:

The conjugate base of compound B by removal of  $H_c$

 

$H_c$ is less acidic than $H_d$ as the conjugate base formed by the removal of $H_c$ has $8\mathrm{\pi }$ electrons and is anti-aromatic. The lone pair and the negative charge are present on the alkene carbon in the case of the conjugate base formed by the removal of $H_d$ proton.

 

Hence, they are neither aromatic nor antiaromatic.