Q37. - Organic Chemistry

Question

Question: Why is the pKa of the Ha protons in 1-acetylcyclohexene higher than the pK_a  of the Hbprotons?

Step-by-Step Solution

Answer

Answer

 

The pK_a  of  H_a is more than the H_b  of 1-acetylcyclohexene because H_a  is stabilized by extensive resonance with the double-bonded carbons as well as with the carbonyl group whereas  H_b is resonance stabilized only with the double-bonded carbons.

 

              

Step 1: Criteria for acidity

 A compound is considered to be highly acidic when the intermediate structure formed after the proton abstraction is stabilized by resonance or -I effect of the adjacent group(generally an electronegative element). The pK_a and acidity are inversely related to each other.

Step 2: Stability of the given molecules

The negative charge formed after the proton abstraction undergoes extensive resonance along with the exertion of -I effect of the adjacent group. More is the stability of the conjugate base formed, more will be its acidity.

Representation of the resonance stabilization after the abstraction of  the H_a  proton 

Representation of the resonance stabilization after the abstraction of the H_b  proton 

  

    Thus, in the given compound the extent of resonance is more for Ha protons compared to Hb protons.