Organic Chemistry – Some Basic Principles and Techniques · Question 3

Consider the depicted hydrogen (H) in the hydrocarbons given below. The most acidic hydrogen (H) is

1. [figure]
2. [figure]
3. [figure]
4. [figure]

Answer

(2) [figure]

Solution

Acidity of a C–H is decided by the stability of the carbanion left after the marked H is removed.

(B): [figure] [figure]

The charge moves through the exocyclic C=C into the ring. The ring now has two C=C and one carbanion carbon: 6π electrons, an aromatic cyclopentadienyl anion. This gives very large stabilisation.

(A) and (C): the charge on the isopropyl carbon is delocalised only into a cross-conjugated fulvene; the ring always keeps an exocyclic C=C, so it never becomes aromatic.

(D): the ring has an sp3 CH2, so the ring cannot become aromatic.

So the marked H in (B) is the most acidic.

Answer: (B)