Fundamentals of Organic ChemistryChemistryGeneral Organic Chemistry
Identify the more stable carbocation in each of the following pairs (a) and (b). Pair (a): (I) (II) Pair (b): (I) (II)
Options
- A(a) - (I) ; (b) - (I)
- B(a) - (I) ; (b) - (II)
- C(a) - (II) ; (b) - (I)
- D(a) - (II) ; (b) - (II)
Correct answer
D. (a) - (II) ; (b) - (II)
Step-by-step solution
In pair (a), structure (I) is the bicyclo[2.2.1]heptan-1-yl cation and structure (II) is the bicyclo[2.2.2]octan-1-yl cation. Carbocations prefer a planar sp^2 geometry, which is highly strained at bridgehead positions of small bicyclic systems (related to Bredt's rule). Since bicyclo[2.2.2]octane is a larger and more flexible system than bicyclo[2.2.1]heptane, it can better accommodate the required planar geometry with less angle strain. Therefore, the bridgehead carbocation (II) is more stable than (I). In pair (