Rearrangements to Electron-Deficient Carbon
Neighboring Group Effects and
Nonclassical Carbocations

A.  Mustard Gases:

1.  accelerated rates of hydrolysis; first order kinetics

2.  cyclic intermediate

B.  Retention of configuration

Expected (S_N2 attack at both carbons)

Observed:

Mechanism:

Step 1:  S_N2 substitution at first carbon

Step 2:  Neighboring group assistance

 

C.  Nonclassical carbocations

1.  Comparison of classical and nonclassical carbocations.

2.  Example

The cis isomer of the structure shown below dissolves in strong acid to give a deep yellow solution . The trans isomer shows no strong absorption above 300 mµ under the same conditions.

The cis isomer forms a stable nonclassical carbocation, which has a characteristic absorption in the UV. Distribution of the positive charge to the distant p system in the cis isomer, but not in the trans isomer, is possible.

Trans isomer cannot fold to form nonclassical carbocation.