Answer to Question #94077 in Organic Chemistry for Megha Nautiyal

Question

Addition of HBr to propenenitrile does not follow Markownikoff’s rule. Explain.

Answer 1

If consider addition of HBr to propenenitrile as electrophilic addition, two possible carbacations may form:

Carbocation with a positive charge on α-carbon is destabilized due to -I (negative inductive) effect of CN-group that increases positive charge on α-carbon even more. Moreover, resonance structure of this carbocation with a positive charge on a nitrogen atom is also destabilized.

On the other hand, carbocation with a positive charge on β-carbon is more stable as an influence of CN-group on it is much less. This carbocation results in an anti-Markovnikov product.