Why do strong nucleophiles add to the carbon of the carbonyl and weak nucleophiles to beta carbon of an enone?

Question

Why do strong nucleophiles add to the carbon of the carbonyl and weak nucleophiles to beta carbon of an enone?

Answer 1

You mean a C=C double bond is alpha to the carbonyl?  This is classic example of kinetic vs. thermodynamic control.  It isn't quite as simple as strong vs. weak nucleophile, but generally, addition of a weak nucleophile is reversible, while addition of a strong nucleophile is not.  When the addition is reversible, you will end up with the thermodynamically most favored product in which the nucleophile attacks the beta carbon of the C=C double bond.  When the addition is not reversible, you will end up with the product that is more favored kinetically.  Generally, attack of the nucleophile at the carbon atom of the carbonyl happens fastest because this is where the electron density is lowest.  

Hope that helps.  If you are still confused, Google kinetic vs. thermodynamic control.  You will find what you need.