addition to a,b-unsaturated aldehydes and ketones?

Question

When will the conjugate addition take place(1,4 addition) and when is the 1,2 addition?
How to know?

Answer 1

It is very hard to give a rule which will always be followed.

Generally speaking, small compounds, like Hydrogen cyanide, will react across the C=O double bond (in this case giving a cyanohydrin:

RCH=O + H–C≡N --> RCH(OH)CN

Metal hydrides will usually reduce the carbonyl to the corresponding alcohol.

RCH=O + H:(–) --> RCH2OH

Larger size reactants will usually undergo a 1,4 addition. See some examples at the last site below:

Answer 2

1