1,2-addition occurs when the carbonyl oxygen (1) is attached by the electrophile and the carbonyl carbon (2) attaches to the nucleophile

for the 1,4 the 4 is the beta carbon

A reaction in which a negative ion (nucleophile) attacks on a partially positive carbon atom then reaction is known as nucleophilic reaction, it may be substitution reaction or addition reaction.

Formaldehye is H2C=O and has a double bond. The Carbon-oxygen bond is slightly polar and the carbon has partial positive charge and oxygen slightly negative charge. Hence formaldehyde will undergo nucleophilic addition addition reaction across the C=O.

Pyridine will add to carbon 3 in electrophilic reactions, such as Bromine addition. However in a nucleophilic reaction, such as seen in the Chichibabin reaction, carbon #2 and #4 are substituted such as if NH2 - attacked. Draw out the resonance forms and you will see this, or consult any Organic text under heterocyclic Chemistry.In a C3 attack, the electrophile will destabilize the C2 and C4 position, to a great extent since N lacks an octet in one of these resonance forms.In a nucleophilic addition, addition at C2 or C4 allows the negative charge to be shared by Nitrogen thus is preferred to the C3 attack. Hope that helps. Dr Jim Romano CEO Romano Scientific CEO Orgoman.com Class of 1991 NYU

Not necessarily, it depends upon structure and size of molecule, usually free alcohols are more nucleophilic.