CLEMMENSEN REDUCTION
The Clemmensen Reduction involves adding ##Zn(Hg)## dissolved in heated ##HCl## to something reducible. Note though that this process can accidentally chlorinate a double bond also present on the reactant.
The principal action of the Clemmensen Reduction is to reduce a ##-stackrel(O)stackrel(||)C-## to a ##-CH_2-## or ##-stackrel(O)stackrel(||)C-H## to a ##-CH_3##.
So using it on 2-butenal is supposed to turn it into 2-butene.
But when you have ##HCl## a strong acid in the presence of a double bond there’s a good chance it’ll protonate the double bond. At that point a hydrochlorination would happen across the double bond forming an alkyl chloride.
That’s why if you wanted to remove the carbonyl oxygen you should do a similar reduction in basic media and the Wolff-Kischner reduction qualifies.
WOLFF-KISHNER REDUCTION
The basic counterpart to the acidic Clemmensen reduction is the Wolff-Kishner reduction which is run in base. It aims to accomplish the same thing but in different sometimes more favorable conditions.
The general reactants are hydrazine (##H_2N-NH_2##) and a strong base like ##NaOH##.
WOLFF-KISHNER VS. CLEMMENSEN
The main benefit of the Wolff-Kishner reduction is that because it’s in base it wouldn’t accidentally protonate a hydroxyl group or double bond in the reactant and allow side reactions to occur for acid-sensitive reagents.
Confer with a reaction involving a hydroxyaldehyde reactant (Organic Chemistry Bruice Ch. 15.18) and you would see the following conundrum is resolved by choosing one reduction method over the other: