[Daphoenix]

The first step in a Wolff-Kishner reduction is an imine formation in which hydrazine adds to a Carbonyl compound. Imine formation however requires acid in order to generate the good leaving H2O. An ideal pH of about 5 is required, so how come every source I keep reading says the Wolff-Kishner is done under basic conditions?  Clearly it needs acidic conditions first. And if so, then how can this type of reduction be used selectively over the Clemmenson reduction?

[pgk]

1). No! Imine formation can also occur under neutral and alkaline conditions. Acid is just a catalyst that accelerates the reaction because under neutral conditions, the reaction is very slow. Under basic conditions, a strong base is necessary and therefore, alkaline imine formation is not preferred.
2). In Wolff-Kishner reaction, imine is formed under alkaline conditions, followed by nitrogen release, via azo group formation. The carbanions that are formed during the reaction, are immediately neutralized by the alcohol (or any other protic) solvent medium.
>C=O + H2NNH2 → >=NNH-H → >CH-N=N-H → >CH2 + N2
3). Clemmensen reduction needs the preliminary washing of zinc with HCl, followed by the “in situ” preparation of a zinc amalgam with mercury and therefore, it is not advantageous in practice.