Experimentally reducing a functional group cannot be done by which one of the following reagents?

Solution

Chemistry ma’am · 3 steps

A reducing system must actually deliver hydrogen to the substrate. Check which pair cannot do that.

1. What reduction needs. Reducing a functional group means adding hydrogen to it. There are two ways to do that. One is H₂ gas activated on a metal catalyst. The other is a metal that gives electrons together with a proton source such as water, alcohol or acid.

   Reagent	How it works	Reduces?
   Pt-C/H₂	H₂ adsorbs on Pt and adds across C=C or C=O (catalytic hydrogenation)	Yes
   Na/H₂	Na is not a hydrogenation catalyst, and H₂ gas is not a proton source. Heated together they only form the salt NaH	No
   Pd-C/H₂	H₂ activated on Pd (catalytic hydrogenation)	Yes
   Zn/H₂O	Zn gives electrons and water gives protons. It is used as the reductive workup in ozonolysis	Yes

2. Why Na fails here. Sodium reduces things only when paired with a proton donor, as in Na/C₂H₅OH or Birch reduction with Na/liq. NH₃. With H₂ gas alone, nothing can put hydrogen onto the functional group.
3. So. The reagent that cannot reduce a functional group is (b), Na/H₂.

The idea to keep. For reduction, look for H₂ on a catalyst metal (Pt, Pd, Ni) or a metal paired with a proton source (water, alcohol, acid, liquid ammonia). Na with H₂ gas fits neither pattern.

Answer from the NTA final answer key. Solution by PrepWiser.