Reduction of a nitrile to a primary amine requires the conversion of the carbon–nitrogen triple bond into a carbon–nitrogen single bond while simultaneously adding hydrogens to both carbon and nitrogen. The triple bond contains one sigma and two pi bonds; reducing it to a single bond means breaking the two pi components and saturating them. Each pi bond reduction consumes one molecule of hydrogen, so the nitrile group R–C≡N is converted to R–CH2–NH2 by the uptake of two molecules of H2. The option of one molecule is insufficient because it would only partially reduce the group to an imine stage. Three or four molecules would over-reduce, but there are no further unsaturations to saturate in propanenitrile, so those values are impossible. This stepwise reduction through an intermediate imine to the primary amine is the NCERT-described Mendius-type reduction using catalytic hydrogen. As a stoichiometric plausibility check, counting atoms confirms two hydrogen molecules supply exactly the four hydrogen atoms needed to turn one triple bond and its terminus into a –CH2–NH2 unit.