Step 1: Identify the transformation. The reaction converts a primary amide \( RCONH_2 \) to a primary amine \( RNH_2 \) with one less carbon. One carbon is lost as \( CO_2 \). Step 2: Name the reaction. This is the Hofmann bromamide degradation. It is the classic reaction that degrades a primary amide to a primary amine containing one fewer carbon. Step 3: Identify the reagent. The Hofmann bromamide reaction requires bromine in sodium hydroxide solution: \[ Br_2 + NaOH \] Step 4: Outline the mechanism. The amide reacts with \( Br_2/NaOH \) to form an N-bromoamide. Base removes a proton, and the intermediate rearranges via a nitrene-like species to an isocyanate \( R\text{-}N=C=O \). The isocyanate is hydrolysed by NaOH to the amine \( RNH_2 \) with release of \( CO_2 \). Step 5: Eliminate other options. Zn-Hg/HCl (Clemmensen reduction) converts carbonyls to \( -CH_2- \). \( HCl/SnCl_2 \) reduces \( -NO_2 \) to \( -NH_2 \). \( ZnCl/HCl \) is not used for this carbon-loss reaction. None of these produces an amine from an amide with one less carbon. Step 6: State the answer. Reagent P is \( Br_2/NaOH \). \[ \boxed{Br_2/NaOH} \]
