Step 1: This is the Hofmann bromamide degradation, which converts an amide with one less carbon into a primary amine. The mechanism proceeds through a series of intermediates rather than a single step.
Step 2: Treating $CH_3CONH_2$ with $Br_2$ and excess $KOH$ first substitutes a hydrogen on nitrogen with bromine, forming the N-bromoacetamide, $CH_3CONHBr$.
Step 3: Excess base removes the remaining N-H proton, producing a negatively charged bromamide ion, $CH_3CONBr^-$.
Step 4: The N-Br bond breaks heterolytically, releasing $Br^-$ and leaving nitrogen with only six electrons. This electron-deficient, neutral, divalent nitrogen species is a nitrene, specifically an acyl nitrene.
Step 5: Because a nitrene is highly unstable, it does not survive as a discrete species for long. The alkyl group attached to the carbonyl carbon migrates to nitrogen in a concerted 1,2-shift, giving the stable isocyanate $CH_3N=C=O$.
Step 6: Base then hydrolyzes the isocyanate to a carbamate, which decarboxylates to release the final primary amine, $CH_3NH_2$, along with carbonate salt.
The intermediate that defines this mechanism, and distinguishes it from simple substitution routes, is the electron-deficient nitrogen species formed in Step 4.
\[\boxed{\text{Nitrene}}\]