The carbylamine test (isocyanide test) is exclusively given by primary aliphatic and aromatic amines ($\text{R-NH}_2$). When heated with chloroform ($\text{CHCl}_3$) and alcoholic $\text{KOH}$, primary amines form foul-smelling isocyanides (carbylamines):

$$\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH} \xrightarrow{\Delta} \text{R-NC} + 3\text{KCl} + 3\text{H}_2\text{O}$$

• $N$-Methylaniline is a secondary amine ($2^\circ$).
• $N,N$-Dimethylaniline is a tertiary amine ($3^\circ$).
• Triethylamine is a tertiary amine ($3^\circ$).
• $p$-Methylbenzylamine ($\text{CH}_3-\text{C}_6\text{H}_4-\text{CH}_2\text{NH}_2$) contains a primary amino group ($-\text{CH}_2\text{NH}_2$, $1^\circ$) and therefore gives a positive carbylamine test.

Deduction of the compound's structure:

• The carbonyl compound $\text{C}_3\text{H}_6\text{O}$ could be either propanal (an aldehyde) or acetone / propan-2-one (a ketone).
• Because it does not reduce Tollens' reagent, it is not an aldehyde; hence it must be acetone ($\text{CH}_3\text{COCH}_3$).
• Acetone contains the carbonyl group ($\gt \text{C=O}$), so it reacts with hydroxylamine to form an oxime: $(\text{CH}_3)_2\text{C=N-OH}$.
• Acetone possesses a methyl ketone group ($\text{CH}_3\text{CO}-$), so when treated with iodine and aqueous sodium hydroxide, it undergoes the iodoform reaction to form a yellow precipitate of iodoform ($\text{CHI}_3$).

In the gaseous phase, there is no solvation effect or steric hindrance due to hydration. The basic character of amines is determined solely by the electron-donating inductive effect ($+I$ effect) of alkyl groups. The greater the number of methyl groups, the higher the electron density on nitrogen and the more readily it accepts a proton:

$$3^\circ\text{ amine} \gt 2^\circ\text{ amine} \gt 1^\circ\text{ amine} \gt \text{NH}_3$$ $$(\text{CH}_3)_3\text{N} \gt (\text{CH}_3)_2\text{NH} \gt \text{CH}_3\text{NH}_2 \gt \text{NH}_3$$