Which of the following is not a correct statement for primary aliphatic amines?

We begin by recalling some well-established facts about aliphatic primary amines.

(i) Gabriel\,phthalimide\,synthesis: This method converts potassium phthalimide to a primary amine after two simple steps (alkylation and hydrolysis). Only the first hydrogen on nitrogen is replaced, so the product obtained is always a primary amine and never a secondary or tertiary one. Hence primary aliphatic amines can be prepared by this synthesis.

(ii) Basicity order in aqueous medium: The general order for aliphatic amines is secondary $$\gt \text{primary}\gt \text{tertiary}.$$ This comes from a compromise between the $$+I$$ effect (which favours basicity) and solvation of the protonated base (which disfavours steric crowding). Therefore a primary amine is indeed less basic than the corresponding secondary amine.

(iii) Hydrogen bonding (intermolecular association): A primary amine has two N-H bonds whereas a secondary amine possesses only one. The more N-H bonds present, the more hydrogen bonds can form between two molecules: No.\ of hydrogen bonds $$\propto\text{No.\ of N-H bonds}.$$ Consequently the intermolecular association in primary amines is actually greater than in secondary amines, not less.

(iv) Reaction with nitrous acid, HONO: For an aliphatic primary amine RNH$$_2$$, diazotisation gives the diazonium ion R-N$$_2^+$$, which immediately loses N$$_2$$ to generate the carbocation R$$^+$$. Water present in the reaction mixture captures this carbocation and furnishes the corresponding alcohol ROH. The overall transformation is sketched below:

$$\mathrm{RNH_2+HONO\;\longrightarrow\; R^+ + N_2 + 2H_2O\; ;\qquad R^+ + H_2O \longrightarrow ROH + H^+}.$$

For higher homologues (ethylamine, propylamine, etc.) the reaction proceeds cleanly to the alcohol. In the special case of methylamine, the methyl diazonium ion is so unstable that several side reactions compete; the major gaseous product is N$$_2$$, but the amount of methanol obtained is negligible. Hence in laboratory practice we say that “all primary aliphatic amines give the corresponding alcohol on treatment with nitrous acid except methylamine.”

Now we examine each option in the light of the above discussion:

Option A is consistent with fact (i) and is therefore a correct statement.

Option B matches fact (ii) and is likewise correct.

Option C claims that primary amines associate less than secondary amines, whereas fact (iii) tells us the reverse. So Option C is incorrect.

Option D agrees with fact (iv); methylamine is indeed treated as an exception in standard texts, so the statement is regarded as correct.

Only Option C fails to conform to the established chemistry.

Hence, the correct answer is Option C.