Q.Which order of boiling points of the following compounds is correct?

Boiling points are governed by intermolecular forces; primary amines form extensive hydrogen bonds while tertiary amines cannot. The assertion is false (trimethylamine boils lower than propylamine), but the reason is true (primary amines do hydrogen-bond more extensively). The correct answer is (D).

The lowest members of the aliphatic amine family are gases at room temperature with a sharp, fishy smell resembling ammonia.

Simple aliphatic amines such as methylamine, dimethylamine, and trimethylamine are gases under ordinary conditions (their boiling points are low, close to or below room temperature) and have a characteristic pungent, fishy odour, similar to (but distinguishable from) ammonia — this smell is also noticeable in decaying fish, w …

Boiling point depends on the strength and extent of intermolecular hydrogen bonding: O–H bonds hydrogen-bond more strongly than N–H bonds, and fewer N–H bonds mean weaker association.

All three compounds have similar molecular mass, so the difference in boiling point comes from hydrogen bonding:

• C2H5OH (ethanol): the O–H bond is highly polar (O is more electronegative than N), so alcohols form the strongest intermolecular hydrogen bonds among the three. Boiling point ≈ 78 °C.
• C2H5NH2 (ethylamine, a primary amine): has two N–H bonds available for hydrogen bonding, so molecules associate reasonably well, but N–H hydrogen bonds are weaker than O–H. Boiling point ≈ 17 °C. …

Lower aliphatic amines like ethylamine dissolve readily in water via hydrogen bonding, but aromatic amines like aniline are only sparingly soluble because the bulky, hydrophobic benzene ring dominates their behaviour.

The statement is True.

Amines can form hydrogen bonds with water through their N–H bonds and the lone pair on nitrogen. Solubility in water depends on the balance between this hydrogen-bonding ability and the hydrophobic bulk of the rest of the molecule.

• Ethylamine (C$_2$H$_5$NH$_2$) has a small ethyl group, so the polar –NH$_2$ group dominates; it is completely miscible with water. …

Boiling point depends on the strength of intermolecular forces; only amines with an N–H bond can hydrogen-bond with each other, and primary amines have two such N–H bonds versus none in tertiary amines.

A primary amine, $R-NH_2$, has two N–H bonds. Because nitrogen is electronegative, these N–H bonds are polarised, and one molecule's N–H can hydrogen-bond to the lone pair on another molecule's nitrogen. This intermolecular hydrogen bonding is a relatively strong force that must be overcome for the liquid to boil, raising the boiling point.

A tertiary amine, $R_3N$, has no N–H bond at all — all three positions on nitrogen are occupied by carbon substituents. Tertiary amine molecules therefore cannot hydrogen-bond with each other (there is no acidic H to donate), and can only interact through weaker dipole-dipole interactions and van der Waals forces.

As a result, for amines of comparable molecular mass, the order of boiling points is: …