Arrange the following in increasing order of their boiling points:
A. CH3CH2CH2CHO
B. CH3CH2CH2CH2OH
C. C2H5OC2H5
D. CH3CH2CH2CH2CH3
E. CH3CH2COOH
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C < D < A < B < E
These five compounds have similar molecular masses (72 to 74), so their boiling points follow the strength of the intermolecular forces.
(C) \( \ce{C2H5OC2H5} \) (ethoxyethane, 307.6 K) — first (lowest). The C–O–C group is only weakly polar, so its boiling point is close to that of an alkane of similar mass.
(D) \( \ce{CH3(CH2)3CH3} \) (n-pentane, 309.1 K) — second. It has only weak dispersion forces; its boiling point is about the same as that of the ether.
(A) \( \ce{CH3CH2CH2CHO} \) (butanal, about 349 K) — third. The polar C=O groups give dipole-dipole attraction, but there is no hydrogen bonding between aldehyde molecules.
(B) \( \ce{CH3(CH2)3OH} \) (butan-1-ol, 390 K) — fourth. The −OH groups form intermolecular hydrogen bonds.
(E) \( \ce{CH3CH2COOH} \) (propanoic acid, about 414 K) — fifth (highest). The acid molecules are held by more extensive hydrogen bonding (dimers), which is not broken completely even in the vapour.
Shortcut: The acid (E) must come last; only Option (4) ends with E.
NCERT (2026-27 edition):
Physical Properties – “The C–O bonds in ethers are polar and thus, ethers have a net dipole moment. The weak polarity of ethers do not appreciably affect their boiling points which are comparable to those of the alkanes of comparable molecular masses but are much lower than the boiling points of alcohols as shown in the following cases:
n-Pentane, CH3(CH2)3CH3: b.p. 309.1 K; Ethoxyethane, C2H5–O–C2H5: b.p. 307.6 K; Butan-1-ol, CH3(CH2)3–OH: b.p. 390 K”
Physical Properties – “… Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature. The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding.”
Physical Properties – “Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase.”