What is the decreasing order of boiling point of the following and why?
CH3CH2OH (a) OHCH2CH2OH (b) OHCH2CH(OH)CH2OH (c)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (c), (b), (a) because of hydrogen bonding
Alcohols have much higher boiling points than hydrocarbons of similar mass because their –OH groups form intermolecular hydrogen bonds, which must be broken before the liquid boils. Each extra –OH group gives a molecule more hydrogen bonds with its neighbours, so the boiling point rises steeply with the number of –OH groups:
(c) \( \ce{OHCH2CH(OH)CH2OH} \): glycerol (propane-1,2,3-triol), three –OH groups, b.p. about 563 K.
(b) \( \ce{OHCH2CH2OH} \): ethylene glycol (ethane-1,2-diol), two –OH groups, b.p. about 470 K.
(a) \( \ce{CH3CH2OH} \): ethanol, one –OH group, b.p. 351 K.
Molecular mass alone does not decide this order; it follows the number of –OH groups, so Options (2) and (4) fail on the reason as well.
Shortcut: Glycerol (c), with three –OH groups, must come first; only Option (1) starts with (c).