Read the passage carefully and answer the Questions.
Ethers with the general formula \( C_nH_{2n+2}O \) and functional group C-O-C may be divided as symmetrical and unsymmetrical. Lower symmetrical ether may be obtained by the dehydration of alcohols at low temperatures in the presence of protic acids, such as \( H_2SO_4 \) and \( H_3PO_4 \) whereas at high temperature intramolecular dehydration takes place. Formation of ether from primary alcohols is also accompanied by the formation of small amount of alkenes. Due to the steric hindrance, secondary and tertiary alcohols undergo elimination reaction rather than substitution reaction yielding alkene as major product. Ethers can also be obtained by heating alkyl halides with sodium or potassium alkoxides. Substituted Ethers (secondary & tertiary) can also be prepared using this method. If primary halides are replaced with secondary and tertiary halides, alkenes are the major products.
The compound with the lowest boiling point is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Ethers
The boiling point of a compound depends on the strength of the forces between its molecules. For compounds of similar molar mass the order is set by the kind of attraction each class can show:
Alcohols have an O–H group and form intermolecular hydrogen bonds, so they boil highest.
Aldehydes and ketones have the strongly polar C=O group and are held by dipole-dipole attraction, so they boil well above hydrocarbons but below alcohols.
Ethers have only weakly polar C–O bonds and no O–H group, so they cannot hydrogen bond with one another. Their boiling points are close to those of alkanes of similar molar mass.
For example, for compounds of molar mass 58–60: methoxyethane boils at 281 K, propanal at 322 K, acetone at 329 K and propan-1-ol at 370 K. So, of the four classes, ethers have the lowest boiling point.
Why the other options are incorrect
Option (1) Alcohols — Incorrect. Hydrogen bonding between their molecules gives alcohols the highest boiling points of the four.
Option (2) Aldehydes — Incorrect. The polar C=O group gives dipole-dipole attraction, so aldehydes boil higher than ethers of the same molar mass.
Option (4) Ketones — Incorrect. Ketones are also held by dipole-dipole attraction of the C=O group and boil a little above the aldehydes of the same molar mass.
NCERT (2026-27 edition):
Physical Properties – …
Physical Properties – “… 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.”