Chemistry · Alcohols, Phenols And Ethers · NEET
Ethanol (C2H5OH, mass 46) and propane (C3H8, mass 44) have almost the same molecular mass, but ethanol boils at 351 K while propane boils near 231 K. The reason is that ethanol has an -OH group, so its molecules form intermolecular hydrogen bonds with each other. Propane has no -OH, so it is held only by weak van der Waals forces. Breaking hydrogen bonds needs much more heat, so ethanol boils far higher. NEET loves this exact comparison, so remember: same mass, but -OH wins.
An ether (like methoxymethane, CH3-O-CH3) has an oxygen but NO hydrogen attached to that oxygen. Hydrogen bonding needs an H directly bonded to O, N or F. Since the ether's oxygen has no O-H, ether molecules cannot hydrogen-bond to each other. So ethers are held mainly by weak forces, just like alkanes, and their boiling points are close to alkanes and much lower than alcohols. NCERT says the ether's b.p. is intermediate but close to the alkane, well below the alcohol.
Intermolecular hydrogen bonding is the main reason, but two smaller factors also matter. First, adding more carbon atoms raises the boiling point of alcohols because van der Waals forces grow with size. Second, branching lowers the boiling point because a branched molecule is more spherical, has less surface contact, and weaker van der Waals forces. But when you compare an alcohol with an ether or alkane of the SAME mass, hydrogen bonding is the deciding factor.
Yes, within a series of straight-chain alcohols, boiling point rises as you add carbon atoms because van der Waals forces increase with molecular size. But if you compare two alcohols of the same formula, the more branched one boils lower. Example: butan-1-ol (straight) boils higher than 2-methylpropan-2-ol (branched). So think two rules: more carbons = higher b.p., more branching = lower b.p.
The same -OH group that raises boiling point also lets alcohols form hydrogen bonds WITH water molecules. That is why small alcohols like methanol and ethanol mix with water in all proportions. As the carbon chain gets longer, the water-hating (hydrophobic) part grows and solubility falls. So hydrogen bonding explains both the high boiling point and the water solubility of small alcohols - one idea, two NEET facts.
Carboxylic acids have higher boiling points than aldehydes, ketones and even alcohols of comparable molecular mass. This is due to their:
Statement I: The boiling points of aldehydes and ketones are higher than hydrocarbons of comparable molecular masses because of weak molecular association due to dipole-dipole interactions. Statement II: The boiling points of aldehydes and ketones are lower than the alcohols of similar molecular masses due to the absence of H-bonding.
Statement I: The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane. Statement II: When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
For the same molecular mass, the order is alcohol > ether > alkane. Example: ethanol (351 K) > methoxymethane (dimethyl ether) > propane (about 231 K). The alcohol is highest because of intermolecular hydrogen bonding; the ether is intermediate but close to the alkane; the alkane is lowest with only van der Waals forces.
Hydrogen bonding needs a hydrogen atom bonded directly to O, N or F. In an ether the oxygen is bonded to two carbon atoms, so there is no O-H bond. Without an O-H, ether molecules cannot hydrogen-bond to one another, so they boil low like alkanes. (They CAN accept a hydrogen bond from water, which is why ethers still dissolve a little in water.)
Branching decreases the boiling point. A branched alcohol is more compact and spherical, so it has a smaller surface area of contact and weaker van der Waals forces. So butan-1-ol boils higher than the more branched 2-methylpropan-2-ol of the same formula.
As you add more carbon atoms, the molecule has a larger surface area, so van der Waals forces between molecules become stronger. Stronger forces need more heat to break, so the boiling point rises steadily as the alcohol chain grows longer.
Yes. NEET repeatedly asks which compound boils highest and why, often using statement-based questions on hydrogen bonding, dipole-dipole forces and branching. Knowing that only -OH (and -COOH) give intermolecular hydrogen bonds lets you answer these quickly without memorising numbers.
