Lesson 5 of 13 · 6 min
Physical properties
NCERT §7.4.3
In the distillation room, the first few litres off the still are collected in a separate can and never sold. The operator says they carry the methanol, which boils before the ethanol does.
The lesson in notes
In short
Alcohols and phenols are an alkyl or aryl group joined to –OH. The –OH part is polar and forms hydrogen bonds; the hydrocarbon part is not.
Boiling points rise as the number of carbons grows, because van der Waals forces grow. Within isomers, branching lowers the boiling point because the surface area of contact shrinks.
Alcohols and phenols boil much higher than hydrocarbons, ethers and haloarenes or haloalkanes of similar mass, because their molecules are linked by intermolecular hydrogen bonds: ethanol boils far above propane, with methoxymethane in between.
Worked order: n-butane < ethoxyethane < pentanal < pentan-1-ol. The alcohol has hydrogen bonding; the aldehyde has dipole–dipole forces; the ether only weak dipoles; the alkane only dispersion forces.
Worked order, highest boiling point first: pentan-1-ol > butan-1-ol > butan-2-ol > propan-1-ol > ethanol > methanol. Longer chains boil higher, and the branched butan-2-ol sits below its straight-chain isomer.
Water solubility comes from hydrogen bonds between –OH and water. It falls as the hydrophobic alkyl or aryl part grows. The lower alcohols dissolve in water in all proportions.