Lesson 1 of 13 · 6 min

Classification

NCERT §7.1

The lab's solvent shelf has ethanol, propan-2-ol, glycerol, phenol, benzyl alcohol and diethyl ether. All six contain C–O single bonds, yet the chemist asks Nila to store phenol away from the alcohols.

The story this chapter follows: Nila's summer at the distillery

Nila, a Class 12 student, spends her summer break in the quality lab of the distillery attached to a sugar mill near her town. Molasses from the mill is fermented to ethanol there, and her first job is a bench trial: 180 g of glucose fermented in a flask, which should give 92 g of ethanol. Over the summer that same 92 g, 2.00 mol, is tested, dehydrated, oxidised and turned into ether.

The lesson in notes

In short

An alcohol has an –OH group on an aliphatic carbon; a phenol has –OH directly on a carbon of an aromatic ring. Replacing the H of –OH by an alkyl or aryl group gives an ether, R–O–R′.

Counting –OH groups sorts alcohols and phenols into monohydric (one), dihydric (two), trihydric (three) and polyhydric (many). Ethane-1,2-diol is dihydric and propane-1,2,3-triol is trihydric.

Monohydric alcohols with the –OH on an sp³ carbon are primary (1°), secondary (2°) or tertiary (3°), matching the kind of carbon that holds the –OH.

Allylic alcohols carry –OH on the sp³ carbon beside a C=C; benzylic alcohols carry it on the sp³ carbon joined to an aromatic ring. Each of these can itself be 1°, 2° or 3°.

When –OH sits on an sp² carbon of a C=C the compound is a vinylic alcohol, such as CH₂=CH–OH. Phenols are the aromatic case of –OH on an sp² carbon.

Ethers are symmetrical when both groups on oxygen are the same (C₂H₅OC₂H₅) and unsymmetrical, or mixed, when they differ (C₂H₅OCH₃, C₂H₅OC₆H₅).

These classes are the starting points for detergents (alcohols), antiseptics (phenols) and fragrances (ethers). Furniture-polishing spirit is mostly ethanol, and sugar, cotton and paper are all built from –OH compounds.