Alcohols and the –OH group
Alcohols contain the hydroxyl group –OH on a carbon chain. They are classified by how many carbons are attached to the carbon bearing the OH: primary (one), secondary (two) or tertiary (three) — a distinction that decides how they oxidise. The O–H group lets alcohols hydrogen-bond, so they have higher boiling points and better water solubility than alkanes of similar size.
Oxidation — the key reaction
Oxidising alcohols (e.g. with acidified potassium dichromate, which turns orange to green) depends on the class:
Primary alcohols oxidise to an aldehyde, then further to a carboxylic acid. Secondary alcohols oxidise to a ketone (and no further). Tertiary alcohols resist oxidation — there is no C–H on the OH carbon to remove. So the oxidation product tells you the class of alcohol.
| Alcohol class | Oxidation product | Note |
|---|---|---|
| Primary | aldehyde → carboxylic acid | two stages |
| Secondary | ketone | stops there |
| Tertiary | no reaction | no H on the OH carbon |
Other reactions
Alcohols also undergo dehydration (losing water over a hot catalyst to form an alkene — an elimination), combustion (a clean fuel, e.g. ethanol), and esterification — reacting with a carboxylic acid to form a sweet-smelling ester plus water.
Phenols and ethers
Phenol has an –OH attached directly to a benzene ring, and that changes everything: unlike alcohols, phenol is weakly acidic. The ring pulls electron density from the O–H and stabilises the ion formed when it loses H⁺, so phenol reacts with sodium hydroxide (an alcohol does not). It is used to make plastics, antiseptics and aspirin.
Ethers (R–O–R) have oxygen between two carbon groups. They are fairly unreactive and volatile, which makes them useful as solvents.
Oxidation identifies the alcohol
Warm an alcohol with acidified dichromate: orange→green with a primary (→acid) or secondary (→ketone) alcohol; no colour change with a tertiary alcohol. The result reveals the class.
A worked example
Propan-2-ol and 2-methylpropan-2-ol are both warmed with acidified potassium dichromate. What happens to each, and why?
- Propan-2-ol is a secondary alcohol (two carbons on the OH carbon).Classify by carbons attached to the OH-bearing carbon.
- It oxidises to a ketone (propanone); the dichromate turns orange → green.Secondary alcohols oxidise to ketones.
- 2-methylpropan-2-ol is a tertiary alcohol (three carbons on the OH carbon).No hydrogen remains on the OH carbon.
- It is NOT oxidised; the dichromate stays orange.Tertiary alcohols resist oxidation — nothing to remove.
The secondary alcohol → propanone (orange→green); the tertiary alcohol is unreactive (stays orange). The colour change distinguishes them.
Making and dehydrating alcohols
Ethanol is made two ways: by fermentation of sugars with yeast (a slow, batch process from a renewable crop) and by the hydration of ethene with steam and a phosphoric acid catalyst (a fast, continuous process from crude oil). Each has trade-offs in cost, purity and sustainability.
Alcohols also react in the other direction: heated with a catalyst such as concentrated sulfuric acid or hot aluminium oxide, an alcohol is dehydrated, losing water to form an alkene — an elimination reaction that is the reverse of hydration.
Phenol's distinctive chemistry
Although it has an –OH group, phenol is not just another alcohol. The ring pulls electron density from the O–H and stabilises the ion left when it loses H⁺, so phenol is weakly acidic — acidic enough to react with sodium hydroxide, but too weak to react with carbonates (unlike a carboxylic acid). The relationship runs both ways: the oxygen also enriches the ring, so phenol undergoes electrophilic substitution readily, decolourising bromine water without a catalyst.
Phenol is used to make antiseptics, resins and, historically, the first plastics.
Acid strength ladder
Carboxylic acids > carbonic acid > phenol > alcohols. Phenol reacts with NaOH but not with sodium carbonate; a carboxylic acid reacts with both — a neat way to tell them apart.