Reactions of alcohols

Four routes out of an alcohol

Alcohols sit at the centre of the reaction map, and there are four things you can do to one.

• Substitution → a haloalkane
• Elimination → an alkene
• Oxidation → an aldehyde, ketone or carboxylic acid
• Esterification → an ester

Substitution: replacing the –OH with a halogen

Three reagents do this, and all three appear on the standard's list.

• Concentrated HCl gives a chloroalkane:

$$\text{R}-\text{OH} + \text{HCl} \rightarrow \text{R}-\text{Cl} + \text{H}_2\text{O}$$

• HBr gives a bromoalkane:

$$\text{R}-\text{OH} + \text{HBr} \rightarrow \text{R}-\text{Br} + \text{H}_2\text{O}$$

• Thionyl chloride, $\text{SOCl}_2$, gives a chloroalkane:

$$\text{R}-\text{OH} + \text{SOCl}_2 \rightarrow \text{R}-\text{Cl} + \text{SO}_2 + \text{HCl}$$

• $\text{SOCl}_2$ is the best of the three, because both by-products are gases that escape from the mixture, so the reaction goes to completion and the product is easy to purify.
• The functional group stays on the same carbon — substitution never moves it.

Elimination: removing water to make an alkene

• Reagent: concentrated sulfuric acid, with heat.

$$\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{CH}{=}\text{CH}_2 + \text{H}_2\text{O}$$

• The –OH and a hydrogen from the neighbouring carbon are removed as water, and a double bond forms between those two carbons.

• This is also called dehydration, because water is lost.

• If the two neighbouring carbons are different, two alkenes form — see the next page.

• Do not confuse the eliminating reagents. Concentrated $\text{H}_2\text{SO}_4$ eliminates from an alcohol; KOH in alcohol eliminates from a haloalkane.

Oxidation

• Reagents: acidified permanganate, $\text{MnO}_4^-/\text{H}^+$, or acidified dichromate, $\text{Cr}_2\text{O}_7^{2-}/\text{H}^+$.
• What you get depends entirely on whether the alcohol is primary, secondary or tertiary — covered in full on the oxidation page.

Esterification

• An alcohol reacts with a carboxylic acid, catalysed by concentrated sulfuric acid, to give an ester and water.

$$\text{R}-\text{OH} + \text{R}'\text{COOH} \rightleftharpoons \text{R}'\text{COO}-\text{R} + \text{H}_2\text{O}$$

• The reaction is reversible, so it does not go to completion.
• An alcohol also reacts with an acyl chloride to give an ester, and that reaction is not reversible, so it gives a better yield.

Which reactions depend on the class of alcohol

Reaction	Depends on 1°/2°/3°?
substitution	No — all classes react
elimination	No — all classes with a neighbouring H react
oxidation	YES — this is the one
esterification	No

• So classify the alcohol before answering an oxidation question, and do not waste time doing so for the others.