Reactions of alcohols

Alcohols react in two ways: the O–H bond breaks (the alcohol acts as an acid or a nucleophile), or the C–O bond breaks (the OH group is replaced or removed).

O–H breaking. With reactive metals such as sodium, alcohols give an alkoxide and hydrogen gas. With carboxylic acids (and a little concentrated H₂SO₄) they form esters: esterification.

Replacing OH by a halogen. HX, PCl₃, PCl₅ or SOCl₂ all turn R–OH into R–X. With HX the order of reactivity is 3° > 2° > 1°. This is the Lucas test: shake the alcohol with HCl and ZnCl₂. A tertiary alcohol goes cloudy at once, a secondary one in about five minutes, and a primary one stays clear at room temperature.

Dehydration. Heat an alcohol with concentrated H₂SO₄ (for ethanol, at 443 K) and a molecule of water is removed: an OH from one carbon and an H from the next, leaving a C=C double bond. Tertiary alcohols lose water most easily. When more than one alkene is possible, the more substituted one is the main product (Saytzeff's rule).

The mechanism explains much of this. The acid first adds H⁺ to the OH, so it can leave as a water molecule, leaving a carbocation. A carbocation can rearrange: a hydrogen (hydride) or a methyl group moves across from the next carbon to make a more stable carbocation (3° is more stable than 2°, which is more stable than 1°).

Oxidation depends on the type of alcohol:

• A primary alcohol gives an aldehyde under controlled conditions (PCC, or CrO₃ in anhydrous medium), but a carboxylic acid with a strong oxidising agent such as acidified KMnO₄ or K₂Cr₂O₇.
• A secondary alcohol gives a ketone.
• A tertiary alcohol has no H on its OH carbon, so it resists oxidation.

Passing an alcohol vapour over hot copper at 573 K also removes hydrogen: primary alcohols give aldehydes and secondary ones give ketones, while tertiary ones lose water and give alkenes.