Hydroxy Compounds

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Cambridge International AS & A Level Chemistry (9701)

Hydroxy Compounds

The big idea: Alcohols sit at the centre of the AS organic map: they are made from alkenes, halogenoalkanes, carbonyls, acids and esters, and they turn into all of those in return. Whether an alcohol is primary, secondary or tertiary decides what oxidation does to it.

Summary — What This Chapter Covers

Six routes to alcohols: hydration, cold KMnO₄, hydrolysis of halogenoalkanes, reduction of carbonyls and acids, hydrolysis of esters.
Reactions: combustion, substitution to halogenoalkanes, sodium, oxidation, dehydration, esterification.
Primary → aldehyde (distil) → carboxylic acid (reflux); secondary → ketone; tertiary not oxidised.
Alkaline iodine gives yellow CHI₃ with a CH₃CH(OH)– group.
Alcohols are weaker acids than water.

1. Alcohols

16.1

Making alcohols

Starting material	Reagent and conditions	Example
alkene	steam H₂O(g), H₃PO₄ catalyst	CH₂=CH₂ + H₂O → C₂H₅OH
alkene	cold dilute acidified KMnO₄	CH₂=CH₂ + [O] + H₂O → HOCH₂CH₂OH (a diol)
halogenoalkane	NaOH(aq), heat	C₂H₅Br + OH⁻ → C₂H₅OH + Br⁻
aldehyde or ketone	NaBH₄ or LiAlH₄	CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃
carboxylic acid	LiAlH₄ (in dry ether)	CH₃COOH + 4[H] → CH₃CH₂OH + H₂O
ester	dilute acid or dilute alkali, heat	CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH

Classifying alcohols

Count the carbons attached to the C–OH carbon: one (or none, in methanol) = primary, two = secondary, three = tertiary. A molecule with more than one –OH is classified at each group, e.g. propane-1,2-diol, HOCH₂CH(OH)CH₃, has one primary and one secondary –OH.

16.1

Reactions of alcohols

Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Substitution to halogenoalkanes: HX(g); KCl + concentrated H₂SO₄ or H₃PO₄; PCl₃ and heat; PCl₅; SOCl₂.
Sodium: 2C₂H₅OH + 2Na → 2C₂H₅O⁻Na⁺ + H₂. Bubbles of hydrogen; the sodium dissolves; less vigorous than sodium with water. The product is sodium ethoxide.
Dehydration to an alkene: heated Al₂O₃, or concentrated H₂SO₄/H₃PO₄ and heat.
Esterification: with a carboxylic acid, concentrated H₂SO₄ catalyst, heat. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Oxidation with acidified K₂Cr₂O₇ or KMnO₄

Alcohol	Distil product off as it forms	Heat under reflux	Dichromate colour
primary	aldehyde: CH₃CH₂OH + [O] → CH₃CHO + H₂O	carboxylic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O	orange → green
secondary	ketone: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O		orange → green
tertiary	not oxidised		stays orange

Distillation removes the aldehyde (lower boiling point, no hydrogen bonding between its molecules) before it can be oxidised further. Reflux keeps it in the flask until it becomes the acid. With KMnO₄ the colour change is purple → colourless.

The tri-iodomethane (iodoform) test

Warm the alcohol with alkaline aqueous iodine (I₂ with NaOH(aq)). A pale yellow precipitate of CHI₃ (with an antiseptic smell) shows a CH₃CH(OH)– group. The alcohol CH₃CH(OH)R is oxidised, then cleaved, to CHI₃ and the carboxylate ion RCO₂⁻. Ethanol (R = H) gives a positive test; so does every secondary alcohol with –OH on C2.

Acidity

weaker acids than water ⁻ ⁻ ⁺ ⁻ ⁺ ₂

Worked Example

₄ ₁₀

Worked Example

₂ ₄ _r ₃ ₃ ₂ ₅

Practice Question

Which of these give a positive tri-iodomethane test: methanol, ethanol, propan-1-ol, propan-2-ol?

Memorise

Oxidation

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Hydroxy Compounds