Alcohols

MDCAT Chemistry · Chemistry · Alcohols and phenols. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Alcohols contain an –OH group on a saturated carbon atom. They are primary, secondary or tertiary according to whether the carbon bearing the –OH is joined to one, two or three other carbon atoms. Most questions turn on this classification.

Two bonds can break. When the O–H bond breaks (with sodium, or in ester formation), primary alcohols react fastest. When the C–O bond breaks (with HX or Lucas reagent, or in dehydration), tertiary alcohols react fastest.

Alcohol molecules form hydrogen bonds with each other and with water. This gives them high boiling points and makes the small ones mix fully with water.

Rules to remember

Oxidation with acidified K₂Cr₂O₇ (orange to green): primary → aldehyde → carboxylic acid; secondary → ketone; tertiary → no oxidation
Lucas reagent (concentrated HCl + anhydrous ZnCl₂): tertiary turns cloudy at once, secondary in 5 to 10 minutes, primary stays clear at room temperature
Concentrated H₂SO₄: about 170 °C with excess acid gives an alkene; about 140 °C with excess alcohol gives an ether
Esterification: RCOOH + R′OH ⇌ RCOOR′ + H₂O (concentrated H₂SO₄). The acid loses –OH and the alcohol loses H
2ROH + 2Na → 2RONa + H₂. ROH + SOCl₂ → RCl + SO₂ + HCl. ROH + PCl₅ → RCl + POCl₃ + HCl
Iodoform test (I₂ + NaOH, yellow precipitate of CHI₃): positive for CH₃CH(OH)–, so ethanol and propan-2-ol react but methanol does not
Made by: hydration of alkenes, hydrolysis of alkyl halides with aqueous NaOH, Grignard reagent + carbonyl compound, reduction of aldehydes, ketones and acids, fermentation (ethanol)

Common mistake

Students write the same reactivity order for every reaction. With sodium the order is primary > secondary > tertiary, but with HX or Lucas reagent it is tertiary > secondary > primary, so first decide which bond is breaking.

Worked example

6.0 g of propan-1-ol, C₃H₇OH, is completely dehydrated by excess concentrated H₂SO₄ at 170 °C. What mass of propene is formed? (H = 1, C = 12, O = 16)

Equation: C₃H₇OH → C₃H₆ + H₂O, a 1 : 1 ratio.
M_r of propan-1-ol = 36 + 8 + 16 = 60, so moles = 6.0 ÷ 60 = 0.10 mol.
M_r of propene = 36 + 6 = 42, so mass = 0.10 × 42 = 4.2 g.

Answer: 4.2 g of propene

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