12 min readPathway sheetLucas test as a 2-mark 'distinguish' question; mechanism of dehydration of ethanol as a 3-mark answer; oxidation products in 1-mark MCQs; often part of a 5-mark answer; also a NEET favourite

Reactions of alcohols: C-O bond cleavage

Alcohols swap -OH for X with HX (Lucas test) and lose water to alkenes by acid dehydration. Oxidation turns 1° alcohols into aldehydes then acids, and 2° into ketones.

Lucas reagentConc HCl + anhydrous ZnCl₂: 3° alcohols turn cloudy at once, 1° not at room temperature; in the lab, 2° take about 1 to 5 minutes

Dehydration conditions1°: conc H₂SO₄, 443 K; 2°: 85% H₃PO₄, 440 K; 3°: 20% H₃PO₄, 358 K

Ease of dehydration3° > 2° > 1°, the order of carbocation stability

Hot copper, 573 K1° alcohol → aldehyde, 2° → ketone (dehydrogenation); 3° → alkene (dehydration)

Reaction with hydrogen halides, and the Lucas test

1
R-OH + HX → R-X + H₂O
Order of reactivity: 3° > 2° > 1°
2★
(CH₃)₃C-OH + HCl → (CH₃)₃C-Cl + H₂O
Lucas reagent, room temperature
3
Shake the alcohol with Lucas reagent (conc HCl + anhydrous ZnCl₂). The alcohol dissolves, but the alkyl chloride does not, so it shows as turbidity (cloudiness).
4★
3°: turbid at once. 2°: turbid in about 1 to 5 minutes (from the lab manual; your textbook gives only the 3° and 1° results). 1°: no turbidity at room temperature.
3° alcohols form the stable 3° carbocation fastest

★ key step

Dehydration, and its mechanism

1★
C₂H₅OH → CH₂=CH₂ + H₂O
Conc H₂SO₄ at 443 K. Ethene is removed as it forms, to push the equilibrium forward
2
Step 1, protonation: CH₃CH₂-OH + H⁺ → CH₃CH₂-O⁺H₂
Fast: the alcohol takes up a proton (protonated alcohol)
3★
Step 2, carbocation: CH₃CH₂-O⁺H₂ → CH₃C⁺H₂ + H₂O
Slowest, rate-determining step: water leaves
4
Step 3, loss of a proton: CH₃C⁺H₂ → CH₂=CH₂ + H⁺
The acid used in step 1 is given back in step 3
5
CH₃CH(OH)CH₃ → CH₃CH=CH₂ + H₂O (85% H₃PO₄, 440 K); (CH₃)₃COH → (CH₃)₂C=CH₂ + H₂O (20% H₃PO₄, 358 K)
2° and 3° alcohols need milder conditions

★ key step

Oxidation

Oxidation of an alcohol removes hydrogen from the O-H and from the carbon carrying it.

1
RCH₂OH + [O] → RCHO + H₂O; RCHO + [O] → RCOOH
[O] stands for oxygen from the oxidising agent. A strong oxidising agent such as acidified KMnO₄ takes a 1° alcohol all the way to the carboxylic acid
2★
CH₃CH=CHCH₂OH + [O] → CH₃CH=CHCHO + H₂O
PCC (pyridinium chlorochromate) stops at the aldehyde and leaves the C=C alone. CrO₃ in an anhydrous medium also gives aldehydes
3
RCH(OH)R' + [O] → RCOR' + H₂O
CrO₃ oxidises a 2° alcohol to a ketone
4
3° alcohols resist oxidation. Under strong conditions (KMnO₄, high temperature), C-C bonds break, giving a mixture of acids with fewer carbon atoms.

★ key step

Vapours passed over copper at 573 K

Alcohol	Reaction	Product
1°, RCH₂OH	Dehydrogenation: RCH₂OH → RCHO + H₂	Aldehyde
2°, R₂CHOH	Dehydrogenation: R₂CHOH → R₂CO + H₂	Ketone
3°, (CH₃)₃COH	Dehydration: (CH₃)₃COH → (CH₃)₂C=CH₂ + H₂O	Alkene (2-methylpropene)

In real life

Breath analysers used to check drunk driving work by oxidising the ethanol in a driver's breath. Older kits used orange acidified dichromate that turns green; newer ones use a fuel cell.

Exam angle

2 marks: “How will you distinguish between propan-2-ol and 2-methylpropan-2-ol using the Lucas test?” Shake each with conc HCl + anhydrous ZnCl₂. 2-Methylpropan-2-ol (3°) turns cloudy at once; propan-2-ol (2°) turns cloudy only after about 1 to 5 minutes.

3 marks: “Ethanol gives ethene with conc H₂SO₄ at 443 K. Write the three steps of the mechanism and name the slow step.” Step 1: C₂H₅OH + H⁺ → C₂H₅O⁺H₂. Step 2 (slow): C₂H₅O⁺H₂ → CH₃CH₂⁺ + H₂O. Step 3: CH₃CH₂⁺ → CH₂=CH₂ + H⁺. Conditions: conc H₂SO₄, 443 K.

2 marks: “What happens when the vapours of (i) propan-2-ol and (ii) 2-methylpropan-2-ol are passed over copper at 573 K?” (i) Dehydrogenation: CH₃CH(OH)CH₃ → CH₃COCH₃ + H₂ (propanone). (ii) Dehydration: (CH₃)₃COH → (CH₃)₂C=CH₂ + H₂O (2-methylpropene).

Remember it

Lucas clock: 3° now, 2° soon, 1° no. Dehydration: protonate, lose water (slow), lose H⁺.