JEE Mains Chemistry · Alcohols, Phenols and Ethers

Reactions of Alcohols: Substitution, Dehydration and Rearrangement

An alcohol reacts at its O–H bond (esters, acetylation) or at its C–O bond once the OH is made a leaving group (halides, Lucas test, dehydration); in acid the carbocation shifts a hydride or methyl to become more stable before the most substituted alkene forms.

All Alcohols, Phenols and Ethers notes JEE Mains Chemistry strategy

Why this matters

Fifteen PYQs, all multiple choice, and three from 2026; seven of them come from 2021 papers. Four ask what happens at the OH group: acetylation mass, thionyl chloride on a molecule with both an alcoholic and a phenolic OH, hot copper and the acrolein test. Eleven predict the alkene or halide formed through a carbocation, nearly all from an alcohol in acid, and most of them hinge on a hydride shift, a methyl shift or a ring expansion.

Concept 1 of 2: Reactions at the O–H and C–O bonds of alcohols

Two bonds can break. The O–H bond breaks when the alcohol acts as an acid or a nucleophile, as in ester formation and acetylation. The C–O bond breaks once the OH is turned into a leaving group, as with HX, PCl₅ or SOCl₂. In a phenol the C–O bond has partial double-bond character, so it does not break this way.

Definition

Acetylation with acetic anhydride or $\mathrm{CH_3COCl}$ : each OH becomes $\mathrm{OCOCH_3}$ , so the molar mass rises by 42 g mol $^{-1}$ per OH ( $\mathrm{CH_3CO}$ , 43, replaces H, 1).
Halides: HX, $\mathrm{PCl_3}$ , $\mathrm{PCl_5}$ and $\mathrm{SOCl_2}$ replace an alcoholic OH by halogen; $\mathrm{SOCl_2}$ releases $\mathrm{SO_2}$ and HCl as gases. A phenolic OH is untouched.
Lucas reagent (conc. HCl + anhydrous $\mathrm{ZnCl_2}$ ): a 3° alcohol turns cloudy at once, a 2° in about five minutes, a 1° gives no cloudiness at room temperature.
Oxidation: 1° → aldehyde with PCC, → acid with $\mathrm{KMnO_4}$ or acidified dichromate; 2° → ketone; 3° resists.
Hot copper, 573 K: 1° → aldehyde, 2° → ketone (dehydrogenation); 3° → alkene (dehydration), since it has no H on the carbinol carbon.
Glycerol + $\mathrm{KHSO_4}$ , heat: loses two waters to give acrolein, $\mathrm{CH_2{=}CH{-}CHO}$ , with a pungent smell. This is the test for glycerol.

Molar mass after complete acetylation

M_{\text{product}} = M + 42n \qquad (n = \text{number of OH groups})

Worked example

Sorbitol,

\mathrm{C_6H_{14}O_6}

(molar mass 182 g mol

^{-1}

), is fully acetylated with excess acetic anhydride. The product has molar mass 434 g mol

^{-1}

. How many OH groups does sorbitol have?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 24 Jan 2026 Shift 1 · Q40Moderate

Example 1 · Alcohols, Phenols and Ethers · Reactions of Alcohols: Substitution, Dehydration and Rearrangement

A hydroxy compound (X) with molar mass 122 g

mol^{- 1}

is acetylated with acetic anhydride, using a large excess of the reagent ensuring complete acetylation of all hydroxyl groups. The product obtained has a molar mass of

290\text{ }g{\text{ }mol}^{- 1}

. The number of hydroxyl groups present in compound

(X)

is :

Ethanol is primary: no quick Lucas turbidity

Only a tertiary alcohol clouds Lucas reagent at once. A primary alcohol such as ethanol stays clear at room temperature.

Hot copper dehydrates a tertiary alcohol

A 3° alcohol has no H on its carbinol carbon, so Cu at 573 K cannot make a ketone from it.

\mathrm{(CH_3)_3COH}

gives 2-methylpropene instead, and that alkene can then react further in acid.

Acrolein is pungent, not fruity

The acrolein test for glycerol works by its sharp, irritating smell. Fruity smells belong to esters.

Drill 3 more on reactions at the o–h and c–o bonds of alcohols

Concept 2 of 2: Acid dehydration and carbocation shifts

In acid the OH is protonated and leaves as water, and a carbocation is left behind. Before it loses a proton, the cation looks at its neighbours: if moving a hydride or a methyl group (or a ring bond) gives a more stable cation, it moves. Only then does it lose a β-hydrogen, to give the most substituted alkene.

Definition

Ease of dehydration: 3° > 2° > 1°. Typical conditions: 1°, conc. $\mathrm{H_2SO_4}$ at 443 K; 2°, 85% $\mathrm{H_3PO_4}$ at 440 K; 3°, 20% $\mathrm{H_3PO_4}$ at 358 K.
Mechanism (E1): protonate OH → lose $\mathrm{H_2O}$ → carbocation → lose $\mathrm{H^+}$ .
1,2-shifts: a hydride or methyl moves to the cationic carbon when that gives a 3° (or benzylic) cation.
Ring expansion: a cation on a carbon next to a small ring can take a ring bond, turning a four-membered ring into a five-membered one or a five into a six.
The alkene is the most substituted one (Saytzeff), trans rather than cis, and conjugated with a ring when it can be.
A diol with excess acid loses two waters and gives a diene.
The same shifts happen when HX adds to an alkene and when an alcohol reacts with HX by $\mathrm{S_N1}$ .
Contrast: a bulky base such as $\mathrm{(CH_3)_3CO^-K^+}$ on an alkyl halide gives the less substituted (Hofmann) alkene.

E1 dehydration with a possible shift

\mathrm{ROH} \xrightarrow{\mathrm{H^+}} \mathrm{ROH_2^+} \xrightarrow{-\mathrm{H_2O}} \mathrm{R^+} \xrightarrow{\text{1,2-shift if better}} \mathrm{R'^+} \xrightarrow{-\mathrm{H^+}} \text{most substituted alkene}

Worked example

Predict the major product when 3-methylbutan-2-ol,

\mathrm{CH_3CH(OH)CH(CH_3)_2}

, is heated with conc.

\mathrm{H_2SO_4}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2021 · Paper 22 · Q39Moderate

Example 2 · Alcohols, Phenols and Ethers · Reactions of Alcohols: Substitution, Dehydration and Rearrangement

The major product formed in the following reaction is:

Check the neighbours before drawing the alkene

If the carbon next to a 2° cation is tertiary or quaternary, expect a shift. The product then has a different carbon skeleton from the alcohol, and the unshifted alkene is always among the options.

A primary alcohol can rearrange too

Cyclohexylmethanol in hot acid does not stop at a primary cation. A 1,2-hydride shift from the ring carbon gives the 3° 1-methylcyclohexyl cation, and 1-methylcyclohexene forms.

Acid on an alcohol is Saytzeff; a bulky base on a halide is Hofmann

1-Methylcyclohexanol with

\mathrm{H_3PO_4}

gives 1-methylcyclohexene (inside the ring). The matching chloride with

\mathrm{(CH_3)_3COK}

gives mostly methylenecyclohexane (outside the ring).

Drill 10 more on acid dehydration and carbocation shifts

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Reactions at the O–H and C–O bonds of alcohols
Molar mass after complete acetylation
$M_{\text{product}} = M + 42n \qquad (n = \text{number of OH groups})$
Acid dehydration and carbocation shifts
E1 dehydration with a possible shift
$\mathrm{ROH} \xrightarrow{\mathrm{H^+}} \mathrm{ROH_2^+} \xrightarrow{-\mathrm{H_2O}} \mathrm{R^+} \xrightarrow{\text{1,2-shift if better}} \mathrm{R'^+} \xrightarrow{-\mathrm{H^+}} \text{most substituted alkene}$

Watch out for (6)

Ethanol is primary: no quick Lucas turbidity
→ Reactions at the O–H and C–O bonds of alcohols
Hot copper dehydrates a tertiary alcohol
→ Reactions at the O–H and C–O bonds of alcohols
Acrolein is pungent, not fruity
→ Reactions at the O–H and C–O bonds of alcohols
Check the neighbours before drawing the alkene
→ Acid dehydration and carbocation shifts
A primary alcohol can rearrange too
→ Acid dehydration and carbocation shifts
Acid on an alcohol is Saytzeff; a bulky base on a halide is Hofmann
→ Acid dehydration and carbocation shifts

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