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MHT-CET Chemistry · Alcohols, Phenols and Ethers

Chemical Reactions of Alcohols and Acidity

An alcohol reacts by breaking its O–H bond (esterification, acidity) or its C–O bond (HX, PX₃, dehydration); tertiary alcohols react fastest with HX and Lucas reagent through a carbocation, dehydration follows Saytzeff with hydride shifts, and Grignard reagents build alcohols from carbonyls.

All Alcohols, Phenols and Ethers notesMHT-CET Chemistry strategy

Why this matters

14 PYQs, 2 HARD. Five are Lucas reagent and the HX reactivity order (tertiary first), plus which reaction breaks C–O; five are dehydration — hot Cu on a tertiary alcohol, the Saytzeff alkene from 2-methylhexan-3-ol with a hydride shift, the two-step propan-1-ol → propene → propan-2-ol; four are the Grignard back-calculation, which alcohol methanal cannot give, and acidity (p-nitrophenol strongest). Three cards.

Concept 1 of 3

Lucas Reagent and the HX Reactivity Order

Intuition

Lucas reagent is concentrated HCl with anhydrous ZnCl₂. It swaps OH for Cl through a carbocation, so the alcohol that makes the most stable cation reacts fastest: tertiary turns cloudy at once, secondary in minutes, primary not at room temperature. Any reaction that replaces OH — HX, PCl₃, PCl₅, SOCl₂ — breaks the C–O bond; esterification with an acid, anhydride or acyl chloride breaks the O–H bond.

Definition

  • Lucas reagent = conc. HCl + anhydrous ZnCl2\text{ZnCl}_2. Reactivity: 3° > 2° > 1°; with halo acids methanol is slowest of all: 3° > 2° > CH3CH2OH\text{CH}_3\text{CH}_2\text{OH} > CH3OH\text{CH}_3\text{OH}.
  • C–O bond breaks: R-OH+PCl3\text{R-OH} + \text{PCl}_3, PCl5\text{PCl}_5, SOCl2\text{SOCl}_2, HX, dehydration. O–H bond breaks: reaction with Na, with carboxylic acids, acetic anhydride, acetyl chloride (esters).
  • PCC oxidises a 1° alcohol to the ALDEHYDE only: propan-1-ol → propanal (C₃H₆O) and, with PCl₃, → 1-chloropropane (C₃H₇Cl). Acidified dichromate takes it on to the acid.
  • Distinguishing test in one line: Lucas — turbidity time.

Lucas test

R-OH→conc. HCl / ZnCl2R-Cl (turbid);3∘ instant>2∘ minutes>1∘ none\text{R-OH} \xrightarrow{\text{conc. HCl / ZnCl}_2} \text{R-Cl}\ (\text{turbid});\quad 3^\circ \text{ instant} > 2^\circ \text{ minutes} > 1^\circ \text{ none}

Worked example

Three unlabelled alcohols are butan-1-ol, butan-2-ol and 2-methylpropan-2-ol. How does the Lucas test tell them apart?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Alcohols, Phenols and EthersEASY
Which from following is called as Lucas reagent?

[Q96 · 11th May Shift 1 · 2023]

Marking esterification as C–O cleavage

In R–OH + R'COOH the alcohol keeps its oxygen (the ester is R'CO–O–R); it is the O–H bond that breaks. C–O breaks only when the whole OH leaves — HX, PX₃, SOCl₂, dehydration.
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Concept 2 of 3

Dehydration (Saytzeff, With Rearrangement) and Oxidation

Intuition

Concentrated H₂SO₄, or Al₂O₃ at 623 K, or hot copper for a tertiary alcohol, removes water to give an alkene — the more substituted one (Saytzeff). Because the route runs through a carbocation, a 1,2-hydride shift to a more stable cation happens first: 2-methylhexan-3-ol gives 2-methylhex-2-ene, not hex-3-ene. Hot copper dehydrogenates 1° and 2° alcohols instead (to aldehyde and ketone) and dehydrates 3° ones.

Definition

  • 2-Methylhexan-3-ol + conc. H2SO4\text{H}_2\text{SO}_4: 2° cation at C3 → hydride shift → 3° cation at C2 → 2-methylhex-2-ene (CH3)2C=CH-CH2CH2CH3(\text{CH}_3)_2\text{C=CH-CH}_2\text{CH}_2\text{CH}_3.
  • 2-Methylbutan-2-ol (or -1-ol via a shift) → 2-methylbut-2-ene; 2-methylpropan-2-ol vapour over hot Cu → 2-methylpropene.
  • Propan-1-ol →Al2O3,623 K\xrightarrow{\text{Al}_2\text{O}_3, 623\text{ K}} propene →H2SO4; H2O\xrightarrow{\text{H}_2\text{SO}_4;\ \text{H}_2\text{O}} propan-2-ol (Markovnikov re-hydration).
  • Hot Cu, 573 K: 1° → aldehyde, 2° → ketone, 3° → alkene. PCC: 1° → aldehyde. K2Cr2O7/H+\text{K}_2\text{Cr}_2\text{O}_7/\text{H}^+: 1° → acid.

Dehydration

R2CH-CH(OH)R→conc. H2SO4, ΔR2C=CHR+H2O(Saytzeff, after any hydride shift)\text{R}_2\text{CH-CH(OH)R} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ \Delta} \text{R}_2\text{C=CHR} + \text{H}_2\text{O} \quad (\text{Saytzeff, after any hydride shift})

Worked example

Give the major alkene from 3-methylbutan-2-ol with concentrated H₂SO₄.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Alcohols, Phenols and EthersMODERATE
Identify the major product formed when 2-Methylhexan-3-ol is heated with concentrated sulphuric acid.

[Q75 · 9th May Shift 2 · 2023]

Eliminating from the carbon that carried the OH

The secondary cation rearranges before it loses a proton. Hex-3-ene from 2-methylhexan-3-ol is the drawing beside the right answer; the hydride shift moves the double bond to the branched carbon.
Drill 4 more on dehydration (saytzeff, with rearrangement) and oxidation

Concept 3 of 3

Alcohols From Grignard Reagents; Acidity of Alcohols and Phenols

Intuition

R–MgX adds R to a carbonyl carbon: methanal gives a PRIMARY alcohol (R–CH₂OH), another aldehyde a secondary one, a ketone a tertiary one. Working backwards, 3-methylpentan-3-ol with an ethyl Grignard came from butanone. Acidity runs phenol far above water, which sits above the alcohols, and an electron-withdrawing nitro group makes p-nitrophenol the strongest of the lot.

Definition

  • HCHO+RMgX→RCH2OH\text{HCHO} + \text{RMgX} \to \text{RCH}_2\text{OH} (1°): ethanol, propan-1-ol, butan-1-ol — but never propan-2-ol, which needs ethanal + CH3MgX\text{CH}_3\text{MgX}.
  • RCHO+R’MgX→\text{RCHO} + \text{R'MgX} \to 2° alcohol; R2CO+R’MgX→\text{R}_2\text{CO} + \text{R'MgX} \to 3° alcohol. 3-Methylpentan-3-ol = butanone + C2H5MgBr\text{C}_2\text{H}_5\text{MgBr}.
  • Acidity: p-nitrophenol > phenol > water > ethanol > tert-butyl alcohol — alkyl groups (+I) weaken, nitro (−R) strengthens by stabilising the phenoxide.
  • Alcohols react with Na to give H₂ (O–H cleavage); with H2SO4\text{H}_2\text{SO}_4 at low temperature an alkyl hydrogen sulphate — sodium lauryl sulphate C12H25OSO3−Na+\text{C}_{12}\text{H}_{25}\text{OSO}_3^-\text{Na}^+, an ANIONIC detergent used in toothpaste and shampoo, is such an ester.

Grignard to alcohol

HCHO→1∘;RCHO→2∘;R2CO→3∘(then H3O+)\text{HCHO} \to 1^\circ;\quad \text{RCHO} \to 2^\circ;\quad \text{R}_2\text{CO} \to 3^\circ \quad (\text{then H}_3\text{O}^+)

Worked example

Which carbonyl and Grignard pair gives 2-methylbutan-2-ol, and which gives butan-2-ol?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Alcohols, Phenols and EthersHARD
Identify the compound A in the following sequence: A→C2H5MgBr/dry ether→H3O+A \xrightarrow{C_2H_5MgBr/\text{dry ether}} \xrightarrow{H_3O^+} 3-Methylpentan-3-ol

[Q72 · 14th May Shift 2 · 2024]

Counting the Grignard carbon into the wrong place

3-Methylpentan-3-ol has C3 bearing OH, methyl, ethyl, ethyl. One ethyl came from the Grignard, so the ketone had methyl + ethyl on the carbonyl: butanone, not propanone.
Drill 3 more on alcohols from grignard reagents; acidity of alcohols and phenols

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 (3)

  • Lucas Reagent and the HX Reactivity Order

    Lucas test

    R-OH→conc. HCl / ZnCl2R-Cl (turbid);3∘ instant>2∘ minutes>1∘ none\text{R-OH} \xrightarrow{\text{conc. HCl / ZnCl}_2} \text{R-Cl}\ (\text{turbid});\quad 3^\circ \text{ instant} > 2^\circ \text{ minutes} > 1^\circ \text{ none}
  • Dehydration (Saytzeff, With Rearrangement) and Oxidation

    Dehydration

    R2CH-CH(OH)R→conc. H2SO4, ΔR2C=CHR+H2O(Saytzeff, after any hydride shift)\text{R}_2\text{CH-CH(OH)R} \xrightarrow{\text{conc. H}_2\text{SO}_4,\ \Delta} \text{R}_2\text{C=CHR} + \text{H}_2\text{O} \quad (\text{Saytzeff, after any hydride shift})
  • Alcohols From Grignard Reagents; Acidity of Alcohols and Phenols

    Grignard to alcohol

    HCHO→1∘;RCHO→2∘;R2CO→3∘(then H3O+)\text{HCHO} \to 1^\circ;\quad \text{RCHO} \to 2^\circ;\quad \text{R}_2\text{CO} \to 3^\circ \quad (\text{then H}_3\text{O}^+)

Watch out for (3)

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