MHT-CET Chemistry · Alcohols, Phenols and Ethers

Classification of Alcohols and Phenols

An alcohol is classed by the carbon that carries OH — primary, secondary or tertiary, and allylic (next to C=C), benzylic (on a ring carbon) or vinylic (on the C=C itself) — and by how many OH groups it has: monohydric, dihydric, trihydric; a phenol has OH directly on the benzene ring.

All Alcohols, Phenols and Ethers notes MHT-CET Chemistry strategy

Why this matters

32 PYQs, 1 HARD — the paper's favourite way into this chapter. Of the twenty-one set from 2021 to 2025, twelve ask which drawn or named alcohol is primary, secondary or tertiary allylic, secondary benzylic, or vinylic; seven ask which named phenol or polyol is dihydric, trihydric, isomeric with catechol, or not phenolic at all; two are the formula and the anti-Markovnikov preparation of a named alcohol. Three cards.

Concept 1 of 3: Allylic, Benzylic and Vinylic Alcohols, and 1°/2°/3°

Two labels stack on the OH carbon. First, how many carbons it carries: one — primary, two — secondary, three — tertiary. Second, what sits next to it: a C=C one bond away — allylic; a benzene ring directly attached — benzylic; OH on the double-bond carbon itself — vinylic (an enol). So but-3-en-2-ol is a SECONDARY ALLYLIC alcohol and 2-methylbut-3-en-2-ol a tertiary allylic one.

Definition

Primary allylic: $\text{CH}_2\text{=CH-CH}_2\text{OH}$ (prop-2-en-1-ol, allyl alcohol); $\text{CH}_3\text{CH=CH-CH}_2\text{OH}$ (but-2-en-1-ol, crotyl alcohol).
Secondary allylic: $\text{CH}_2\text{=CH-CH(OH)-CH}_3$ (but-3-en-2-ol); $\text{CH}_3\text{CH=CH-CH(OH)CH}_3$ .
Tertiary allylic: $\text{CH}_2\text{=CH-C(CH}_3)_2\text{OH}$ (2-methylbut-3-en-2-ol).
Vinylic: OH on the sp² carbon — but-2-en-2-ol. Benzylic: $\text{C}_6\text{H}_5\text{CH}_2\text{OH}$ (1°), $\text{C}_6\text{H}_5\text{CH(OH)CH}_3$ (2°), $\text{C}_6\text{H}_5\text{C(OH)(CH}_3)_2$ (3°).
NOT allylic: but-3-en-1-ol and pent-3-en-1-ol (OH two carbons from the C=C); NOT benzylic: 2-phenylethanol.

Two labels on the OH carbon

\text{degree: } 1^\circ/2^\circ/3^\circ \text{ by carbons attached};\quad \text{allylic: C=C next door};\ \text{benzylic: ring next door};\ \text{vinylic: OH on C=C}

Worked example

Classify pent-1-en-3-ol, 1-phenylpropan-1-ol and 2-methylprop-1-en-1-ol.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 10th May Shift 2 · Q83Moderate

Example 1 · Alcohols, Phenols and Ethers · Classification of Alcohols and Phenols

Which of the following is a tertiary allylic alcohol?

Calling but-3-en-1-ol allylic

Count the bonds: OH carbon → CH₂ → CH=. Two carbons away is homoallylic, not allylic. Allylic means the OH carbon is bonded directly to a double-bond carbon.

Drill 11 more on allylic, benzylic and vinylic alcohols, and 1°/2°/3°

Concept 2 of 3: Monohydric, Dihydric, Trihydric: the Named Diols and Phenols

Count the OH groups. Ethylene glycol and the three benzenediols are dihydric; glycerol and the two benzenetriols are trihydric; crotyl alcohol, with one OH, is monohydric even though it is an alkene. Among the benzenediols, catechol (1,2), resorcinol (1,3) and quinol (1,4) are isomers of one another.

Definition

Dihydric: ethylene glycol $\text{HOCH}_2\text{CH}_2\text{OH}$ ; catechol, resorcinol, quinol (hydroquinone) — benzene-1,2-, 1,3-, 1,4-diol.
Trihydric: glycerol (propane-1,2,3-triol, drawn as three carbons each with OH and NO double bond); pyrogallol (1,2,3) and phloroglucinol (1,3,5).
Phloroglucinol is the planted 'dihydric' option — it has three OH. Crotonyl alcohol is the planted 'dihydric' among glycol, resorcinol, quinol — it has one.
Phenolic OH = OH directly on a benzene ring: curcumin, eugenol, gallic acid have it; ascorbic acid does not (enediol on a furanone ring).

Compound	OH count	Class	Note
Crotonyl (crotyl) alcohol	1	Monohydric, allylic	Not dihydric despite the C=C
Ethylene glycol	2	Dihydric alcohol	Intramolecular H-bond
Catechol / resorcinol / quinol	2	Dihydric phenols	1,2 / 1,3 / 1,4 — isomers of each other Pyrogallol and phloroglucinol carry three OH groups, so a pair of dihydric phenols is drawn from these three only.
Glycerol (propylene glycerol)	3	Trihydric alcohol	Propane-1,2,3-triol
Pyrogallol / phloroglucinol	3	Trihydric phenols	1,2,3 / 1,3,5 Phloroglucinol is NOT dihydric.
Ascorbic acid	—	Not a phenol	No OH on a benzene ring

Dihydric means two OH; the benzenediols are each other's isomers.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 14th May Shift 1 · Q90Moderate

Example 2 · Alcohols, Phenols and Ethers · Classification of Alcohols and Phenols

Which of the following is a pair of dihydric phenols?

Reading '-ol' endings as OH counts

Phloroglucinol ends like a diol but has three OH groups; pyrogallol too. Count from the structure or the IUPAC name (benzene-1,3,5-triol), never from the common name.

Drill 6 more on monohydric, dihydric, trihydric: the named diols and phenols

Concept 3 of 3: Molecular Formula and the Preparations of Alcohols

A saturated acyclic alcohol is ( ext{C}_n ext{H}_{2n+2} ext{O}) — a ring or a C=C costs two hydrogens. Alcohols come from alkenes by acid-catalysed hydration (Markovnikov, OH on the more substituted carbon) or by hydroboration–oxidation (anti-Markovnikov, OH on the terminal carbon), from carbonyls by reduction or Grignard, and from halides by aqueous alkali.

Definition

$\text{C}_5\text{H}_{12}\text{O}$ is pentan-3-ol (or any saturated pentanol); cyclopentanol is $\text{C}_5\text{H}_{10}\text{O}$ , cyclopent-3-en-1-ol $\text{C}_5\text{H}_8\text{O}$ .
Hydroboration–oxidation ( $\text{B}_2\text{H}_6$ , then $\text{H}_2\text{O}_2/\text{OH}^-$ ): anti-Markovnikov — but-1-ene → butan-1-ol.
Acid hydration ( $\text{H}_2\text{SO}_4$ , then water): Markovnikov — propene → propan-2-ol; but-1-ene → butan-2-ol.
Aldehyde + $\text{LiAlH}_4$ / $\text{NaBH}_4$ → 1° alcohol; ketone → 2° alcohol. Grignard on methanal → 1°, on other aldehydes → 2°, on ketones → 3°.
R–X + aqueous KOH → R–OH.

The two hydrations

\text{RCH=CH}_2 \xrightarrow{\text{B}_2\text{H}_6;\ \text{H}_2\text{O}_2/\text{OH}^-} \text{RCH}_2\text{CH}_2\text{OH};\qquad \text{RCH=CH}_2 \xrightarrow{\text{H}_2\text{SO}_4;\ \text{H}_2\text{O}} \text{RCH(OH)CH}_3

Worked example

Give the product of propene by (i) hydroboration–oxidation and (ii) acid-catalysed hydration.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 22 April Shift I · Q93Moderate

Example 3 · Alcohols, Phenols and Ethers · Classification of Alcohols and Phenols

Hydroboration-oxidation of but-1-ene forms

Putting the OH on the wrong carbon in hydroboration

Hydroboration is the ANTI-Markovnikov route: boron, then OH, goes to the less substituted carbon. But-1-ene gives butan-1-ol, not butan-2-ol — the acid route gives that.

Drill 1 more on molecular formula and the preparations of alcohols

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)

Allylic, Benzylic and Vinylic Alcohols, and 1°/2°/3°
Two labels on the OH carbon
$\text{degree: } 1^\circ/2^\circ/3^\circ \text{ by carbons attached};\quad \text{allylic: C=C next door};\ \text{benzylic: ring next door};\ \text{vinylic: OH on C=C}$
Molecular Formula and the Preparations of Alcohols
The two hydrations
$\text{RCH=CH}_2 \xrightarrow{\text{B}_2\text{H}_6;\ \text{H}_2\text{O}_2/\text{OH}^-} \text{RCH}_2\text{CH}_2\text{OH};\qquad \text{RCH=CH}_2 \xrightarrow{\text{H}_2\text{SO}_4;\ \text{H}_2\text{O}} \text{RCH(OH)CH}_3$

Reference tables (1)

Monohydric, Dihydric, Trihydric: the Named Diols and Phenols6 rows

Compound	OH count	Class	Note
Crotonyl (crotyl) alcohol	1	Monohydric, allylic	Not dihydric despite the C=C
Ethylene glycol	2	Dihydric alcohol	Intramolecular H-bond
Catechol / resorcinol / quinol	2	Dihydric phenols	1,2 / 1,3 / 1,4 — isomers of each other Pyrogallol and phloroglucinol carry three OH groups, so a pair of dihydric phenols is drawn from these three only.
Glycerol (propylene glycerol)	3	Trihydric alcohol	Propane-1,2,3-triol
Pyrogallol / phloroglucinol	3	Trihydric phenols	1,2,3 / 1,3,5 Phloroglucinol is NOT dihydric.
Ascorbic acid	—	Not a phenol	No OH on a benzene ring

Dihydric means two OH; the benzenediols are each other's isomers.

Watch out for (3)

Calling but-3-en-1-ol allylic
→ Allylic, Benzylic and Vinylic Alcohols, and 1°/2°/3°
Reading '-ol' endings as OH counts
→ Monohydric, Dihydric, Trihydric: the Named Diols and Phenols
Putting the OH on the wrong carbon in hydroboration
→ Molecular Formula and the Preparations of Alcohols

Test yourself on Alcohols, Phenols and Ethers

20 past MHT-CET questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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Related notes

Halogen Derivatives — allylic and benzylic halides, the same labels →