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

22 PYQs, 1 HARD — the paper's favourite way into this chapter. Thirteen 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°

Intuition

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.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1Alcohols, Phenols and EthersMODERATE

Which of the following is a tertiary allylic alcohol?

[Q80 · 10th May Shift 2 · 2023]

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

Concept 2 of 3

Monohydric, Dihydric, Trihydric: the Named Diols and Phenols

Intuition

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 Resorcinol is the isomer of catechol the paper keys.
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.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2Alcohols, Phenols and EthersMODERATE

Which of the following is a pair of dihydric phenols?

[Q90 · 14th May Shift 1 · 2024]

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

Intuition

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.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 3Alcohols, Phenols and EthersMODERATE

Hydroboration-oxidation of but-1-ene forms

[Q93 · 22 April Shift I · 2025]

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 Resorcinol is the isomer of catechol the paper keys.
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

Drill every past-year question on this subtopic

22 questions from the bank — paginated, with cart and Word-export support.

Drill the 22 questions

Drill every past-year question on this subtopic

Related notes