Intermediate

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Alcohols — Classification, Preparation & Physical Properties

Master Alcohols for JEE & NEET. Covers 1°/2°/3° classification, IUPAC names, hydroboration, Grignard, LAH/NaBH4 reduction, fermentation, H-bonding and solubility.

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Alcohols, phenols, and ethers are three intimately related classes of oxygen-containing organic compounds. Alcohols contain the hydroxyl group ( $- OH$ ) attached to a saturated carbon; phenols have $- OH$ directly on an aromatic ring; and ethers have two organic groups linked by an oxygen ( $R - O - R^{'}$ ). Together these compounds appear in almost every domain — from industrial solvents and fuels to pharmaceuticals and everyday household chemicals. For JEE and NEET, the chapter is rich in reaction mechanisms, reagents, and comparative chemistry. This first topic establishes the classification, IUPAC nomenclature, all preparation methods, and the physical properties of alcohols — the foundation on which the reaction chemistry is built.

1. Classification of Alcohols

A. Based on Nature of Carbon Bearing –OH

Type	Structure	Example
Primary (1°)	$- OH$ on carbon attached to 1 other carbon	${CH}_{3} {CH}_{2} OH$ (ethanol)
Secondary (2°)	$- OH$ on carbon attached to 2 other carbons	$({CH}_{3})_{2} CHOH$ (propan-2-ol)
Tertiary (3°)	$- OH$ on carbon attached to 3 other carbons	$({CH}_{3})_{3} COH$ (2-methylpropan-2-ol)

B. Based on Number of –OH Groups

Monohydric: One $- OH$ group — e.g., ethanol ( $C_{2} H_{5} OH$ )
Dihydric (glycols): Two $- OH$ groups — e.g., ethane-1,2-diol (ethylene glycol)
Trihydric: Three $- OH$ groups — e.g., propane-1,2,3-triol (glycerol)
Polyhydric: More than three $- OH$ groups

2. IUPAC Nomenclature of Alcohols

Replace the terminal "e" of the alkane name with "-ol". Number the chain to give the $- OH$ group the lowest locant.

Structure	IUPAC Name	Common Name
${CH}_{3} OH$	Methanol	Wood alcohol
$C_{2} H_{5} OH$	Ethanol	Grain alcohol
$({CH}_{3})_{2} CHOH$	Propan-2-ol	Isopropyl alcohol
${HOCH}_{2} {CH}_{2} OH$	Ethane-1,2-diol	Ethylene glycol
${HOCH}_{2} {CH(OH)CH}_{2} OH$	Propane-1,2,3-triol	Glycerol/Glycerin

3. Preparation of Alcohols

Method 1 — Hydration of Alkenes

(a) Acid-catalysed hydration (Markovnikov addition of water):

${CH}_{2} {=CH}_{2} + H_{2} O \overset{H_{3} {PO}_{4}, 300 ° C}{\to} {CH}_{3} {CH}_{2} OH$

Follows Markovnikov's rule — $- OH$ adds to the more substituted carbon.

(b) Hydroboration–oxidation (anti-Markovnikov):

$alkene \overset{1. B_{2} H_{6}, THF}{\to} \overset{2. H_{2} O_{2} / NaOH}{\to} anti-Markovnikov alcohol$

Syn addition; $- OH$ on less substituted carbon; no rearrangement. JEE key: the product is the opposite regiochemistry to acid hydration.

Method 2 — From Grignard Reagents

$R-MgX + carbonyl compound \overset{1. dry ether}{\to} \overset{2. H_{3} O^{+}}{\to} alcohol$

Carbonyl compound	Product
Formaldehyde ( $HCHO$ )	Primary alcohol (one carbon longer)
Any other aldehyde ( $RCHO$ )	Secondary alcohol
Ketone ( $R_{2} C=O$ )	Tertiary alcohol

Method 3 — Reduction of Carbonyl Compounds

Reagent	Substrate → Product
${LiAlH}_{4}$ (LAH) in dry ether	Aldehyde → 1° alcohol; Ketone → 2° alcohol; Ester/carboxylic acid → 1° alcohol
${NaBH}_{4}$ in alcohol	Milder; reduces aldehydes and ketones only (not esters/acids)
$H_{2} / Ni$ (catalytic hydrogenation)	Aldehyde → 1° alcohol; Ketone → 2° alcohol

Method 4 — From Alkyl Halides (Nucleophilic Substitution)

$R-X + NaOH(aq) \overset{Δ}{\to} R-OH + NaX$

$S_{N} 2$ for primary halides; $S_{N} 1$ for tertiary halides. Not preferred for tertiary halides (elimination competes).

Method 5 — Fermentation (Industrial method for ethanol)

$C_{6} H_{12} O_{6} \overset{invertase/zymase}{\to} 2 C_{2} H_{5} OH + 2 {CO}_{2}$

Glucose → Fructose (invertase) → Ethanol + ${CO}_{2}$ (zymase). Temperature: 25–30°C. Max ~15% ethanol (yeast dies above this).

4. Physical Properties of Alcohols

Boiling Points

Alcohols have much higher boiling points than alkanes of similar molecular mass due to intermolecular hydrogen bonding ( $O - H \dots O$ ).
Boiling point increases with: (a) molecular weight/chain length, (b) number of $- OH$ groups.
Branching decreases boiling point (less surface area for H-bonding).

Compound	Mol. Mass	B.P. (°C)
Propane	44	–42
Methanol	32	65
Ethanol	46	78
Propan-1-ol	60	97
Propan-2-ol	60	82

Solubility in Water

Lower alcohols ( $C_{1}$ – $C_{3}$ ) are completely miscible with water — they form H-bonds with water.
Solubility decreases as chain length increases (hydrophobic alkyl part dominates).
Methanol, ethanol, propanol: completely miscible. Butanol onwards: partially soluble. Higher alcohols: practically insoluble.

State at Room Temperature

$C_{1}$ – $C_{11}$ alcohols: liquids.
$C_{12}$ and above: waxy solids.
Lower alcohols have a characteristic smell and burning taste.

Preparation Methods — Quick Decision Chart

Target alcohol	Best method	Key feature
1° alcohol (anti-Markovnikov)	Hydroboration-oxidation	${BH}_{3} / THF$ then $H_{2} O_{2} / {OH}^{-}$ ; syn; no rearrangement
1° alcohol (Markovnikov not possible here)	Grignard + HCHO	Adds one carbon, gives 1° alcohol
2° alcohol	Acid hydration of alkene OR Grignard + aldehyde	Markovnikov regiochemistry for acid hydration
3° alcohol	Grignard + ketone	Two R groups from ketone + R from Grignard
Industrial ethanol	Fermentation of glucose	Zymase enzyme; 25–30°C; max ~15%

JEE / NEET Power Tips — Alcohols: Classification & Preparation

Hydroboration-oxidation gives anti-Markovnikov, syn, and no-rearrangement addition. All three features together distinguish it from every other addition. JEE frequently tests all three in one question.
${LiAlH}_{4}$ vs ${NaBH}_{4}$ : LAH reduces everything (aldehydes, ketones, esters, carboxylic acids, amides). NaBH4 reduces only aldehydes and ketones — milder and selective. Can't use NaBH4 to reduce esters.
Grignard reagents are destroyed by water, alcohols, or any acidic H. Reaction must be done in dry ether. After addition, aqueous $H_{3} O^{+}$ is added in a separate workup step to protonate the alkoxide.
Tertiary alcohol + acid catalyst → easy dehydration. $3 ° > 2 ° > 1 °$ in ease of dehydration (same as carbocation stability). This means strong acid/high temperature is needed for primary alcohols.
Solubility in water decreases with chain length. The $- OH$ group is hydrophilic; the alkyl group is hydrophobic. As the chain grows, hydrophobic character dominates.

Practice Questions

Q1 (JEE Main / NEET): Which of the following correctly predicts the major product of the hydroboration-oxidation of propene?

A) Propan-2-ol
B) Propan-1-ol
C) 1-bromopropane
D) 2-bromopropane

Answer: B) Propan-1-ol.

Explanation: Hydroboration-oxidation results in the apparent anti-Markovnikov addition of water across the double bond. The hydroxyl group ( $- OH$ ) adds to the less substituted carbon (C-1 of propene). By contrast, an acid-catalysed hydration would yield propan-2-ol (following Markovnikov's rule).

Q2 (JEE Main): What type of alcohol is produced when a Grignard reagent reacts with acetaldehyde ( ${CH}_{3} CHO$ )?

Explanation:

The reaction proceeds via nucleophilic addition followed by acidic workup:

$R-MgX + {CH}_{3} CHO \to_{2. H_{3} O^{+}}^{1. dry ether} {R-CH(OH)-CH}_{3}$

In the final product, the carbon bearing the $- OH$ group is directly attached to two other alkyl carbon atoms (the 'R' group from the Grignard and the methyl group from acetaldehyde). Therefore, it yields a secondary (2°) alcohol.

General rule: Grignard + formaldehyde = primary alcohol; Grignard + any other aldehyde = secondary alcohol; Grignard + ketone = tertiary alcohol.

Q3 (NEET): Arrange the following compounds in increasing order of their boiling points: propan-1-ol, propan-2-ol, butane.

Explanation:

Butane: A non-polar alkane with only weak Van der Waals forces (B.P. $\approx - 1^{\circ} C$ ).
Propan-2-ol: Forms intermolecular hydrogen bonds, but branching decreases the effective surface area and slightly sterically hinders H-bonding compared to straight chains (B.P. $\approx 82^{\circ} C$ ).
Propan-1-ol: A straight-chain alcohol with extensive intermolecular hydrogen bonding and higher surface area (B.P. $\approx 97^{\circ} C$ ).

Increasing B.P.: butane < propan-2-ol < propan-1-ol.

Q4 (JEE Main): Which of the following reagents reduces an ester to give a primary alcohol?

A) ${NaBH}_{4}$
B) ${LiAlH}_{4}$
C) $H_{2} / Ni$
D) $Zn-Hg/HCl$

Answer: B) ${LiAlH}_{4}$ .

Explanation: Lithium aluminium hydride ( ${LiAlH}_{4}$ ) is a highly powerful reducing agent capable of reducing esters (and carboxylic acids) to primary alcohols. Sodium borohydride ( ${NaBH}_{4}$ ) is a milder agent and cannot reduce esters. Catalytic hydrogenation ( $H_{2} / Ni$ ) does not easily reduce esters under normal conditions. Clemmensen reduction ( $Zn-Hg/HCl$ ) is used strictly to reduce aldehydes/ketones to alkanes ( $- {CH}_{2} -$ ), not to alcohols.

Q5 (Board / NEET): Write the IUPAC name of $({CH}_{3})_{2} {C(OH)CH}_{2} {CH}_{3}$ . What classification of alcohol is it?

Explanation:

First, identify the longest carbon chain containing the carbon attached to the hydroxyl group. The longest chain is 4 carbons long (a butane backbone). Numbering from left to right gives the $- OH$ group the lowest locant (C-2).

At C-2, there is both a methyl group and a hydroxyl group.
IUPAC Name: 2-methylbutan-2-ol.

Because the carbon bonded to the $- OH$ group is directly attached to three other carbon atoms (two methyls and one ethyl), it is classified as a Tertiary (3°) alcohol.

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Est. Read Time26 minutes

CategoryAlcohols Phenols And Ethers

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Alcohols — Classification, Preparation & Physical Properties

1. Classification of Alcohols

A. Based on Nature of Carbon Bearing –OH

B. Based on Number of –OH Groups

2. IUPAC Nomenclature of Alcohols

3. Preparation of Alcohols

Method 1 — Hydration of Alkenes

Method 2 — From Grignard Reagents

Method 3 — Reduction of Carbonyl Compounds

Method 4 — From Alkyl Halides (Nucleophilic Substitution)

Method 5 — Fermentation (Industrial method for ethanol)

4. Physical Properties of Alcohols

Boiling Points

Solubility in Water

State at Room Temperature

Preparation Methods — Quick Decision Chart

Practice Questions

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Practice Questions

Topic Information

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