Alcohols: Classification, Preparation, and Reactions

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Topic 5 — Key Facts for Kenyatta University (Kenya) Core concept: Alcohols contain the –OH (hydroxyl) functional group attached to an sp³-hybridised carbon; they are classified as primary (1°), secondary (2°), or tertiary (3°) based on the carbon bearing the –OH group High-yield point: 1° alcohols undergo oxidation to aldehydes then carboxylic acids; 2° alcohols oxidise to ketones; 3° alcohols do not oxidise without breaking the C–C bond; Lucas test (ZnCl₂/HCl) distinguishes them by reaction rate ⚡ Exam tip: The Lucas test reaction times are: 3° alcohols react immediately (cloudy within 1 min), 2° alcohols react in 5–15 min, 1° alcohols do not react at room temperature — frequently tested

---

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Alcohols: The Hydroxyl Functional Group

Alcohols are organic compounds containing one or more –OH (hydroxyl) groups attached to an sp³-hybridised carbon atom. They are the organic derivatives of water (H–O–H) where one hydrogen is replaced by an alkyl group.

Classification of Alcohols

Alcohols are classified by the number of carbon substituents on the carbon bearing the –OH group:

Type	Definition	Example	Oxidation Product
Primary (1°)	–OH on carbon with 1 alkyl group	CH₃OH (methanol), CH₃CH₂OH (ethanol)	Aldehyde → Carboxylic acid
Secondary (2°)	–OH on carbon with 2 alkyl groups	CH₃–CH(OH)–CH₃ (propan-2-ol)	Ketone
Tertiary (3°)	–OH on carbon with 3 alkyl groups	(CH₃)₃C–OH (2-methylpropan-2-ol)	No oxidation (C–C bond must break)

Nomenclature of Alcohols

IUPAC naming:

1. Select the longest chain containing the –OH group
2. Number to give the –OH the lowest possible locant
3. Use suffix -ol for the functional group
4. If –OH is the principal functional group, the chain is named as an alkanol

Examples:

• Methanol (CH₃OH): One carbon, –OH group
• Ethanol (CH₃CH₂OH): Two carbons
• Propan-1-ol (CH₃CH₂CH₂OH): Three carbons, –OH at C1
• Propan-2-ol (CH₃–CH(OH)–CH₃): Three carbons, –OH at C2
• Butan-1-ol (CH₃CH₂CH₂CH₂OH): Four carbons, –OH at C1
• 2-methylpropan-2-ol (tert-butanol): Four carbons with branching, –OH at C2 of branched chain

Physical Properties of Alcohols

Hydrogen Bonding: Alcohols can form hydrogen bonds because the –OH group contains:

• An electronegative oxygen that attracts the hydrogen (δ⁺)
• A hydrogen attached to oxygen, capable of forming a hydrogen bond with another electronegative atom

This hydrogen bonding explains why alcohols have:

• Higher boiling points than alkanes, ethers, and alkyl halides of similar molecular weight
• Water solubility for small alcohols (methanol, ethanol, propanol are completely miscible with water)

Boiling Point Trends:

Alcohol	Molecular Weight	Boiling Point (°C)	Solubility in Water
Methanol	32	65	Miscible
Ethanol	46	78	Miscible
Propan-1-ol	60	97	Miscible
Butan-1-ol	74	117	7.9 g/100mL
Pentan-1-ol	88	138	2.2 g/100mL
Hexan-1-ol	102	157	0.6 g/100mL

As the hydrocarbon chain length increases, water solubility decreases because the hydrophobic alkyl portion increasingly dominates.

⚡ Exam Tip: The boiling point of ethanol (78°C) is much higher than that of dimethyl ether (C₂H₆O, −24°C) even though they are isomers. This is due to hydrogen bonding in ethanol.

---

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Preparation of Alcohols

1. From Alkenes — Hydroboration-Oxidation

Reagents: Borane (BH₃·THF) followed by hydrogen peroxide and sodium hydroxide (H₂O₂/NaOH)

Reaction:

R–CH=CH₂ + BH₃ → R–CH₂–CH₂–BH₂ → (R–CH₂–CH₂–)₃B
(R–CH₂–CH₂–)₃B + H₂O₂/NaOH → R–CH₂–CH₂–OH   (anti-Markovnikov alcohol)

Key Feature: Anti-Markovnikov addition of water — the OH group adds to the less substituted carbon (the terminal carbon). This is the opposite regioselectivity of acid-catalysed hydration.

Mechanism:

• BH₃ is an electron-deficient molecule (boron has only 6 valence electrons)
• It adds to the less substituted carbon of the alkene (steric approach control)
• Oxidation replaces the B–C bond with an O–C bond

⚡ Exam Tip: This reaction converts terminal alkenes to primary alcohols — the only method that reliably gives primary alcohols from alkenes.

2. From Carbonyl Compounds — Reduction

Aldehydes and ketones are reduced to alcohols:

• Aldehyde + NaBH₄ or H₂/Pd → primary alcohol
• Ketone + NaBH₄ or H₂/Pd → secondary alcohol

Reagents:

• Sodium borohydride (NaBH₄): Selective reducing agent; reduces aldehydes and ketones but not esters, carboxylic acids, or amides
• Lithium aluminium hydride (LiAlH₄): Stronger reducing agent; reduces aldehydes, ketones, esters, carboxylic acids; highly reactive with water (must be used in anhydrous conditions)
• Hydrogen and catalyst (H₂/Pd or H₂/Ni): General reduction for aldehydes and ketones

Examples:

CH₃CHO + [H] → CH₃CH₂OH   (ethanol from acetaldehyde)
CH₃COCH₃ + [H] → CH₃CH(OH)CH₃   (propan-2-ol from acetone)

3. From Alkyl Halides — Nucleophilic Substitution

Reaction: Alkyl halide + OH⁻ (from NaOH or KOH) → alcohol + halide ion

Mechanism:

• SN1 for tertiary and some secondary alkyl halides: Rate = k[R–X][H₂O]; carbocation intermediate
• SN2 for primary and some secondary alkyl halides: Rate = k[R–X][OH⁻]; backside attack on carbon bearing the leaving group

SN2 Example:

CH₃CH₂–Br + NaOH → CH₃CH₂–OH + NaBr

⚡ Exam Tip: If the alkyl halide is tertiary, an E2 elimination competes with SN1 substitution. A bulky base (like t-BuOK) favours elimination; a weak base (like NaOH) favours substitution.

4. From Esters — Reduction

Esters can be reduced to alcohols (two primary alcohols from a fat):

• LiAlH₄: R–COOR’ + 4[H] → R–CH₂OH + R’–CH₂OH
• NaBH₄ does NOT reduce esters (only LiAlH₄ does)

Example from fat (triglyceride) hydrolysis:

Fat (ester of glycerol) + LiAlH₄ → Glycerol + fatty alcohols

5. From Grignard Reagents — Reaction with Carbonyl Compounds

Grignard reagents (R–MgBr) react with carbonyl compounds to give alcohols:

With formaldehyde (HCHO): R–MgBr + HCHO → R–CH₂–OH (primary alcohol) With aldehydes: R–MgBr + R’–CHO → R–CH(R’)–OH (secondary alcohol) With ketones: R–MgBr + R’–CO–R” → R–C(R’)(R”)–OH (tertiary alcohol)

⚡ Exam Tip: This is a key carbon-carbon bond-forming reaction. The Grignard reagent will not react if there are any acidic hydrogens or reactive groups (like –OH, –NH₂, –COOH) present in the molecule. These must be protected first.

Reactions of Alcohols

1. Oxidation Reactions

Oxidation of Primary Alcohol: R–CH₂–OH → R–CHO → R–COOH

Oxidation of Secondary Alcohol: R–CH(OH)–R’ → R–CO–R’

Oxidation of Tertiary Alcohol: No oxidation under these conditions without C–C bond cleavage (tertiary alcohols are resistant to oxidation)

Oxidising Agents:

• Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄): Orange Cr(VI) → Green Cr(III); used as a colour test
• Acidified potassium permanganate (KMnO₄/H₂SO₄): Purple Mn(VII) → Brown Mn(IV) MnO₂
• ** PCC (pyridinium chlorochromate)**: Mild oxidant; stops at aldehyde for primary alcohols
• Jones reagent (CrO₃/H₂SO₄): Oxidises primary alcohols to acids cleanly

⚡ Exam Tip: In the laboratory, the K₂Cr₂O₇/H₂SO₄ oxidation of ethanol is used in breathalyser tests. Ethanol is oxidised to acetic acid, changing the orange colour of dichromate to green. The intensity of green colour is proportional to the ethanol concentration in breath.

2. Reactions with Metals

Alcohols react with alkali metals (Na, K) and alkaline earth metals (Mg) to produce hydrogen gas and alkoxide salts:

2CH₃CH₂OH + 2Na → 2CH₃CH₂O⁻Na⁺ + H₂↑

Rate of reaction: Methanol > Primary > Secondary > Tertiary (steric hindrance effect)

⚡ Exam Tip: The reaction of Na with ethanol is less vigorous than with methanol. If a small piece of Na is added to ethanol, it reacts slowly with evolution of hydrogen gas. This is a demonstration of the decreasing acidity of alcohols as the alkyl chain grows.

3. Esterification (Fischer Esterification)

Alcohol + carboxylic acid → ester + water (acid-catalysed, reversible):

CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O
           Ethanol + Acetic acid → Ethyl acetate + Water

Mechanism:

1. Protonation of the carbonyl oxygen of the carboxylic acid
2. Nucleophilic attack by the alcohol on the carbonyl carbon
3. Proton transfer to form a tetrahedral intermediate
4. Elimination of water to give the ester

⚡ Exam Tip: Esterification is a reversible reaction. Removing water or using an excess of one reactant drives the equilibrium toward the ester product. Using a dehydrating agent (like concentrated H₂SO₄) is common in the laboratory.

4. Dehydration Reactions

Alcohols can be dehydrated (eliminate water) to give alkenes or ethers under acidic conditions:

To give alkenes ( Elimination — E1 or E2):

• Conditions: Concentrated H₂SO₄ at 170°C
• Mechanism: Protonation of –OH → loss of water → carbocation → elimination of H⁺ to give alkene
• Follows Zaitsev’s rule (most substituted alkene predominates)

CH₃–CH(OH)–CH₃ → CH₃–CH=CH₂ + H₂O   (propene from propan-2-ol)

To give ethers (Substitution):

• Conditions: Concentrated H₂SO₄ at 140°C
• Mechanism: Protonation of –OH → SN1 attack by another alcohol molecule → ether + water

2CH₃CH₂OH → CH₃CH₂–O–CH₂CH₃ + H₂O   (diethyl ether)

⚡ Exam Tip: Temperature determines the product. Low temperature (140°C) favours ether; high temperature (170°C) favours alkene. This is a classic examination question.

5. Substitution with HX — Formation of Alkyl Halides

Alcohol + HX → alkyl halide + H₂O

The reactivity of HX: HI > HBr > HCl > HF (reverse of acidity order; I⁻ is the best nucleophile)

Mechanism: The –OH group is protonated, then replaced by X⁻ via SN1 or SN2 depending on the alcohol type:

• 3° and 2° alcohols: SN1 (via carbocation)
• 1° alcohols: SN2

Reagent choice for alkyl halide formation:

• HCl (with ZnCl₂ catalyst — Lucas reagent): Tertiary > secondary >> primary
• HBr (generated in situ from KBr + H₂SO₄): Works for all alcohols
• PBr₃ or SOCl₂: Best for primary alcohols (avoids rearrangement)

---

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.