Alcohols

Full Notes

Alcohols are organic compounds that contain a carbon atom bonded to a hydroxy, OH group. They can be made in several ways and are important in organic synthesis processes.

Hydration of Alkenes

• Reagents: H₂O(g), H₃PO₄ catalyst
• Conditions: 300°C, 60 atm

Example CH₂=CH₂ + H₂O → CH₃CH₂OH

Cold KMnO₄ Oxidation of alkenes (to Diols)

• Reagents: Cold dilute acidified KMnO₄
• Reaction: CH₂=CH₂ → CH₂OH–CH₂OH

Explanation: KMnO₄ adds –OH groups across the C=C, forming a diol; also used as a test for unsaturation as a colour change of purple to colourless is observed.

Substitution of Halogenoalkanes

• Reagents: NaOH(aq), heat

Explanation: OH⁻ acts as a nucleophile and replaces the halogen in a nucleophilic substitution.

Example CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr

Reduction of Aldehydes/Ketones

• Reagents: NaBH₄ (aqueous) or LiAlH₄ (in dry ether)

LiAlH₄ is a more powerful reducing agent than NaBH₄, because of this if LiAlH₄ is used no water can be present and the reaction must be carried out in dry ether.

Examples
CH₃CHO + 2[H] → CH₃CH₂OH

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Matt’s exam tip

Remember reduction in organic chemistry reduction is the gaining of a carbon-hydrogen bond. To provide the hydrogen needed, we use reducing agents (such as NaBH₄ and LiAlH₄) and show hydrogen from a reducing agent in equations as [H].

Reduction of Carboxylic Acids

• Reagent: LiAlH₄ (only)

Explanation: LiAlH₄ is strong enough to reduce acids to primary alcohols, NaBH₄ can’t.

Example CH₃COOH + 4[H] → CH₃CH₂OH + H₂O

Hydrolysis of Esters

• Reagents: Dilute HCl or NaOH, heat under reflux

Explanation: The ester bond breaks, forming an alcohol and either a carboxylic acid (if acidic conditions are used) or carboxylate salt (if alkaline conditions are used).

Examples
CH₃COOCH₂CH₃ + H₂O → CH₃COOH + CH₃CH₂OH (acid hydrolysis)

CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH (alkaline hydrolysis)

Matt’s exam tip

Pay careful attention to the conditions used for ester hydrolysis in a question. A carboxylic acid can’t form in alkaline conditions (because OH^- ions are present that would instantly take a H⁺ from the carboxylic acid, forming H₂O), instead the carboxylate salt of the carboxylic acid gets formed.

Reactions of Alcohols

Combustion

Alcohols (especially shorter chain alcohols) burn cleanly to form CO₂ and water making them useful as fuels.

Example CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O

Substitution – Forming Halogenoalkanes

• Reagents: HX(g), PCl₅, SOCl₂, or KCl + conc. H₂SO₄

Example CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl

Reaction with Sodium

Explanation: The OH group of the alcohol loses a H⁺ ion (acts as an acid) and the H⁺ ion gets reduced by the Na(s) forming H₂ gas.. This forms a negatively charged O⁻ group (making an alkoxide) and Na⁺ ion which are attracted to each other. The reaction confirms the weak acidity of alcohols - see below for more detail on the acidity of alcohols.

Example 2CH₃CH₂OH + 2Na → 2CH₃CH₂O⁻Na⁺ + H₂(g)

Oxidation with Acidified K₂Cr₂O₇ / KMnO₄

Primary and Secondary alcohols can be oxidised by oxidising agents. The products formed depend on the classification of alcohol - alcohols can be primary, secondary or tertiary based on the number of carbons bonded to the C-OH group.

Matt’s exam tip

Remember for organic chemistry, oxidation can be considered a carbon atom gaining a bond to a more electronegative element (usually oxygen). To provide the oxygen needed, we use an oxidising agent and show oxygen from the oxidising agent in equations as [O]. Common oxidising agents include acidified K₂Cr₂O₇ and acidified KMnO₄.

For primary alcohols:

Primary alcohols oxidise first to aldehydes, then to carboxylic acids using heat and acidified potassium dichromate (VI) as an oxidising agent.

Observation: Orange Cr₂O₇²⁻ (dichromate) turns green (Cr³⁺ formed).

Formation of Aldehyde (Distillation):

Distillation is needed to obtain the aldehyde because the aldehyde has a low boiling point and will easily evaporate once formed, leaving the reaction mixture as a vapour.

Example CH₃CH₂OH + [O] → CH₃CHO + H₂O

(Ethanol → Ethanal)

Formation of Carboxylic Acid (Reflux):

Reflux is needed to obtain the carboxylic acid because the aldehyde must be continually condensed and forced to re-enter the reaction mixture, enabling it to be further oxidised.

Example CH₃CHO + [O] → CH₃COOH

(Ethanal → Ethanoic Acid)

For secondary alcohols:

Observation: Orange Cr₂O₇²⁻ (dichromate) turns green (Cr³⁺ formed).

Example CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

(Propan-2-ol → Propanone)

Matt’s exam tip

Remember only Primary and Secondary alcohols can be oxidised by mild oxidising agents such as Acidified K₂Cr₂O₇. When this happens, a colour change of orange to green occurs. If no colour change happens, the alcohol must be a tertiary alcohol.

Dehydration to Alkenes

• Reagents: Al₂O₃ catalyst or conc. H₂SO₄
• Conditions: Heat

Explanation: Alcohol loses water to form an alkene; reverse of hydration.

Example CH₃CH₂OH → CH₂=CH₂ + H₂O

Esterification

• Reagents: Carboxylic acid + conc. H₂SO₄

Explanation: A reversible reaction forming esters, used for perfumes and flavours.

Example CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O

Iodoform Test (CH₃CH(OH)–R Group)

The presence of a CH3CH(OH)- group can be detected by heating the alcohol with iodine (I2) in alkaline conditions (OH- ions).

• Reagent: I₂ in alkali
• Positive result: Yellow precipitate of CHI₃

Explanation: Identifies secondary alcohols with a methyl group on the OH bonded carbon.

Example
CH₃CH(OH)CH₃ + 3I₂ + 4NaOH → CHI₃ + CH₃COONa + 3NaI + 3H₂O

Acidity of Alcohols

Alcohols are much weaker acids than water. They are less able to lose H⁺ ions.

This is because the alkoxide ion (RO⁻) formed after losing a H⁺ ion is less stable than the hydroxide ion (OH⁻) formed from water. This is due to the electron-donating nature of the alkyl group, which increases the electron density on the negatively charged oxygen, making it less stable and less likely to form. In contrast, the hydroxide ion is more stable, so water more readily donates a proton, making it a stronger acid than alcohols.

Summary

• Alcohols can be prepared by alkene hydration, dihydroxylation with cold KMnO₄, nucleophilic substitution of halogenoalkanes, reductions (carbonyls/acids), and ester hydrolysis.
• Reactions include clean combustion, substitution to halogenoalkanes, sodium → alkoxide + H₂, oxidation (primary to aldehyde/carboxylic acid and secondary to ketone, dehydration to alkenes, and esterification.
• Primary/secondary/tertiary classification determines oxidation behaviour; dichromate turns from orange to green for primary/secondary.
• Iodoform test (I₂/alkali) gives yellow CHI₃ for CH₃CH(OH)–R group.
• Alcohols are weak acids. RO⁻ is less stable than OH⁻, so alcohols are less acidic than water.