Lesson 13: Alcohols and Haloalkanes

Alcohols and haloalkanes are the great interconversion hub of organic chemistry — from ethanol in drinks and fuels to the CFCs that damaged the ozone layer. Their reactions showcase the two fundamental polar mechanisms, S_N1 and S_N2, whose kinetics you met in Lesson 5. This lesson covers classification and preparation of alcohols, their oxidation and dehydration, nucleophilic substitution of haloalkanes with both mechanisms, hydrolysis rates and uses of haloalkanes, and the environmental story of CFCs and the ozone layer.

Alcohols — classification and preparation

Alcohols contain the –OH functional group. Classification depends on the carbon bearing the OH: primary (1°, bonded to one other carbon, e.g. ethanol), secondary (2°, e.g. propan-2-ol), tertiary (3°, e.g. 2-methylpropan-2-ol). Classification determines oxidation products — the single most tested fact about alcohols. Check each alcohol by counting the carbons attached to the C–OH carbon.

Ethanol is made by fermentation (glucose → ethanol + CO₂, yeast, warm, anaerobic — a batch process giving low concentrations, about 12%, because ethanol poisons the yeast; renewable from sugar cane) or by hydration of ethene (steam, H₃PO₄, 300 °C, 60 atm — continuous, pure product, but from non-renewable crude oil). Comparing the two routes — renewability, purity, rate, conditions, environmental impact — is a standard 6-mark evaluation: fermentation suits countries with abundant sugar cane; hydration suits petrochemical economies. Examiners reward conclusions that depend on context rather than blanket verdicts.

Alcohols have high boiling points for their size because of hydrogen bonding, and small alcohols mix with water (hydrogen bonding with water molecules). They burn cleanly, so ethanol is used as a biofuel additive.

Oxidation and dehydration of alcohols

Oxidation with acidified potassium dichromate(VI) (orange → green) distinguishes the classes. Primary alcohols: gentle heating with distillation gives aldehydes (distil the product off to prevent further oxidation); reflux gives carboxylic acids. Secondary alcohols: reflux gives ketones, which resist further oxidation. Tertiary alcohols: no reaction (there is no H on the C–OH carbon to remove). The observations and product identities are tested together.

Mechanistically, oxidation removes two hydrogens (one from O, one from C). Write equations with [O] for the oxidising agent: CH₃CH₂OH + [O] → CH₃CHO + H₂O; CH₃CHO + [O] → CH₃COOH. The distillation/reflux distinction is practical chemistry: distil to collect the volatile aldehyde before it oxidises further; reflux to force complete oxidation to the acid.

Dehydration (elimination) with concentrated H₂SO₄ or H₃PO₄, warmed, removes H₂O to form alkenes: CH₃CH₂OH → CH₂=CH₂ + H₂O. This E1-type elimination goes via a carbocation, so for unsymmetrical alcohols the more substituted alkene predominates (Zaitsev's rule). Dehydration links alcohols back to alkenes, completing a two-way interconversion for synthesis questions.

Nucleophilic substitution — S_N1 and S_N2

Haloalkanes undergo nucleophilic substitution: the δ+ carbon (Cδ⁺–Xδ⁻) is attacked by nucleophiles (OH⁻, CN⁻, NH₃). Two mechanisms. S_N2 is concerted — the nucleophile attacks as the C–X bond breaks, through a transition state (one step; rate = k[haloalkane][nucleophile], second order). S_N1 is stepwise — the C–X bond breaks first, forming a carbocation (slow, rate-determining step), then the nucleophile attacks rapidly (rate = k[haloalkane], first order).

Tertiary haloalkanes react by S_N1: the tertiary carbocation is stabilised by three electron-donating alkyl groups, so ionisation is feasible; the planar carbocation is attacked from either side, giving a racemic mixture if the product is chiral. Primary haloalkanes react by S_N2: the carbocation would be too unstable, and the unhindered carbon allows backside attack. Secondary haloalkanes use a mixture of both. This is exactly what the rate equations of Lesson 5 predict — kinetic evidence for mechanism.

Reactivity trends: C–I > C–Br > C–Cl (bond enthalpy decreases down the group, so C–I breaks most easily) and tertiary > secondary > primary (carbocation stability and steric hindrance). The test uses aqueous AgNO₃ in ethanol: precipitates form fastest for tertiary iodides and slowest for primary chlorides — the rate of hydrolysis mirrors S_N reactivity. Hydrolysis with hot aqueous OH⁻ converts haloalkanes to alcohols.

Elimination vs substitution, and chain extension

Haloalkanes with hot concentrated ethanolic OH⁻ undergo elimination to alkenes, while aqueous OH⁻ gives substitution to alcohols — conditions determine the pathway, a favourite exam distinction. For synthesis: want an alcohol? Aqueous OH⁻, warm. Want an alkene? Ethanolic OH⁻, heat.

Substitution by CN⁻ (KCN in ethanol, heat) extends the carbon chain by one: the nitrile can then be hydrolysed (HCl(aq), heat) to a carboxylic acid, or reduced to an amine. Two-step synthesis questions use this constantly: bromoethane → propanenitrile → propanoic acid. Track the carbon count at every step. Excess NH₃ in ethanol (heat) gives amines by the same substitution pattern.

Uses of haloalkanes: CFCs were refrigerants and propellants (inert, non-toxic, non-flammable — seemingly ideal); chlorinated solvents are used for dry cleaning; haloalkanes are key synthesis intermediates. But the CFCs' very stability let them reach the stratosphere, where UV releases Cl• radicals that destroy ozone catalytically.

CFCs and the ozone layer

The ozone layer absorbs harmful UV radiation. CFCs (e.g. CCl₂F₂) released at the surface drift to the stratosphere, where UV causes homolytic fission: CCl₂F₂ → •CClF₂ + Cl•. The chlorine radical destroys ozone catalytically:

Cl• + O₃ → ClO• + O₂
ClO• + O₃ → Cl• + 2O₂

The Cl• is regenerated, so one radical destroys thousands of ozone molecules — a chain reaction, exactly like the propagation steps of Lesson 12. Consequences of ozone depletion: increased UV reaching Earth causes skin cancers, cataracts and crop damage; the Antarctic ozone hole appears each spring. The Montreal Protocol (1987) phased out CFCs — the most successful environmental treaty ever — and the ozone layer is recovering.

Evaluation points: CFCs were adopted because they seemed safe (non-toxic, inert) — the harm was unforeseen, illustrating the need for long-term testing. Replacements (HCFCs, then HFCs) solved ozone depletion, but HFCs are strong greenhouse gases, now being phased down under the Kigali Amendment. A 6-mark question might ask you to discuss the trade-offs in refrigerant choice across the generations.

Data treatment — measuring hydrolysis rates with silver nitrate

The AgNO₃/ethanol test converts mechanism theory into timings. Add each haloalkane to aqueous silver nitrate in ethanol at the same temperature and record the time for a precipitate to appear:

Interpretation. Down the halogens, the C–X bond weakens (C–Cl 346, C–Br 290, C–I 228 kJ mol⁻¹), so bond breaking in the RDS gets faster: I > Br > Cl. Between classes, the tertiary compound reacts fastest despite containing bromine, because its S_N1 ionisation forms a stabilised tertiary carbocation; the primary compounds must wait for the concerted S_N2 displacement. Present the data and the two separate explanations (bond enthalpy for the halogen trend; carbocation stability/mechanism for the class trend) — exam mark schemes keep them in separate marking points.

Going deeper — competing pathways and the synthesis map

Substitution and elimination compete for the same substrate, and A-level questions probe the boundary systematically. The variables that tip the balance: solvent (aqueous favours substitution; ethanolic favours elimination), temperature (heat favours elimination — it produces more particles, an entropically favoured outcome), base strength (concentrated, bulky bases abstract protons more readily), and substrate class (tertiary haloalkanes eliminate easily because the alkene product is more substituted and stable). For alcohols, the dehydrating agent's concentration plays the same role: concentrated H₂SO₄ or H₃PO₄ eliminates; the alcohol simply distils unchanged from dilute acid. Whenever a question specifies unusual conditions, it is signalling the pathway — read "ethanolic, concentrated, heat" as elimination before you even see the substrate.

Two more synthesis connections complete the alcohol hub. First, the breathalyser is applied oxidation chemistry: the original devices passed breath through acidified dichromate(VI), and the extent of the orange → green colour change measured the ethanol; modern evidential instruments use infrared absorption (Lesson 14) or fuel-cell oxidation of ethanol instead, but the redox logic is identical. Second, consider the full journey of one carbon skeleton through the hub: ethene →(HBr) bromoethane →(aqueous OH⁻) ethanol →(conc H₂SO₄, warm) back to ethene, or →(CN⁻, then H₃O⁺) propanoic acid, or →([O], reflux) ethanoic acid →(SOCl₂) ethanoyl chloride →(ethanol) ethyl ethanoate. Six functional groups, one reaction map. If you can walk that map in both directions, naming reagents and conditions at every arrow without notes, the organic synthesis paper holds no surprises — routes are only ever two or three steps sampled from maps of exactly this kind.

One quantitative footnote on the fuel comparison: fermentation ethanol is produced as a dilute aqueous solution (around 12%) and must be distilled to fuel grade, spending energy that offsets some of its renewable advantage — which is why life-cycle analyses, not slogans, decide whether a given country's bioethanol actually cuts emissions; sugar-cane ethanol in Brazil clears the bar comfortably, grain ethanol elsewhere less clearly.

The reverse direction matters too: alcohols are converted to haloalkanes with PCl₅ (steamy fumes of HCl — also the –OH test), SOCl₂, or concentrated HBr/H₂SO₄ mixtures. Together with hydrolysis, these give the two-way link alcohol ⇌ haloalkane that anchors the interconversion map. A practical consequence: because PCl₅ reacts with any –OH (including carboxylic acids and water), apparatus must be dry — a detail synthesis questions test as "state one precaution".

Common mistakes

• Reflux vs distillation swapped. Distilling a primary alcohol oxidation gives the aldehyde (it escapes before further oxidation); refluxing gives the carboxylic acid. State the apparatus word — it is the marking point.
• "Alcoholic KOH" and "aqueous KOH" confused. Ethanolic OH⁻ + heat → elimination (alkene); aqueous OH⁻ + warm → substitution (alcohol). The solvent decides the product.
• Saying S_N1 is "faster because the carbocation is stable" without the kinetics. Link it to the rate equation: rate depends only on [haloalkane], showing ionisation alone is the RDS.
• CFC equations missing the regeneration of Cl•. The catalytic cycle needs both propagation steps; a single Cl• + O₃ step earns half credit.

Exam Experience

Q1. 2-bromo-2-methylpropane is warmed with aqueous sodium hydroxide. The reaction is first order in the haloalkane and zero order in hydroxide. (a) Name the mechanism and explain what the rate equation tells you about its rate-determining step. [3] (b) Name the organic product. [1] (c) When the same compound is heated with concentrated ethanolic potassium hydroxide, a different product forms. Name it and state the type of reaction. [2]

Mark scheme. (a) S_N1 [1]; the rate depends only on the haloalkane concentration, so only the haloalkane is involved in the rate-determining step [1]; the RDS is the slow ionisation (heterolytic fission of C–Br) forming a tertiary carbocation, after which OH⁻ attacks rapidly [1]. (b) 2-methylpropan-2-ol [1]. (c) 2-methylpropene [1]; elimination (dehydrohalogenation) [1].