OCR A-Level Chemistry: Alcohols, Haloalkanes and Analysis — Complete Revision Guide (H432)
OCR A-Level Chemistry: Alcohols, Haloalkanes and Analysis
Alcohols and haloalkanes are the two reactive functional groups that unlock the rest of organic synthesis at A-Level. Once you can recognise an electron-rich double bond and predict an electrophilic-addition product from basic organic and hydrocarbons, the next step is the chemistry of single-bond functional groups that act as nucleophilic-substitution and elimination substrates. Alcohols sit at a synthetic crossroads — oxidisable to aldehydes, ketones or carboxylic acids; dehydratable to alkenes; substitutable to haloalkanes — while haloalkanes invert that connection by being hydrolysed back to alcohols. This course closes with the spectroscopy basics (infrared and mass spectrometry) that turn structure prediction into structure verification.
H432 examiners reward this module heavily because it is the place where the AS-level organic-mechanism vocabulary becomes synthetically useful. Every multi-step synthesis question on Paper 2 — turn molecule A into molecule B in three or four steps — passes through an alcohol or a haloalkane intermediate. Every analytical-elucidation question on Paper 2 ends with IR or MS data that constrains the candidate's structural hypothesis. Every synoptic environmental-chemistry question on Paper 3 reaches back to the CFC-ozone radical mechanism. The fluency reward for the candidate is therefore double: the synthesis pathways collapse into a handful of single-step transformations chained together, and the analytical work becomes a recognition task against a small library of characteristic IR bands and MS-fragment losses. Candidates who treat the module as discrete reactions struggle on the multi-step synthesis items; candidates who internalise it as a reaction map find the same questions short.
Course 6 of the H432 Chemistry learning path on LearningBro, Alcohols, Haloalkanes and Analysis, develops the AS-level synthesis vocabulary the rest of the spec will rely on. It builds in four phases: alcohol classification and the three principal reaction types (oxidation, dehydration, substitution); haloalkane nucleophilic substitution and its mechanism distinctions; CFCs and ozone chemistry as the environmental case study; and a final phase on practical synthesis techniques and the two analytical methods that pair with them. It sits adjacent to Basic Organic and Hydrocarbons and feeds directly into Carbonyls, Polymers and Spectroscopy on the OCR A-Level Chemistry learning path.
Guide Overview
The Alcohols, Haloalkanes and Analysis course is built as a sequence of lessons that move from alcohol chemistry through haloalkane chemistry and CFCs into practical and analytical techniques.
- Alcohol Classification and Physical Properties
- Oxidation of Alcohols
- Dehydration of Alcohols
- Substitution Reactions of Alcohols to Haloalkanes
- Haloalkane Nucleophilic Substitution
- Hydrolysis of Haloalkanes
- SN1 vs SN2 and Substitution vs Elimination
- CFCs and Ozone Depletion
- Organic Synthesis Practical Techniques
- Infrared Spectroscopy
- Mass Spectrometry Basics
OCR H432 Specification Coverage
This course addresses OCR H432 Module 4.2.1 (alcohols), Module 4.2.2 (haloalkanes), Module 4.2.3 (organic synthesis) and parts of Module 4.2.4 (analytical techniques). Each is mapped to one or more lessons (refer to the official OCR specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Primary/secondary/tertiary alcohols; physical properties | OCR H432 Module 4.2.1 | Alcohol Classification and Physical Properties |
| Oxidation of alcohols to aldehydes, ketones and carboxylic acids | OCR H432 Module 4.2.1 | Oxidation of Alcohols |
| Dehydration of alcohols to alkenes | OCR H432 Module 4.2.1 | Dehydration of Alcohols |
| Substitution to form haloalkanes | OCR H432 Module 4.2.1 | Substitution Reactions of Alcohols to Haloalkanes |
| Nucleophilic substitution of haloalkanes; hydrolysis | OCR H432 Module 4.2.2 | Haloalkane Nucleophilic Substitution; Hydrolysis of Haloalkanes |
| Reaction-pathway selection; elimination vs substitution | OCR H432 Module 4.2.2 | SN1 vs SN2 and Substitution vs Elimination |
| Atmospheric chemistry of CFCs and ozone | OCR H432 Module 4.2.2 | CFCs and Ozone Depletion |
| Practical synthesis techniques | OCR H432 Module 4.2.3 | Organic Synthesis Practical Techniques |
| Infrared spectroscopy of organic compounds | OCR H432 Module 4.2.4 | Infrared Spectroscopy |
| Mass spectrometry for molecular ion and fragmentation | OCR H432 Module 4.2.4 | Mass Spectrometry Basics |
Module 4.2 is heavily examined on Paper 2 (Synthesis and Analytical Techniques) and synoptically on Paper 3. Practical-skills questions on reflux, distillation and recrystallisation are routine, as are IR-and-MS combined structure-elucidation items.
Topic-by-Topic Walkthrough
Alcohols: Classification, Oxidation, Dehydration and Substitution
The alcohol classification lesson develops the primary/secondary/tertiary scheme according to how many alkyl groups are bonded to the carbon bearing the OH (one, two, three respectively). Methanol is special (no alkyl groups; sometimes classed as zero° but behaves like primary). Hydrogen bonding makes alcohols miscible with water at short chain lengths but increasingly hydrophobic as the chain grows. The oxidation lesson develops the use of acidified potassium dichromate K₂Cr₂O₇/H₂SO₄ — orange to green colour change as Cr(VI) is reduced to Cr(III). Primary alcohols give an aldehyde when distilled off as formed, or a carboxylic acid when refluxed; secondary alcohols give a ketone (no further oxidation under these conditions); tertiary alcohols do not oxidise (no H on the carbon bearing OH).
The dehydration lesson develops the elimination of water with hot concentrated H₂SO₄ or H₃PO₄ to give an alkene. The mechanism is E1 — protonation, loss of water, loss of H⁺ from an adjacent carbon — and Zaitsev's rule favours the more substituted alkene (more alkyl groups stabilise the developing C=C). The substitution lesson develops the H⁺-promoted conversion to a haloalkane: HCl/HBr/HI directly, NaBr + H₂SO₄ for bromoalkanes, NaI + H₃PO₄ for iodoalkanes (H₂SO₄ would oxidise iodide). The alcohol acts as a nucleophile via its lone pair, then becomes the leaving group as water after protonation.
Haloalkane Nucleophilic Substitution
The haloalkane nucleophilic substitution lesson develops the canonical reaction: a nucleophile (OH⁻, CN⁻, NH₃) attacks the δ+ carbon of a C-X bond, expelling the X⁻ leaving group. The mechanism is the textbook curly-arrow exercise — lone pair on Nu attacks C, C-X bond pair retreats to X. The hydrolysis lesson develops the rate trend: C-I (weakest, ~228 kJ mol⁻¹) hydrolyses fastest, C-Br moderately, C-Cl slowest, C-F essentially not at all — and this is the bond-strength explanation, not the bond-polarity explanation (the polarity argument predicts the opposite trend). The standard experimental demonstration uses ethanolic AgNO₃ to give the silver halide precipitate (white AgCl, cream AgBr, yellow AgI) as the rate readout. The reaction with KCN/ethanol extends the carbon chain by one (haloalkane → nitrile, hydrolysable to carboxylic acid); the reaction with NH₃ in ethanol under pressure gives a primary amine.
The SN1 vs SN2 and substitution vs elimination lesson develops the two mechanistic options. SN2 is concerted (one-step, bimolecular rate-determining step, backside attack with inversion) and is favoured for primary haloalkanes with strong nucleophiles in polar aprotic solvents. SN1 is stepwise (carbocation intermediate, unimolecular rate-determining step, racemisation) and is favoured for tertiary haloalkanes with weak nucleophiles in polar protic solvents. Elimination (E2) competes with SN2 when the nucleophile is bulky or strongly basic; the warm aqueous OH⁻ substitution gives an alcohol whereas hot ethanolic OH⁻ favours elimination to the alkene. The substrate-and-conditions matrix is a Paper 2 mark-bearing decision.
CFCs and Ozone Depletion
The CFCs and ozone lesson develops the atmospheric-chemistry case study. CFCs (chlorofluorocarbons such as CCl₃F and CCl₂F₂) were used as refrigerants, aerosol propellants and blowing agents because of their inertness and low toxicity at ground level. In the stratosphere, however, short-wavelength UV homolytically cleaves the C-Cl bond, releasing Cl• radicals that catalytically destroy ozone: Cl• + O₃ → ClO• + O₂, then ClO• + O₃ → Cl• + 2O₂. A single Cl• can destroy thousands of O₃ molecules before being scavenged. The Montreal Protocol (1987) phased out CFCs, replaced by HFCs (no Cl, lower ozone-depleting potential) and HCFCs (transitional). The mechanism is reusable as a model for any radical chain in atmospheric chemistry.
Practical Techniques and Analytical Spectroscopy
The practical techniques lesson develops reflux (boiling with vertical condenser to keep volatile reactants in the flask), distillation (separation by boiling point, used to collect a product as soon as it forms or to purify it after the reaction), liquid-liquid separation in a separating funnel (washing the organic layer with aqueous solutions to remove acid or base), drying with an anhydrous salt (Na₂SO₄ or MgSO₄), and recrystallisation for purifying a solid (dissolve hot in minimum solvent, filter hot, cool, filter and dry). Each technique is anchored in a PAG and is the basis of reliable practical-skills mark-bearing items.
The infrared spectroscopy lesson develops IR as a vibrational technique that identifies functional groups from characteristic absorption bands. The H432 spec set is: O-H broad (alcohol 3200-3550, carboxylic acid broader 2500-3300), N-H (3300-3500), C-H (2850-3300), C=O (1680-1750, sharp, intense), C=C (1620-1680), and the fingerprint region below 1500 used for matching to library spectra. The mass spectrometry basics lesson develops the molecular ion M⁺ at the right-most peak (giving the molar mass) and fragmentation peaks at lower m/z that reveal substructure. Loss of 15 = CH₃; loss of 17 = OH; loss of 28 = CO or C₂H₄; loss of 29 = CHO; loss of 43 = COCH₃. Combined IR and MS data routinely give an unambiguous structure even when only a molecular formula is supplied.
A Typical H432 Paper 2 Question
A standard Paper 2 prompt gives candidates a starting alcohol and asks them to design a two- or three-step synthesis to a target compound (often a carboxylic acid, a ketone, an alkene or a nitrile-derived acid), then verify the final product against supplied IR and MS data. The route is fixed: identify the carbon-count change (does the target have the same number of carbons as the starting alcohol, or one more?), choose the functional-group transformation for each step (oxidation, dehydration, substitution, hydrolysis, KCN to add a carbon), specify reagents and conditions for each step, then walk through the supplied IR spectrum looking for a broad O-H or sharp C=O around 1700 cm⁻¹ and the supplied MS for the molecular ion at the right m/z. The discriminator at the top band is the explicit naming of reagents (acidified K₂Cr₂O₇, not "an oxidising agent"), the explicit identification of distillation versus reflux for aldehyde versus carboxylic acid endpoints, and the explicit use of the molecular ion m/z plus loss patterns (loss of 17 implies OH; loss of 45 implies COOH) to verify rather than merely propose the product.
Synoptic Links
Alcohols and haloalkanes are the synthetic backbone that connects the rest of organic chemistry. The oxidation framework returns in carbonyls, polymers and spectroscopy where the aldehydes and ketones produced here are characterised by Tollens', Fehling's and 2,4-DNP tests, and where the carboxylic acids enter into ester and acyl chloride chemistry. The nucleophilic-substitution machinery generalises into the carbonyl nucleophilic addition framework of the same course, and into the electrophilic-aromatic-substitution machinery of transition elements and aromatic. The IR and MS toolkit pairs with the ¹³C and ¹H NMR techniques of carbonyls, polymers and spectroscopy to give a four-instrument analytical workflow.
Paper 3 'Unified chemistry' items deploy this module in two characteristic ways. The first is the multi-step environmental or industrial synthesis: a question may give a feedstock alcohol such as ethanol and ask candidates to design a synthesis of a target pharmaceutical or polymer precursor over four or five steps, naming reagents and predicting practical issues at each stage. The second is the analytical fingerprint question: candidates are given a structurally unknown organic compound together with combustion data, an IR spectrum and an MS spectrum, and asked to propose a structure. The discriminating moves at the top band are explicit use of the percentage-composition arithmetic from atoms, moles and equations to get the empirical formula, then the MS molecular ion to scale to a molecular formula, then the IR functional-group identification to narrow the candidate structures, then a final fragmentation-pattern check to confirm. This four-step elucidation workflow is the synoptic skill that examiners reward.
What Examiners Reward
Top-band marks on this module cluster around precision of reagent and condition specification, and around the explicit linkage between mechanism and observation. For oxidation questions, examiners want the reagent named in full (acidified potassium dichromate, often with the K₂Cr₂O₇/H₂SO₄ formulae) plus the explicit colour change orange-to-green and the explicit distillation-vs-reflux distinction for aldehyde-vs-carboxylic-acid outcomes. For nucleophilic substitution questions, examiners want curly arrows from the lone pair on the nucleophile to the δ+ carbon, from the C-X bond to the halide; and the explicit identification of SN1 versus SN2 by substrate class (primary → SN2, tertiary → SN1, secondary → mixed). For elimination versus substitution decisions, they want the explicit citation of conditions (aqueous-warm-dilute favours substitution; ethanolic-hot-concentrated favours elimination) and the explicit identification of Zaitsev product as the more-substituted alkene. For IR and MS interpretation, they want the wavenumber range cited (not just "OH band"), the band shape (broad versus sharp), and the explicit derivation of fragments from named bond cleavages.
Common pitfalls cluster around six recurring mistakes. First, drawing the curly arrow from H rather than from a lone pair or bond — every mechanism arrow originates at electron density, not at an atom. Second, writing the oxidation of a primary alcohol all the way to a carboxylic acid under distillation conditions when distillation removes the aldehyde before further oxidation can occur. Third, predicting hydrolysis order for haloalkanes by C-X polarity (which gives the wrong answer); the correct argument is bond strength (C-I weakest, fastest). Fourth, omitting the chain-propagation steps in the CFC-ozone radical mechanism and accounting only for initiation. Fifth, identifying a broad IR absorption at ~3000 cm⁻¹ as both O-H and C-H without recognising that the carboxylic acid O-H is broad enough to subsume the C-H envelope. Sixth, misreading the M⁺ peak as M+1 (which exists at 1.1% per carbon due to ¹³C isotope) and underestimating the molecular mass by one.
Practical Activity Groups (PAGs)
This course anchors PAG 6 (Synthesis of an organic liquid) through the reflux-distillation procedure used to prepare a haloalkane from an alcohol or an aldehyde/carboxylic acid from a primary alcohol, PAG 7 (Qualitative analysis of organic functional groups) through the AgNO₃ hydrolysis test for haloalkanes and the oxidation observations for alcohols, and PAG 10 (Research skills) elements through IR-and-MS spectral interpretation. The reflux-and-distillation procedure also threads forward as the foundation of every subsequent organic preparation in the spec.
Going Further
Undergraduate analogues of this material extend in several directions. First, the SN1/SN2 mechanism continuum becomes the full physical-organic-chemistry picture with linear free-energy relationships, kinetic isotope effects and stereochemical analysis of inversion versus racemisation. Second, the carbonyl oxidation chemistry generalises into the modern toolbox of chemoselective oxidants (TEMPO, Swern, Dess-Martin) that bypass the chromium reagents on the spec. Third, the IR and MS techniques generalise into 2D NMR (COSY, HSQC, HMBC) and tandem MS — both standard in any undergraduate or research lab. Oxbridge-style interview prompts on this material include: "Why does a primary alcohol oxidise to an aldehyde under distillation but to a carboxylic acid under reflux?" "Predict whether the elimination or substitution pathway dominates when 2-bromopropane is heated with ethanolic KOH versus aqueous KOH." "Suggest two reasons why HFCs are a better replacement for CFCs than HCs even though HCs contain no chlorine."
Worked Examples in Depth
Organic questions at this stage reward precise reagents-and-conditions recall and disciplined mechanism drawing. The worked examples below rehearse the three highest-value skills: designing a multi-step synthesis, choosing between substitution mechanisms, and confirming a structure from spectra.
Example 1 — a two-step synthesis with a carbon-count increase. Convert 1-bromopropane (C3H7Br) into butanoic acid (C3H7COOH, four carbons).
The target has one more carbon than the starting material, so a chain-extension step is essential — and only one reagent on the spec adds a carbon: cyanide.
- Step 1 (chain extension): 1-bromopropane + KCN in ethanol, heated under reflux → butanenitrile (C3H7CN) + KBr. This is nucleophilic substitution; the CN− nucleophile attacks the δ+ carbon.
- Step 2 (hydrolysis): butanenitrile + dilute H2SO4 (aqueous), heated under reflux → butanoic acid + ammonium salt.
The top-band moves are naming both the reagent and the condition (reflux, ethanolic for step 1), and spotting at the outset that the carbon count forces the KCN route rather than a simpler oxidation.
Example 2 — the substitution-mechanism decision. Predict the mechanism when (a) 1-bromobutane and (b) 2-bromo-2-methylpropane each react with aqueous hydroxide.
- (a) 1-bromobutane is a primary haloalkane. Primary substrates react by SN2: a single concerted step, rate depending on both the haloalkane and OH−, backside attack giving inversion of configuration. There is no stable primary carbocation, so the stepwise route is unavailable.
- (b) 2-bromo-2-methylpropane is a tertiary haloalkane. Tertiary substrates react by SN1: rate-determining loss of the halide to form a relatively stable tertiary carbocation, followed by rapid attack of OH−. The rate depends only on the haloalkane concentration.
The discriminator is linking mechanism to substrate class and justifying it by carbocation stability — a bare "SN1/SN2" label without the reason scores less.
Example 3 — structure elucidation from combined data. An unknown compound has molecular formula C3H6O. Its infrared spectrum shows a strong, sharp absorption at about 1715 cm−1 and no broad band around 3200–3550 cm−1. Its mass spectrum shows the molecular ion at m/z=58 and a strong fragment at m/z=43. Deduce the structure.
Work through the data in order. The IR band at ∼1715 cm−1 is the C=O stretch, and the absence of a broad O–H band rules out both alcohols and carboxylic acids — so this is a carbonyl compound (aldehyde or ketone). The molecular ion at 58 matches C3H6O (36+6+16=58). The fragment at 43 corresponds to a loss of 15 (a CH3 group), leaving CH3CO+ (the acylium ion, mass 43). Loss of a methyl to give an acylium ion is characteristic of a methyl ketone, so the compound is propanone, CH3COCH3. The chain of reasoning — carbonyl from IR, formula from M+, then the 58→43 loss of 15 — is exactly the four-step elucidation workflow examiners reward.
Exam Technique for Organic Questions
Organic marks are won on precision and mechanism discipline. Four habits pay off across every Paper 2 question:
- Name reagents in full, with conditions. "An oxidising agent" scores nothing; "acidified potassium dichromate (K2Cr2O7/H2SO4), heated under reflux" scores. Distillation-versus-reflux is itself a mark for the aldehyde-versus-carboxylic-acid endpoint.
- Draw curly arrows from electron density, never from an atom. Every arrow starts at a lone pair or a bond and ends where the new bond forms (or on the atom taking the electrons). An arrow drawn from an H atom instead of a lone pair loses the mechanism mark automatically.
- Quote IR bands with a wavenumber range and a shape. "O–H, broad, 3200–3550 cm⁻¹" earns more than "OH band". For carboxylic acids, note the O–H is broad enough to overlap the C–H region.
- State the colour change for observation questions. Orange-to-green for dichromate oxidation; the silver-halide precipitate colours (white AgCl, cream AgBr, yellow AgI) for the haloalkane hydrolysis test.
For multi-step synthesis, always begin by comparing the carbon count of start and target — that single check tells you immediately whether a chain-extension (KCN) step is required.
Mini-FAQ
Why does the hydrolysis rate follow C–I fastest and C–F slowest? The controlling factor is bond enthalpy, not bond polarity. The C–I bond is the weakest and breaks most readily, so iodoalkanes hydrolyse fastest; the C–F bond is the strongest and barely reacts. (Bond polarity would predict the opposite order, which is why quoting the polarity argument loses the mark.)
When does elimination beat substitution? Conditions decide it for a given substrate: warm, dilute, aqueous hydroxide favours substitution to the alcohol; hot, concentrated, ethanolic hydroxide favours elimination to the alkene. A bulky or strongly basic nucleophile also pushes towards elimination.
How can one Cl∙ radical destroy thousands of ozone molecules? The chlorine radical is regenerated in the second propagation step (ClO∙+O3→Cl∙+2O2), so it acts catalytically, cycling many times before a termination step removes it. Accounting only for the initiation step is a common mark-loss.
Why can the M⁺ peak be misread by one mass unit? Natural carbon contains about 1.1% carbon-13 per carbon atom, producing a small M+1 peak just to the right of the true molecular ion. Mistaking M+1 for M⁺ inflates the molar mass by one; conversely, treating the tallest peak as M⁺ (when a fragment is more abundant) underestimates it.
Authorship and Sign-off
This guide was authored independently by John Haigh, paraphrasing OCR H432 Modules 4.2.1, 4.2.2, 4.2.3 and 4.2.4 as descriptive use. No verbatim spec text, mark-scheme phrasing, examiner-report quotation, or past-paper question reference appears. The worked examples are original.
Start at the Alcohols, Haloalkanes and Analysis course and work through every lesson in sequence. Once the alcohol and haloalkane reaction sets, the SN1/SN2 decision matrix, and the IR/MS spectral toolkit are automatic, every later organic synthesis becomes a chain of these elementary steps — and the analytical interpretation items resolve into pattern recognition rather than guesswork.