OCR A-Level Chemistry: Basic Organic and Hydrocarbons — Complete Revision Guide (H432)
OCR A-Level Chemistry: Basic Organic and Hydrocarbons
Basic organic chemistry is the gateway to half of the A-Level Chemistry specification. Once you can balance equations, predict shapes from VSEPR and identify electron-rich and electron-poor sites from electronegativity, organic chemistry becomes the systematic study of how carbon-based functional groups react. OCR A-Level Chemistry A (H432) builds this systematic view in two stages — a foundational treatment of alkanes, alkenes, isomerism and mechanism vocabulary at AS, and a sweep through alcohols, haloalkanes, carbonyls, aromatics and polymers at A2. This course is the AS foundation that the rest of the organic spec assumes.
H432 examiners weight Module 4.1 heavily because the curly-arrow vocabulary established here is reused in every later organic mechanism: nucleophilic substitution at saturated carbon, nucleophilic addition to carbonyls, electrophilic substitution of arenes, condensation polymerisation, and the redox chemistry of alcohols. Candidates who emerge from Module 4.1 able to draw a correct curly arrow from a lone pair or a π bond into a δ+ carbon — and able to articulate which species is the electrophile and which is the nucleophile — find every later mechanism question collapses into the same diagrammatic exercise with new substrate identities. Candidates who arrive at Module 4.2 still unsure about the convention struggle on every subsequent mechanism item. The cost of poor fluency here is therefore not local to Module 4.1 but compounds across the rest of the organic spec, which is why examiners revisit the mechanism diagrams in synoptic Paper 3 items as a check on Module 4.1 mastery.
Course 5 of the H432 Chemistry learning path on LearningBro, Basic Organic and Hydrocarbons, develops the introductory organic vocabulary the rest of the path will reuse. It opens with functional groups, nomenclature, formulae and isomerism, then introduces mechanism vocabulary (heterolytic and homolytic fission, electrophile and nucleophile, curly-arrow notation), and develops the chemistry of the two simplest hydrocarbon classes — alkanes (with their inertness and free-radical substitution by halogens) and alkenes (with their reactivity towards electrophiles and their use as addition polymer monomers). It sits adjacent to Acids, Redox, Electrons and Bonding and feeds directly into Alcohols, Haloalkanes and Analysis and onward into Carbonyls, Polymers and Spectroscopy on the OCR A-Level Chemistry learning path.
Guide Overview
The Basic Organic and Hydrocarbons course is built as a sequence of lessons that move from naming and structure through mechanism vocabulary into the chemistry of alkanes and alkenes.
- Functional Groups and Nomenclature
- Formulae and Homologous Series
- Structural Isomerism
- Introduction to Reaction Mechanisms
- Alkanes as Fuels and Combustion
- Free-Radical Substitution of Alkanes
- Alkenes: Structure and Bonding
- E/Z Stereoisomerism
- Electrophilic Addition Reactions of Alkenes
- Addition Polymerisation
OCR H432 Specification Coverage
This course addresses OCR H432 Module 4.1.1 (basic concepts), Module 4.1.2 (alkanes) and Module 4.1.3 (alkenes). The specification organises the topic into the structural vocabulary needed for every organic question, the chemistry of the saturated alkane family, and the chemistry of the unsaturated alkene family with its key C=C electrophilic addition reactions (refer to the official OCR specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| Functional groups, nomenclature, formulae | OCR H432 Module 4.1.1 | Functional Groups and Nomenclature; Formulae and Homologous Series |
| Structural and stereo isomerism | OCR H432 Module 4.1.1 | Structural Isomerism; E/Z Stereoisomerism |
| Mechanism vocabulary; bond fission; curly arrows | OCR H432 Module 4.1.1 | Introduction to Reaction Mechanisms |
| Alkanes; combustion; environmental impact | OCR H432 Module 4.1.2 | Alkanes as Fuels and Combustion |
| Free-radical substitution by halogens | OCR H432 Module 4.1.2 | Free-Radical Substitution of Alkanes |
| Alkene C=C structure and reactivity | OCR H432 Module 4.1.3 | Alkenes: Structure and Bonding |
| Electrophilic addition to alkenes; Markovnikov | OCR H432 Module 4.1.3 | Electrophilic Addition Reactions of Alkenes |
| Addition polymers and disposal | OCR H432 Module 4.1.3 | Addition Polymerisation |
Module 4.1 is heavily examined on Paper 2 (Synthesis and Analytical Techniques), and the mechanism diagrams (especially free-radical substitution and electrophilic addition with Markovnikov-determined product) are routine 4-6 mark items.
Topic-by-Topic Walkthrough
Functional Groups, Nomenclature and Homologous Series
The functional groups and nomenclature lesson covers the IUPAC naming workflow — find the longest carbon chain containing the principal functional group, number from the end nearest that group, prefix substituents in alphabetical order with locants — and the catalogue of functional groups the H432 spec uses: alkane (suffix -ane), alkene (-ene), alcohol (-ol), haloalkane (halo-), aldehyde (-al), ketone (-one), carboxylic acid (-oic acid), ester (-oate), amine (-ylamine or amino-), amide (-amide), nitrile (-nitrile), arene (phenyl- or benzene base). The formulae and homologous series lesson develops empirical, molecular, general (e.g. C_nH_(2n+2) for alkanes), structural, displayed and skeletal formulae and the homologous series concept — same general formula, same functional group, gradation of physical properties with chain length.
Structural Isomerism and Mechanism Vocabulary
The structural isomerism lesson develops the three types: chain isomerism (different carbon skeleton — n-pentane, 2-methylbutane, 2,2-dimethylpropane), positional isomerism (same skeleton, functional group at a different position — but-1-ene vs but-2-ene), and functional group isomerism (different functional group from the same molecular formula — propanal vs propanone, both C₃H₆O). The introduction to mechanisms lesson develops the curly-arrow convention (a double-headed arrow shows the movement of an electron pair, a single-headed arrow shows the movement of one electron in radical chemistry), homolytic fission (each atom takes one electron, generating free radicals) versus heterolytic fission (one atom takes both electrons, generating an ion pair), and the electrophile-nucleophile classification (electrophile = electron-pair acceptor, often δ+ or empty-orbital species; nucleophile = electron-pair donor, often a lone pair or π-bond).
Alkanes: Combustion and Free-Radical Substitution
The alkanes as fuels and combustion lesson develops complete combustion (CO₂ + H₂O), incomplete combustion (CO + H₂O, or C soot + H₂O in very oxygen-poor conditions) and the environmental consequences: CO₂ as a greenhouse gas, CO as a toxic respiratory poison, NOx from high-temperature N₂/O₂ reaction in engines, SO₂ from sulfur-containing fuels causing acid rain. Catalytic converters remove CO, NOx and unburnt hydrocarbons on the same Pt-Pd-Rh surface. The free-radical substitution lesson develops the three-stage mechanism for alkane-halogen reactions in UV light: initiation (UV homolytically splits Cl₂ → 2Cl•), propagation (Cl• + CH₄ → •CH₃ + HCl; then •CH₃ + Cl₂ → CH₃Cl + Cl•), and termination (any two radicals combine). The reaction's main exam-relevant feature is its poor selectivity — chloromethane, dichloromethane, trichloromethane and tetrachloromethane are all formed, and so are C₂H₆ and other coupling products, which is why free-radical halogenation has low atom economy and is not the textbook route to haloalkanes.
Alkenes: Structure, Stereoisomerism and Electrophilic Addition
The alkene structure and bonding lesson develops the C=C double bond as a sigma bond (overlap of sp² hybrid orbitals end-on) plus a pi bond (sideways overlap of unhybridised p orbitals). The pi bond is electron-rich and exposed, making it the site of attack by electrophiles. The sp² hybridisation flattens the C and its three substituents into a plane with 120° angles, which restricts rotation about the C=C bond — this is the basis of geometric (E/Z) isomerism. The E/Z stereoisomerism lesson develops the Cahn-Ingold-Prelog priority rules to assign E (priority groups opposite) or Z (priority groups same side). The cis/trans nomenclature is the simpler historical convention but breaks down when all four substituents differ.
The electrophilic addition lesson develops the canonical three reactions: with HBr (to give a haloalkane), with Br₂ (to give a 1,2-dibromoalkane — the orange-to-colourless test for the C=C), and with concentrated H₂SO₄ followed by water (to give an alcohol; the industrial route to ethanol from ethene). The mechanism for HBr addition to propene illustrates Markovnikov's rule: the electrophile H⁺ adds to the carbon bearing more hydrogens, producing the more stable secondary carbocation, which is then attacked by Br⁻. The result is 2-bromopropane as the major product, not 1-bromopropane. Carbocation stability follows tertiary > secondary > primary because alkyl substituents donate electron density inductively (and through hyperconjugation, beyond the spec).
Addition Polymerisation
The addition polymerisation lesson develops the conversion of an alkene monomer into a saturated polymer. Ethene → poly(ethene), propene → poly(propene), chloroethene → poly(chloroethene) (PVC), tetrafluoroethene → poly(tetrafluoroethene) (PTFE/Teflon). Repeating units are drawn with the C=C reduced to C-C and the substituents preserved. Disposal pathways covered are landfill (slow), incineration with energy recovery (releases CO₂; PVC also releases HCl), recycling (sortable thermoplastics) and biodegradable replacement (starch-based or PLA, which is condensation rather than addition).
A Typical H432 Paper 2 Question
A standard Paper 2 prompt gives candidates an unsymmetrical alkene (typically propene, but-1-ene or methylpropene) and an electrophile (HBr, HCl or H₂SO₄) and asks them to draw the full mechanism showing curly arrows, label the major product, and justify the regiochemistry using carbocation stability. The route is fixed: identify the π bond as the nucleophile and the Hδ+ as the electrophile; draw the curly arrow from the π bond to H, simultaneously cleaving the H-Br bond with the bond pair retreating onto Br; identify the two possible carbocations; assign tertiary/secondary/primary status to each and identify the more stable as the intermediate; draw the second arrow from Br⁻ to the cation carbon; name the major product. The discriminator at the top band is the explicit statement of Markovnikov's outcome in terms of carbocation stability rather than as a memorised rule, plus the explicit acknowledgement that the minor product (anti-Markovnikov, from the less stable carbocation) is still formed in small amount.
Synoptic Links
Basic organic chemistry threads forward into every subsequent organic module. The electrophilic addition framework returns in carbonyls, polymers and spectroscopy as nucleophilic addition to the C=O, with the same curly-arrow style applied to a polarised double bond. The free-radical mechanism returns in alcohols and haloalkanes as the explanation for the CFC-driven destruction of stratospheric ozone. The Markovnikov-determined regioselectivity returns in the Friedel-Crafts and other electrophilic aromatic substitution reactions of transition elements and aromatic. And the addition polymer framework is the comparator for the condensation polymers (polyesters, polyamides, biological polymers) developed in carbonyls, polymers and spectroscopy.
Paper 3 'Unified chemistry' items deploy this module in two characteristic ways. The first is structure-mechanism integration: candidates are given an unfamiliar organic substrate with multiple functional groups and asked to predict the site of reaction with a named reagent. The route is to identify each functional group's electronic character (π-rich, polar, lone-pair-bearing) and to compare relative reactivity, drawing on the mechanism vocabulary built in this module. The second is industrial-route evaluation: candidates are given two proposed syntheses of the same target product, one starting from an alkene and one starting from an alkane, and asked to evaluate them on atom economy, energy demand and waste-product hazard. The discriminating moves at the top band are explicit atom-economy comparisons and explicit identification of free-radical halogenation's poor selectivity as a reason to reject the alkane-based route.
What Examiners Reward
Top-band marks on this module cluster around the precision of curly-arrow placement and the explicit articulation of mechanism vocabulary. For electrophilic addition mechanisms, examiners want the first arrow drawn from the π bond (not from a single carbon), the second arrow drawn from the bond pair to the leaving group atom (not into the bond), and the carbocation intermediate explicitly shown with its formal positive charge. For free-radical substitution mechanisms, they want the explicit labelling of initiation, propagation and termination, the explicit single-headed arrows of homolytic cleavage, and a sample termination step (any two radicals combining). For nomenclature questions, they want the parent chain identified by length and principal functional group, then substituents in alphabetical order with correct locants. For isomerism questions, they want the explicit identification of the type (chain, positional, functional, E/Z) and a sketched example.
Common pitfalls cluster around six recurring mistakes. First, drawing the curly arrow from the H rather than from the H-Br bond pair when an electrophile cleaves heterolytically — the arrow always leaves from electron density. Second, drawing the carbocation intermediate without its positive charge, which costs the carbocation-stability mark. Third, predicting the Markovnikov product correctly but justifying it as "the more substituted carbon attracts the bromine" rather than as carbocation stability. Fourth, omitting the propagation steps in free-radical substitution and presenting only initiation and termination, which misses the chain-reaction explanation. Fifth, assigning E/Z incorrectly because of confusion between Cahn-Ingold-Prelog priority (based on atomic number, with mass as tiebreaker) and the older cis/trans convention. Sixth, writing the addition polymer repeating unit with the C=C still present, rather than redrawing the single C-C with brackets and a subscript n. Each of these is a one- or two-mark deduction that compounds quickly across a multi-part mechanism question.
Practical Activity Groups (PAGs)
This course anchors elements of PAG 6 (Synthesis of an organic liquid) and PAG 7 (Qualitative analysis of organic functional groups) through the bromine-water test for unsaturation (decolourisation in the presence of a C=C double bond) and the combustion observation tests. The alkene electrophilic-addition framework also previews the synthesis of haloalkanes (covered as PAG anchors in alcohols and haloalkanes). Mechanism diagrams in particular are not lab-practical but are mark-bearing on most organic written items. The bromine-water test is also the canonical worked example for the distinction between qualitative (decolourisation observed) and quantitative (rate of decolourisation measured) observational chemistry, and is the conceptual bridge to the more elaborate test for unsaturation by hydrogen-iodine value used in food chemistry. Candidates who can articulate why the addition of bromine to ethene gives 1,2-dibromoethane rather than a substitution product gain the conceptual lever needed for every subsequent electrophilic-addition mark-bearing item.
Going Further
Undergraduate analogues of this material extend in several directions. First, molecular orbital theory replaces the sigma-plus-pi description with bonding and antibonding MOs and explains conjugation in 1,3-butadiene and benzene more rigorously. Second, mechanism theory becomes quantitative through Hammond postulates (transition state resembles the closest stable intermediate), linear free-energy relationships and kinetic isotope effects. Third, polymerisation chemistry generalises into anionic, cationic and Ziegler-Natta coordination polymerisation, giving control over stereochemistry that radical addition cannot deliver. Oxbridge-style interview prompts on this material include: "Why does HBr add to propene to give 2-bromopropane rather than 1-bromopropane?" "Explain why the bromination of methane gives a mixture of products rather than a single chloromethane analogue." "If you replaced one hydrogen in ethene with a CN group, would you expect electrophilic addition of HBr to be faster or slower than for ethene itself?"
Authorship and Sign-off
This guide was authored independently by John Haigh, paraphrasing OCR H432 Modules 4.1.1, 4.1.2 and 4.1.3 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 Basic Organic and Hydrocarbons course and work through every lesson in sequence. Once nomenclature, isomerism, mechanism vocabulary and the alkane/alkene reaction set are automatic, every later organic module becomes a curly-arrow extension of the same logic — and the mechanism-drawing items resolve into a stepwise routine rather than guesswork.