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Spec Mapping — OCR H432 Module 4.1.1 — Basic concepts of organic chemistry, covering naming and representation of organic compounds, classification by functional group, the homologous-series concept, principal-group priority in IUPAC naming, and the relationship between general formula, structural formula, displayed formula and skeletal formula (refer to the official OCR H432 specification document for exact wording).
Organic chemistry is the chemistry of compounds whose framework is built from carbon. With its four valence electrons and modest electronegativity, a single carbon atom can form four strong covalent bonds — to other carbons, to hydrogen, and to a small handful of "heteroatoms" (oxygen, nitrogen, the halogens, sulfur, phosphorus). That single architectural fact gives us catenation (long chains), rings, branches, and a near-limitless library of stable compounds. To navigate that library, chemists do two things at once. They group molecules into homologous series that share a functional group — the small reactive substructure that controls chemistry — and they name each molecule using the International Union of Pure and Applied Chemistry (IUPAC) system, in which the name itself encodes the chain length, the principal group, and the location of every substituent. Get the framework right here and the rest of A-Level organic chemistry — reactions, mechanisms, synthesis routes, NMR interpretation — is just adding pages to a catalogue you already know how to index.
Key Definition: a functional group is an atom or group of atoms within a molecule that is responsible for that molecule's characteristic chemical reactions; molecules sharing a functional group show similar reactivity. A homologous series is a family of compounds sharing a single functional group whose successive members differ by an additional –CH₂– unit and which share a single general formula, gradually changing physical properties, and identical chemical reactions.
Before 1892, organic compounds went by trivial names — wood spirit, ethyl alcohol, glacial acetic acid, glycerine, urea. As tens of thousands of new compounds were synthesised every decade, that naming chaos became untenable: a single substance might wear five different names in five different journals. The Geneva Congress (1892) and its successor body, IUPAC (founded 1919), set out to design a fully systematic nomenclature: a name from which a competent chemist could redraw the structure, unaided, with no lookup table. The cost is verbosity (2-methylpropan-2-ol instead of tert-butyl alcohol), but the gain is universality. OCR A-Level Chemistry uses IUPAC 2013-recommendation names exclusively in mark schemes; a chemically correct but non-IUPAC name (e.g. "isopropanol" for propan-2-ol) loses the naming mark.
| Homologous series | Functional group | Prefix / Suffix | Example | Molecular formula |
|---|---|---|---|---|
| Alkane | C–C single bonds only | (no prefix) / -ane | Ethane | C₂H₆ |
| Alkene | C=C | (alkenyl-) / -ene | Propene | C₃H₆ |
| Haloalkane | C–X (X = F, Cl, Br, I) | fluoro- / chloro- / bromo- / iodo- | 2-chloropropane | C₃H₇Cl |
| Alcohol | –OH | hydroxy- / -ol | Propan-2-ol | C₃H₇OH |
| Aldehyde | –CHO (terminal) | oxo- / -al | Propanal | C₂H₅CHO |
| Ketone | C=O (interior) | oxo- / -one | Propan-2-one | CH₃COCH₃ |
| Carboxylic acid | –COOH (terminal) | carboxy- / -oic acid | Ethanoic acid | CH₃COOH |
| Ester | –COO– | -yl -oate | Methyl ethanoate | CH₃COOCH₃ |
| Acyl chloride | –COCl | (chlorocarbonyl-) / -oyl chloride | Ethanoyl chloride | CH₃COCl |
| Amine | –NH₂ (primary) | amino- / -amine | Methylamine | CH₃NH₂ |
| Nitrile | –C≡N (terminal) | cyano- / -nitrile | Propanenitrile | C₂H₅CN |
| Amide | –CONH₂ | carbamoyl- / -amide | Ethanamide | CH₃CONH₂ |
The "principal" group on a molecule that carries several functional groups is decided by a priority table (see further down). Whichever group sits at the top of the table chooses the suffix; every other group is demoted to a prefix.
graph TD
A[Organic compound] --> B{Contains C=C?}
B -->|Yes| C[Alkene -ene]
B -->|No| D{Contains C=O?}
D -->|No, only C-H and C-C| E[Alkane -ane]
D -->|Yes| F{What is C=O bonded to?}
F -->|-OH| G[Carboxylic acid -oic acid]
F -->|-OR| J[Ester -yl -oate]
F -->|-Cl| K[Acyl chloride -oyl chloride]
F -->|-NH2| L[Amide -amide]
F -->|-H terminal| H[Aldehyde -al]
F -->|-C interior| I[Ketone -one]
A --> M{Contains -OH alone?}
M -->|Yes| N[Alcohol -ol]
A --> O{Contains halogen?}
O -->|Yes| P[Haloalkane halo- prefix]
A --> Q{Contains N?}
Q -->|NH2 only| R[Amine -amine]
Q -->|C triple bond N| S[Nitrile -nitrile]
OCR examiners draw the same molecule four different ways and expect candidates to recognise every form. You will be asked to convert between them.
C4H10O. No structure encoded.C4H10O already (no common factor), but for ethane C2H6 it is CH3.CH3CH2CH2CH2OH.The skeletal form scales — at A-Level it is the preferred representation above C4. Mark schemes accept any of the four forms unless a specific representation is named in the question.
| Stem | Carbons | Stem | Carbons |
|---|---|---|---|
| meth- | 1 | hex- | 6 |
| eth- | 2 | hept- | 7 |
| prop- | 3 | oct- | 8 |
| but- | 4 | non- | 9 |
| pent- | 5 | dec- | 10 |
The final 'e' of the stem is dropped before a suffix beginning with a vowel: propane → propan-2-ol, not "propane-2-ol".
The branch could be on C2 (numbering left-to-right) or C4 (right-to-left). The lowest-locant rule picks C2. Five-carbon chain → stem pentane; no principal functional group → suffix -ane; methyl substituent at C2. Name: 2-methylpentane.
Butane skeleton with a methyl on each of C2 and C3. Two identical substituents on adjacent carbons → prefix 2,3-dimethyl. Use di- for two of the same group. The locant set {2,3} is the same in either numbering direction, so the lowest-set rule is satisfied trivially.
Pentane skeleton; Cl on C2; CH₃ on C3. Alphabetise the prefixes (chloro before methyl). Locant set from the Cl-end: {2,3}; from the other end: {3,4}. Choose {2,3}. Name: 2-chloro-3-methylpentane.
Four-carbon chain with –OH on the second carbon. Principal group is the alcohol → suffix -ol → drop final 'e' of "butane" → butan-, add locant + suffix → butan-2-ol. The –OH locant is written inside the name (modern 2013 IUPAC), not at the front.
Four-carbon chain with –COOH at C1 and –OH at C3. The –COOH outranks the –OH in the priority table, so:
Test yourself by drawing 4-aminopentanoic acid (a γ-amino acid) — same principle.
This molecule has two functional groups: an alkene (C=C between C2 and C3) and an alcohol (–OH on C1). The principal-group priority table places alcohol (rank 9) above alkene (rank 11), so the alcohol drives the suffix. The chain is numbered to give –OH the lowest locant. With both groups in the chain, the name carries two locants: pent-2-en-1-ol. Note the two suffixes are concatenated: -en- (for the C=C) and -ol (for the alcohol). The final 'e' of "pentene" is dropped before the -ol vowel.
Five-carbon chain, principal group is the ketone (–C(=O)– at an interior C). Number to give the ketone the lowest locant: C2 from the methyl end. Substituents: bromo on C4, methyl on C3. Alphabetise (bromo before methyl). Locant set {2,3,4} is lower than {2,3,4} from the other end (a tie at first comparison, but the principal-group locant rule already settled it). Name: 4-bromo-3-methylpentan-2-one.
This is an ester. The parent acid is 2-methylpropanoic acid (the acyl part, written as the suffix "-oate"). The alkyl group from the alcohol that formed the ester (the "alcohol residue") is the prefix word, written separately: "methyl". So the name has two words: methyl 2-methylpropanoate. Note OCR mark schemes accept this space-separated two-word form and reject "methyl-2-methylpropanoate" with an inserted hyphen.
| Rank | Functional group | When principal, suffix | When demoted, prefix |
|---|---|---|---|
| 1 (highest) | Carboxylic acid –COOH | -oic acid | carboxy- |
| 2 | Acid anhydride | -oic anhydride | – |
| 3 | Ester –COOR | -yl -oate | (alkoxycarbonyl-) |
| 4 | Acyl chloride –COCl | -oyl chloride | chlorocarbonyl- |
| 5 | Amide –CONH₂ | -amide | carbamoyl- |
| 6 | Nitrile –CN | -nitrile | cyano- |
| 7 | Aldehyde –CHO | -al | oxo- |
| 8 | Ketone –C(=O)– | -one | oxo- |
| 9 | Alcohol –OH | -ol | hydroxy- |
| 10 | Amine –NH₂ | -amine | amino- |
| 11 | Alkene C=C | -ene | (encoded in stem) |
| 12 | Haloalkane | (no suffix) | halo- |
When in doubt, the principle is: higher-oxidation-state carbonyl groups outrank lower ones. –COOH (acid) outranks –CHO (aldehyde) outranks –C(=O)– (ketone) outranks –OH (alcohol). The reasoning: carboxylic acids carry the carbon at oxidation state +3, aldehydes at +1, ketones at +2 (interior), and alcohols at −1 to −3 depending on substitution.
The priority table is the single most-marked piece of knowledge in OCR's nomenclature section. It is examined directly (give the IUPAC name of compound X) and indirectly (every named product in every synthesis question). A student who has the table internalised gets the suffix right by reflex; a student who does not loses marks in chains across the paper.
The degree of unsaturation (also called index of hydrogen deficiency, IHD) is a quick check on a molecular formula. It counts the total number of rings plus pi-bonds in the molecule:
IHD=22C+2+N−H−X
where C, N, H, X are the atom counts of carbon, nitrogen, hydrogen, and halogen respectively (oxygen does not appear because oxygen is divalent and contributes nothing to the count). Each unit of IHD accounts for one ring or one pi-bond. So:
When OCR feeds you a molecular formula in an unseen-compound question, calculating the IHD first narrows the structural possibilities dramatically. A C₄H₈O₂ acid with IHD = 1 must spend that unit on the C=O of –COOH, leaving no room for further unsaturation — so the carbon skeleton must be fully saturated.
OCR Paper 3 ("Unified Chemistry", 70 marks) loves to surface nomenclature outside its own home topic. A question asking for a Born–Haber cycle of an organic salt forces you to name an anion; a kinetics question with a rate-determining step quoted with IUPAC syntax forces you to assign a numbered position. Connecting threads:
Connects to:
ocr-alevel-chemistry-alcohols-haloalkanes(every named alcohol, haloalkane, aldehyde, ketone, carboxylic acid that course produces relies on this lesson; nomenclature is a marking gate, not optional).ocr-alevel-chemistry-carbonyls-polymers-spectroscopy(NMR multiplicity arguments need an exact IUPAC name to count equivalent protons; mis-naming a 2-bromobutan-2-ol as 2-bromo-2-butanol invalidates the synoptic deduction).ocr-alevel-chemistry-transition-aromatic(substituted benzene nomenclature — chloro-, nitro-, hydroxy-benzene — uses identical principal-group priority logic).
The synoptic skill examined here is chemistry.organic.iupac-nomenclature (already in the platform's 153-skill ontology). Paper 3 has the right to retest this in any context: do not assume "I'll lose the naming mark, never mind" — three nomenclature errors stack and tank a 9-marker.
PAG anchor: PAG 5 (synthesis of an organic liquid). When recording observations on aspirin or 2-bromobutane synthesis you must label every flask with the IUPAC name; a hand-written "iso-butanol" loses the labelling mark.
Question (9 marks): Compound X has the molecular formula C₅H₁₀O₂. A research student characterises X and finds: (i) it gives an effervescent reaction with solid Na₂CO₃ releasing CO₂; (ii) infrared spectroscopy shows a broad absorption between 2500–3300 cm⁻¹ and a sharp absorption at 1715 cm⁻¹; (iii) the molecule contains a single methyl branch on a four-carbon chain.
Deduce the displayed formula of X, give its IUPAC name, and explain how each piece of evidence supports your assignment. [9]
| AO | Marks | What is assessed |
|---|---|---|
| AO1 | 2 | Recall: IR signatures of O–H (broad, 2500–3300) and C=O (1715); meaning of the carbonate reactivity test |
| AO2 | 5 | Application: deduce structure from data; assemble correct IUPAC name with locants; draw displayed formula |
| AO3 | 2 | Evaluation: explicitly link each piece of evidence to the structural feature it supports |
Mid-band response (5/9):
"The 1715 cm⁻¹ peak is C=O and the broad peak at 2500–3300 is O–H, so the compound has a carboxylic acid group. The Na₂CO₃ test confirms an acid because acids react with carbonates to give CO₂. With a methyl branch on a four-carbon chain and C₅H₁₀O₂, I draw (CH₃)₂CHCH₂COOH and call it 3-methylbutanoic acid."
Examiner commentary: M1 IR C=O assignment; M1 IR O–H (broad) assignment; M1 carbonate test linked to carboxylic acid; M1 displayed formula partially drawn (only condensed shown — loses one mark for not using a true displayed representation with every bond); M1 IUPAC name correct. The candidate skips the explicit number-from-the-COOH-end justification and supplies the structural rather than displayed formula, so caps at the C band.
Stronger response (7/9):
"The IR has a sharp peak at 1715 cm⁻¹ which is C=O stretch, and a broad envelope 2500–3300 cm⁻¹ which is the very broad O–H of a carboxylic acid (much wider than alcohol O–H around 3200–3550). Together these point to –COOH. The Na₂CO₃ effervescence (releasing CO₂) confirms an acid stronger than carbonic acid — only carboxylic acids satisfy that at A-Level. With C₅H₁₀O₂ and a single methyl branch on a four-carbon chain, the parent acid is butanoic acid (chain of 4) with the methyl on C3 (numbering from the COOH end, which is C1 by priority-table rule):
H₃C–CH(CH₃)–CH₂–COOH
Displayed formula shows all bonds; IUPAC name is 3-methylbutanoic acid."
Examiner commentary: All five AO1/AO2 marks; one of the AO3 link marks (IR comparison alcohol vs acid). Loses the second AO3 mark for not stating that the carbonate test rules out alcohols (which do not react with Na₂CO₃). Otherwise tight — clear, displayed formula explicit, locant justification given.
Top-band response (9/9):
"Three pieces of evidence converge on a single structure. (1) The 1715 cm⁻¹ IR absorption falls inside the C=O stretch envelope (typically 1680–1750 cm⁻¹ for acids; ketones are higher, esters ~1735 cm⁻¹, amides lower). (2) The very broad 2500–3300 cm⁻¹ envelope is wider and more diffuse than an alcohol O–H (3200–3550 cm⁻¹ and narrower), indicating the strongly hydrogen-bonded O–H of a carboxylic acid dimer. (3) Na₂CO₃ reacting to give CO₂ identifies the compound as more acidic than carbonic acid (pKa 6.4), which rules out alcohols (pKa ~16) and phenols (pKa ~10) but is consistent with carboxylic acids (pKa ~4–5).
The molecular formula C₅H₁₀O₂ has a degree of unsaturation = (2·5+2−10)/2 = 1, accounted for entirely by the C=O. A four-carbon main chain plus methyl branch on a five-carbon backbone gives:
(CH₃)₂CH–CH₂–COOH
Numbering: principal group (–COOH) gets locant 1 (highest priority). Methyl substituent sits on C3.
Displayed formula (every bond explicit):
IUPAC name: 3-methylbutanoic acid."
Examiner commentary: Full marks. Crucially demonstrates the AO3 evaluative move OCR rewards heavily: explicitly rules out alternative interpretations (alcohol, phenol) using pKa data, not just confirms the chosen answer. Degree-of-unsaturation calculation is volunteered (not strictly demanded by the question) and tightens the argument. Displayed formula is genuinely displayed — every C–H bond drawn.
Common ways candidates lose marks on nomenclature questions, written in teacher-voice:
Beyond A-Level, IUPAC publishes a 2013 "Blue Book" (the Recommendations on the Nomenclature of Organic Chemistry) running to over 1,500 pages. Universities introduce additional concepts you can preview now: the R/S system for chirality (an extension of CIP priority into three dimensions, distinguishing the two enantiomers of lactic acid or thalidomide, work developed by Cahn, Ingold and Prelog in the 1950s and 1960s); functional class nomenclature ("methyl acetate" as an acceptable alternative to "methyl ethanoate"); retained names (acetone, formic acid, urea — still permitted in many contexts despite the systematic alternatives); and multiplying prefixes for complex substituents (di-, tri-, tetra- for simple groups; bis-, tris-, tetrakis- for parenthesised complex groups, e.g., 1,3-bis(trifluoromethyl)benzene). A nice Oxbridge-style interview prompt: "How would you name a steroid hormone like testosterone systematically?" — the answer involves an IUPAC steroid nomenclature appendix that runs to 30 pages and is rarely taught at undergraduate level. Recommended reading: Clayden, Greeves & Warren Organic Chemistry (2nd ed.), Chapter 2; the IUPAC online resource at iupac.org for free-to-access naming flowcharts. The 2022 IUPAC update tightens rules on multiple-suffix compounds (e.g., diols, dienes, dialdehydes) and on the handling of "phane" nomenclature for cyclic molecules with internal bridges — useful preview material for first-year chemistry undergraduates. A persistent open research area is the algorithmic generation of unique IUPAC names from machine-readable structure files (SMILES, InChI): software like ChemDraw and OpenBabel implement most of the rules but still fail on edge cases — the unsolved problem of "canonical name" generation is harder than it looks because the IUPAC rules require qualitative chemical judgement that does not always reduce to deterministic algorithm.
Subtle errors that separate A from A*:
A homologous series is a family of compounds sharing one functional group, a single general formula, gradually changing physical properties, and identical chemistry. The functional group sets the suffix in IUPAC names; the longest chain containing the principal group sets the stem; substituents are demoted to alphabetised prefixes with the lowest set of locants. Molecular, structural, displayed and skeletal formulae each show a different level of detail. Master the priority table and the six-step naming procedure here, and every reaction in Modules 4, 5 and 6 will name its product correctly.
Reference: OCR A-Level Chemistry A (H432) Module 4.1.1 — Basic concepts of organic chemistry (refer to the official OCR H432 specification document for exact wording).