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Spec Mapping — OCR H432 Module 4.1.1 — Basic concepts of organic chemistry, covering structural isomerism (chain, position, functional-group) and the distinction between structural isomerism and stereoisomerism (E/Z isomerism is introduced briefly here and developed in Lesson 8) (refer to the official OCR H432 specification document for exact wording).
Isomerism is central to organic chemistry because it explains why two compounds with exactly the same atoms can behave completely differently. The same molecular formula can support several distinct molecules, each with its own connectivity, properties, and reactions. Ethanol and methoxymethane both have the molecular formula C₂H₆O, yet one is a hydrogen-bonded liquid that powers cars and the other is a low-boiling gaseous ether — the difference is purely about which atom is connected to which. At A-Level we restrict attention to structural isomerism in this lesson (chain, position, functional-group) and treat stereoisomerism (E/Z and optical) in later lessons (8 and 17 respectively). Mastering the classification scheme here lets you decode every "draw all the isomers" question on the OCR H432 paper systematically, with no omissions and no duplicates.
Key Definition: structural isomers are compounds with the same molecular formula but different structural formulae — their atoms are connected in a different order. Structural isomers may differ in carbon-chain branching (chain isomers), in functional-group position (position isomers), or in functional-group identity (functional-group isomers).
Key Definition — Isomers: Compounds with the same molecular formula but a different arrangement of atoms.
There are two broad classes of isomers:
graph TD
A[Isomers] --> B["Structural isomers<br/>different atom connectivity"]
A --> C["Stereoisomers<br/>same connectivity,<br/>different 3D arrangement"]
B --> D[Chain isomerism]
B --> E[Position isomerism]
B --> F[Functional group isomerism]
C --> G["E/Z isomerism<br/>around C=C"]
C --> H["Optical isomerism<br/>A2 only"]
At AS level (and this topic), we deal with structural isomerism. Stereoisomerism (E/Z) is covered in Lesson 8.
Key Definition — Structural isomers: Compounds with the same molecular formula but different structural formulae — i.e., their atoms are joined together in a different order.
Chain isomers differ in the arrangement of their carbon skeleton. One may be a straight chain, the other branched, or the branches may be in different positions.
Butane has two chain isomers:
| Isomer | Structure | B.p. / °C |
|---|---|---|
| Butane | CH₃–CH₂–CH₂–CH₃ | −0.5 |
| Methylpropane (isobutane) | (CH₃)₃CH | −12 |
Pentane has three chain isomers:
The more branched the molecule, the more compact its shape, and the lower its boiling point, because London forces act over less contact area.
Hexane has five chain isomers:
The number of chain isomers grows rapidly with carbon count.
Position isomers have the same carbon skeleton and the same functional group, but the functional group is attached at a different position along the chain.
Two position isomers:
Four structural isomers in total, three of which are position isomers of one another:
Functional group isomers have the same molecular formula but contain different functional groups. They therefore belong to different homologous series and have very different chemical and physical properties.
| Molecular formula | Functional group isomers |
|---|---|
| C₂H₆O | Ethanol (CH₃CH₂OH) and methoxymethane (CH₃OCH₃) |
| C₃H₆O | Propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃) |
| C₃H₆O₂ | Propanoic acid (CH₃CH₂COOH) and methyl methanoate (HCOOCH₃) |
| C₃H₆ | Propene (CH₂=CHCH₃) and cyclopropane |
Two functional group isomers:
They have the same molecular formula, but aldehydes can be oxidised to carboxylic acids whereas ketones cannot — a classic basis for the Tollens' and Fehling's tests (covered later in the course).
The huge difference in boiling point (over 100 °C) is because ethanol forms strong hydrogen bonds between molecules, whereas the ether cannot (no O–H bond). The C2 ether has a permanent dipole (the O carries δ⁻ and the two methyl carbons carry δ⁺) so it has some dipole-dipole attraction stronger than equivalent-mass alkane London forces, but the energy penalty for breaking one hydrogen bond (~25 kJ mol⁻¹) is much greater than for breaking one dipole-dipole interaction (~5 kJ mol⁻¹), and ethanol forms multiple hydrogen bonds per molecule. This is a textbook example of functional-group isomerism: same molecular formula, dramatically different intermolecular forces, dramatically different boiling points.
When asked to draw all isomers of a molecular formula, follow a systematic procedure to avoid omissions or duplicates:
There are seven in total:
Alcohols (4):
Ethers (3): 5. Methoxypropane: CH₃OCH₂CH₂CH₃ 6. 2-methoxypropane (or methoxymethylethane): CH₃OCH(CH₃)₂ 7. Ethoxyethane: CH₃CH₂OCH₂CH₃
Isomers 1–4 are chain and position isomers of each other; isomers 1–4 are functional group isomers of 5–7.
The seven C₄H₁₀O isomers form a useful case study for the three classification axes:
| Pair | Same skeleton? | Same group position? | Same functional group? | Class |
|---|---|---|---|---|
| Butan-1-ol vs butan-2-ol | yes (4-C straight) | no | yes (alcohol) | Position isomers |
| Butan-1-ol vs 2-methylpropan-1-ol | no | n/a | yes (alcohol) | Chain isomers |
| Butan-2-ol vs 2-methylpropan-2-ol | no | n/a | yes (alcohol) | Chain isomers (both 2° vs 3°) |
| Butan-1-ol vs ethoxyethane | no | n/a | no (alcohol vs ether) | Functional-group isomers |
| Methoxypropane vs ethoxyethane | yes (both ethers with C–O–C bridge) | n/a (R/R' interchange) | yes (ether) | Functional-class / connectivity isomers |
| 2-methylpropan-1-ol vs 2-methylpropan-2-ol | yes (3-methyl-propane) | no (OH on C1 vs C2) | yes (alcohol) | Position isomers (1° vs 3°) |
Chemically, the most consequential distinction in this set is between primary, secondary and tertiary alcohols: butan-1-ol and 2-methylpropan-1-ol are primary (oxidisable to butanal and 2-methylpropanal respectively); butan-2-ol is secondary (oxidisable to butan-2-one); 2-methylpropan-2-ol is tertiary (not oxidisable under standard conditions). OCR's standard practical PAG 7 distinguishes these by the acidified-dichromate colour change (orange → green for 1° and 2°; no change for 3°), and the four alcohol isomers give three distinct outcomes — a tidy demonstration of position-isomerism affecting reactivity.
For molecular formulae with degree of unsaturation ≥ 1, the enumeration must include cyclic forms. C₄H₈ (IHD = 1) has five structural isomers:
Isomers 1–3 are functional-group isomers of isomers 4–5 (alkene vs cycloalkane). Isomers 1 and 2 are position isomers of each other within the alkene class; isomer 3 is a chain isomer of 1 and 2. Isomers 4 and 5 are chain isomers of each other. Drawing all five is a standard 5-mark "enumerate all structural isomers" question on Paper 2.
Once enumerated, isomers must be distinguishable in the lab — usually by a combination of chemical tests and spectroscopy:
| Test | Distinguishes | Diagnostic outcome |
|---|---|---|
| Bromine water | Alkene vs alkane/cycloalkane | Decolourises (alkene) / no change |
| 2,4-DNP | Carbonyl present vs not | Orange precipitate (aldehyde or ketone) |
| Tollens' reagent | Aldehyde vs ketone | Silver mirror (aldehyde only) |
| Fehling's solution | Aliphatic aldehyde vs ketone | Red Cu₂O precipitate (aliphatic aldehyde) |
| Iodoform (warm I₂/NaOH) | Methyl ketone or CH₃CH(OH)– vs others | Yellow CHI₃ precipitate |
| Na metal | O–H present vs not | H₂ evolution (alcohol, carboxylic acid) |
| Acidified K₂Cr₂O₇ | Oxidisable alcohol vs not | Orange → green (1° or 2° alcohol) |
| IR spectroscopy | Functional group | Diagnostic absorption frequency |
| NMR spectroscopy | Atomic environment | Chemical-shift and multiplicity pattern |
These are the workhorses of OCR Paper 2 and Paper 3 unknown-structure questions. A well-drilled student carries the table mentally and applies it as a decision-tree the moment a test result is given.
Different isomers may:
The thalidomide tragedy of the late 1950s and early 1960s is the canonical biological case: one enantiomer of thalidomide is a mild sedative; the other is teratogenic, causing severe birth defects in children exposed in utero. Both enantiomers were present in the racemic mixture marketed to pregnant women. This is an optical isomerism rather than structural example, but the lesson generalises: same atoms, different connectivity (or different 3D arrangement), entirely different biological action.
A structural-isomerism example with comparable physical-property divergence: ethanol (b.p. 78 °C, miscible with water, hydrogen-bonded) vs methoxymethane (b.p. −24 °C, sparingly soluble in water, dipole-dipole only). The 100+ °C gap in boiling point is the experimental fingerprint of hydrogen bonding — ethanol's O–H allows it, the methoxymethane ether oxygen lacks an O–H so it cannot. Identifying which functional-group isomer is in front of you from a single boiling-point measurement is a standard PAG 7 task.
The number of structural isomers grows non-linearly (and in fact super-exponentially) with carbon count for unrestricted molecular formulae. For alkanes alone:
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