29. An introduction to A Level organic chemistry A2
- Syllabus
- 9701–2028–2029
- Section
- 29
- Level
- A2
A Level organic chemistry A2 conventions and functional group reference
- Syllabus
- 9701–2028–2029
- Topic
- —
- Level
- A2
Recognise the A Level organic functional groups from connectivity
Recognise a functional group from which atoms are directly bonded, not by spotting one atom or relying on a name. In the formulae below, R and R′ are carbon-containing groups, Ar is an arene ring, and X is a halogen.
| Family | Recognition pattern | Decisive connectivity / example |
|---|---|---|
| arene | Ar | contains a benzene ring; e.g. methylbenzene |
| halogenoarene | Ar–X | X is directly bonded to a ring carbon; e.g. chlorobenzene |
| phenol | Ar–OH | OH is directly bonded to a ring carbon; e.g. phenol |
| acyl chloride | R–C(=O)–Cl | Cl is bonded to the carbonyl carbon; e.g. CH₃COCl |
| secondary / tertiary amine | R₂NH / R₃N | N is bonded to two / three carbon groups |
| primary / secondary / tertiary amide | RCONH₂ / RCONHR′ / RCONR′R″ | N is directly bonded to a carbonyl carbon and has zero, one or two N-substituent carbon groups |
| amino acid | H₂N–CHR–CO₂H | amine and carboxyl groups occur in the same molecule; R = H gives glycine |
| Do not confuse… | Discriminating bond |
|---|---|
| halogenoarene vs halogenoalkane | Ar–X vs an sp³ carbon–X bond outside the ring |
| phenol vs aromatic alcohol | Ar–OH vs Ar–CH₂OH |
| acyl chloride vs chloro-ketone | C(=O)–Cl vs Cl elsewhere on the carbon skeleton |
| amide vs amine | amide N is directly bonded to C=O; amine N is not |
The official reference table requires recognition and use of these structures. Systematic naming of secondary and tertiary amines is not required here; later chapters teach their characteristic reactions and comparisons.
Keep a benzene ring abbreviated inside a larger displayed structure
When a benzene ring is part of a molecule, Cambridge does not expect its six carbon atoms and attached ring hydrogens to be expanded as a fully displayed formula. Keep the ring as the conventional benzene hexagon and display the substituent connectivity clearly.
| Molecule | What the representation must show |
|---|---|
| chlorobenzene | a benzene ring with Cl bonded directly to one ring vertex |
| phenol | a benzene ring with OH bonded directly to one ring vertex |
| methylbenzene | a benzene ring with CH₃ bonded to one ring vertex |
| disubstituted arene | two bonds from the correct ring vertices so their relative positions are unambiguous |
If the rest of a molecule is requested as a displayed formula, draw those non-ring atoms and bonds explicitly while leaving the benzene ring in its accepted ring representation. The bond from the ring to each substituent is still part of the required connectivity.
The ring symbol already supplies the six-carbon aromatic framework; it is not an extra atom. Do not omit a substituent bond, attach X or OH through an unintended CH₂ group, or redraw every ring C–H bond when the ring is only part of the molecule.
29.1 Formulas, functional groups and nomenclature
- Syllabus
- 9701–2028–2029
- Topic
- 29.1
- Level
- A2
A functional group's connectivity produces characteristic properties
A functional group is the connected atom arrangement that produces a class's characteristic physical and chemical properties. The rest of the carbon skeleton modifies those properties, but does not replace the functional group's characteristic chemistry.
| Page-47 class | Structural feature | Consequence that distinguishes the class |
|---|---|---|
| arene | delocalised benzene π system | characteristic aromatic rather than alkene-like reaction behaviour |
| phenol | OH directly bonded to an arene ring | ring–OH interaction gives behaviour different from an aliphatic alcohol |
| acyl chloride | polar C=O bonded to Cl | reactive carbonyl derivative with characteristic substitution chemistry |
| amine | N lone pair not directly beside C=O | can accept H⁺ and behave as a base |
| amide | N directly bonded to C=O | N lone pair is delocalised toward the carbonyl, so it is much less available than in an amine |
| amino acid | amine and carboxyl groups in one molecule | can show both basic and acidic behaviour |
Within one homologous class, increasing carbon-chain size can alter boiling point and solubility through intermolecular-force balance. To predict a molecule, identify both the functional group and the size or shape of its carbon framework.
One atom is not a functional-group diagnosis: OH on an arene is phenol, but OH on an sp³ carbon is an alcohol; nitrogen beside C=O is an amide, not an amine.
Translate between general, structural, displayed and skeletal formulae without changing connectivity
| Formula style | What it communicates | Example use for an acyl chloride |
|---|---|---|
| general | the family pattern across different members | RCOCl |
| structural | atom order and grouped connectivity | CH₃CH₂COCl |
| displayed | every atom and every covalent bond in the shown non-benzene structure | show CH₃–CH₂–C with C=O and C–Cl, including all C–H bonds |
| skeletal | carbon framework as lines; C atoms and C-bound H atoms are omitted | a two-segment chain ending at C(=O)Cl |
In a skeletal formula, every unlabelled line end or vertex is carbon; add enough implied H atoms to give carbon four bonds. Heteroatoms such as N, O and Cl are written, and hydrogens attached to heteroatoms are shown when present.
A valid translation preserves carbon count, functional-group connectivity, multiple bonds and overall charge. For CH₃CH₂COCl, every style must still show three carbons and Cl directly bonded to the carbonyl carbon.
A general formula describes a class, not one unique molecule. A molecular formula gives atom counts but may hide connectivity and isomerism; neither can replace a structural formula when bond order matters.
Name bounded aliphatic structures with one systematic workflow
| Step | Naming decision |
|---|---|
| 1 | identify the principal functional group and its suffix |
| 2 | choose the longest parent chain containing that group, or the single parent ring |
| 3 | number from the end that gives the principal group the lowest locant, then the lowest locant set |
| 4 | identify substituents, positions and multiplicative prefixes; alphabetise distinct prefixes |
| 5 | assemble locants, prefixes, parent, unsaturation and suffix with correct punctuation |
| Syllabus boundary | Required range |
|---|---|
| ordinary aliphatic molecules | up to 6 carbon atoms |
| cyclic molecules | one ring only, containing up to 6 carbon atoms |
| esters and amides | up to 6 + 6 carbon atoms in the two carbon-containing parts |
| esters and nitriles | straight chains only |
| secondary and tertiary amines | recognise structures; their naming is not required by the page-47 table |
| Structure | Key parent decision | Systematic name |
|---|---|---|
| CH₃CH₂COCl | 3-carbon acyl chain | propanoyl chloride |
| CH₃CH(NH₂)CO₂H | carboxyl carbon is C1; amino on C2 | 2-aminopropanoic acid |
| CH₃CH₂CONHCH₃ | propanamide parent; methyl substituent is on N | N-methylpropanamide |
| CH₃COOCH₂CH₃ | alcohol-derived ethyl first; acid-derived ethanoate second | ethyl ethanoate |
The parent is not simply the visually longest line if it omits the principal group. In nitriles the C≡N carbon belongs to the parent count; in esters the two sides have different naming roles and are not joined into one chain.
Name one-ring aromatic molecules from the parent group and lowest locants
| Step | Aromatic naming move |
|---|---|
| 1 | choose benzene or the retained functional parent, such as phenol or benzoic acid |
| 2 | assign the parent-group ring carbon as C1 when the parent fixes it |
| 3 | number around the ring in the direction giving the lowest complete set of locants |
| 4 | add substituent locants and di-, tri-, etc.; alphabetise different substituent prefixes where needed |
| Structure description | Numbering | Name |
|---|---|---|
| benzoic acid with NO₂ two ring bonds from C1 | CO₂H-bearing carbon is C1; NO₂ at C3 | 3-nitrobenzoic acid |
| phenol with Br on both adjacent carbons and the opposite carbon | OH-bearing carbon is C1; Br at C2, C4 and C6 | 2,4,6-tribromophenol |
Keep the single benzene ring in its conventional ring form and show each substituent bond at the correct vertex. The spatial drawing and locants must encode the same relative positions.
Do not renumber phenol or benzoic acid from an arbitrary substituent, and do not optimise only the first locant while ignoring the full ordered locant set. This objective is limited to one benzene ring with simple substituents.
29.2 Characteristic organic reactions
- Syllabus
- 9701–2028–2029
- Topic
- 29.2
- Level
- A2
Distinguish electrophilic substitution from addition–elimination by its step sequence
| Mechanism term | Attacking species and site | Characteristic sequence | Net result |
|---|---|---|---|
| electrophilic substitution | an electrophile attacks an electron-rich arene π system | electrophile bonds to the ring; H⁺ or another group is lost; the delocalised π system is restored | one ring atom/group is replaced by the electrophile |
| nucleophilic addition–elimination | a nucleophile attacks the δ⁺ carbonyl carbon of an acyl derivative | nucleophile adds and C=O π electrons move to O; C=O reforms as a leaving group departs | one group on the acyl carbon is replaced by the nucleophile |
Read the name as a mechanism history. ‘Electrophilic’ or ‘nucleophilic’ identifies the electron-pair acceptor or donor that attacks; ‘substitution’ describes the overall replacement; ‘addition–elimination’ records two successive stages even though its overall product also looks substituted.
CX6HX6+BrX2AlBrX3CX6HX5Br+HBr
In bromination, the catalyst helps generate a sufficiently strong electrophile. The benzene ring forms a bond to it and then loses H⁺, restoring aromatic delocalisation; this is substitution rather than permanent addition to the ring.
CHX3COCl+HX2OCHX3COOH+HCl
In acyl-chloride hydrolysis, water attacks the carbonyl carbon to form an addition intermediate. The carbonyl reforms as Cl⁻ leaves; proton transfer gives the overall carboxylic acid and HCl products.
Do not classify from the overall equation alone. Electrophilic addition leaves new groups added across a π bond, whereas electrophilic substitution restores the arene π system; addition–elimination contains a real addition intermediate before the leaving group departs.
29.3 Shapes of aromatic organic molecules; σ and π bonds
- Syllabus
- 9701–2028–2029
- Topic
- 29.3
- Level
- A2
sp² ring carbons create a planar σ framework and one delocalised π system
Each carbon in benzene is sp² hybridised. Its three sp² orbitals form three σ bonds in a trigonal-planar arrangement with bond angles of about 120°, so the six carbon atoms and their attached hydrogen atoms form a planar hexagonal σ framework.
| Bonding part | Orbitals and overlap | Position of electron density | Role in benzene |
|---|---|---|---|
| σ framework | end-on overlap of sp² orbitals for C–C and sp² with H 1s for C–H | along internuclear axes, in the molecular plane | fixes the six-membered planar skeleton; 6 C–C σ and 6 C–H σ bonds |
| delocalised π system | sideways overlap of one unhybridised p orbital on every carbon | continuous regions above and below the ring plane | spreads six π electrons over all six carbon atoms |
sp² hybridisation first provides coplanar σ-bond directions and leaves one p orbital perpendicular to the plane on each carbon. Because all six p orbitals are parallel and adjacent, they overlap around the whole ring; this continuous overlap produces the delocalised π system.
Delocalisation makes all six C–C bonds equivalent, with bond length and character intermediate between localised C–C single and C=C double bonds. Benzene is therefore not three independent alkene units.
The same test applies to other aromatic ring systems: the atoms participating in the conjugated ring must provide aligned adjacent p orbitals for continuous delocalisation. An attached substituent or non-ring atom is not automatically in the same plane unless its own bonding requires it.
Planarity enables parallel p-orbital overlap; it is misleading to say that delocalisation alone creates the sp² geometry. The circle in a benzene symbol represents the delocalised π system, not an extra atom, bond or electron shell.
29.4 Optical isomerism
- Syllabus
- 9701–2028–2029
- Topic
- 29.4
- Level
- A2
Enantiomers usually share properties but differ in biological fit and optical rotation
Enantiomers are non-superimposable mirror images. In an achiral environment they have the same melting point, boiling point and many chemical properties, but they rotate plane-polarised light in opposite directions.
A chiral receptor or enzyme is itself asymmetric, so the two enantiomers can bind differently and produce different biological effects.
Two enantiomers may pass through an ordinary solvent identically yet one fit a biological active site better, like opposite hands fitting a glove.
R and S labels do not by themselves tell you clockwise optical rotation, and ‘same formula’ does not mean the molecules are superimposable.
Optically active samples rotate plane-polarised light; racemates cancel
An optically active sample rotates plane-polarised light. A racemic mixture contains equal amounts of both enantiomers, whose opposite rotations cancel, so the mixture shows no net rotation.
Optical activity describes the sample, not simply the presence of a chiral carbon. A sample enriched in one enantiomer can be active; a perfectly 1:1 mixture is racemic.
A pure (+) enantiomer and pure (−) enantiomer rotate light by equal magnitudes in opposite directions. Mixing equal amounts gives zero observed rotation.
No net rotation does not prove that no chiral molecules are present; it may indicate cancellation in a racemate.
Enantiomers rotate plane-polarised light in opposite directions
The two enantiomers of one substance rotate plane-polarised light by equal angles in opposite directions under the same conditions. One is dextrorotatory (+), the other laevorotatory (−).
The sign of rotation is measured experimentally; it is not predicted from the R/S descriptor or from the direction in which the structure is drawn on a page.
If one enantiomer rotates light +12°, its mirror image rotates it −12° at the same concentration, path length and temperature.
R/S nomenclature and +/− optical rotation are independent labels. Never replace one with the other.
Chiral drug synthesis must control which enantiomer reaches the patient
Drug targets such as receptors and enzymes are chiral. Two drug enantiomers can therefore fit them differently, so one may give the intended effect while the other is less active, inactive or produces a different effect; the outcome must be measured rather than assumed.
| Preparation route | What happens | Why it matters |
|---|---|---|
| achiral synthesis then resolution | a racemic 1:1 mixture is formed and separated into pure enantiomers | supplies the required enantiomer, but separation adds steps and the other enantiomer must be handled or recycled |
| synthesis with a chiral catalyst | the catalyst creates an asymmetric reaction environment that favours formation of one mirror-image pathway | produces predominantly the chosen enantiomer and reduces later separation demand |
| biological chiral catalyst | an enzyme active site binds reactants in one orientation | can provide high stereoselectivity toward one enantiomer |
A pharmaceutical route must identify which enantiomer has the desired biological activity, control or separate the product composition, and verify enantiomeric purity. A racemate cannot be assumed equivalent to the same dose of one pure enantiomer.
A molecule can contain more than one chiral centre, increasing the possible spatial arrangements. This syllabus does not require meso compounds or nomenclature such as diastereoisomers, so do not use those as required explanations here.