29. An introduction to A Level organic chemistry

A functional group is the reactive arrangement of atoms that gives an organic molecule characteristic reactions. Identify it from connectivity, not from the molecule’s name alone.

The syllabus representations include families such as alkenes (C=C), halogenoalkanes (C–X), alcohols (C–OH), aldehydes (–CHO), ketones (>C=O), carboxylic acids (–CO₂H), esters (–CO₂–), amines and nitriles (–C≡N).

Propanal and propanone both contain C=O, but the aldehyde has a terminal –CHO group whereas the ketone has carbonyl carbon bonded to two carbon groups; that difference predicts different oxidation behaviour.

Do not classify a molecule by spotting one atom. The neighbouring bonds and position decide whether the group is, for example, an alcohol, ether, aldehyde or carboxylic acid.

Benzene is represented in A Level structures by a hexagonal ring with alternating bonds or a circle, depending on the required convention. When benzene is part of a molecule, the ring is not normally redrawn as a fully displayed set of C–H bonds.

Attach substituents to the ring at the correct carbon and use the ring symbol consistently in condensed, skeletal and displayed representations.

Phenol can be shown as a benzene ring bonded to –OH; methylbenzene is the same ring bonded to –CH₃. The ring itself supplies the six-carbon aromatic framework.

The circle is not a seventh atom and the alternating-bond drawing is not three isolated double bonds behaving independently; both conventions represent the delocalised ring.

A functional group is the local arrangement of atoms that gives an organic family its characteristic reactions and many of its physical properties.

The carbon skeleton still affects boiling point, solubility and steric access, but the functional group supplies the main reaction pattern. Identify both before predicting behaviour.

Ethanol and ethanoic acid both contain two carbon atoms, yet –OH versus –CO₂H gives very different acidity and reactions. Propene and propane differ by C=C and therefore by addition chemistry.

A functional group is not the whole molecule and one atom alone is not enough to classify it; inspect the complete local connectivity.

General formulae show a family pattern; structural formulae show how atoms are connected; displayed formulae show every bond; skeletal formulae show the carbon framework with most carbon-bound hydrogens omitted.

Translate between styles without changing connectivity. In a skeletal formula, each vertex and line end is a carbon unless labelled otherwise, and hydrogens on heteroatoms are shown.

Ethanol may be written C₂H₆O, CH₃CH₂OH, a displayed C–C–O structure, or a two-carbon skeletal chain ending in –OH. Each representation describes the same molecule.

A general formula cannot distinguish isomers, and a skeletal corner is not an unlabelled oxygen or hydrogen.

Systematic naming selects the parent chain or ring, numbers it to give the principal functional group the lowest possible locant, then adds substituents and unsaturation prefixes.

For the syllabus range, use straight-chain esters and nitriles within the stated carbon limits, and cyclic compounds with one ring of up to six carbons. The suffix identifies the principal functional group.

CH₃CH₂CH₂OH is propan-1-ol, not propan-3-ol: numbering starts at the end nearest –OH. CH₃COOCH₂CH₃ is ethyl ethanoate, named as the alkyl group from the alcohol plus the carboxylate part.

The longest chain is not always numbered from the left, and the first number in an ester name does not describe the acid fragment.

For one benzene ring with simple substituents, name the ring parent and assign numbers that give the set of substituent locants the lowest possible value; list prefixes alphabetically when required.

The principal functional group can determine the parent name, such as benzoic acid or phenol. Start numbering at that group, then choose the direction that minimises the remaining locants.

A nitro group opposite the carboxylic acid is 4-nitrobenzoic acid. Three bromines on phenol at the 2, 4 and 6 positions give 2,4,6-tribromophenol.

Do not number a substituted benzene as if it were a straight chain, and do not choose the direction that merely gives the first substituent a small number while making the full locant set larger.

Electrophilic substitution replaces an atom or group on an electron-rich aromatic ring with an electrophile while restoring aromaticity. Addition–elimination first adds across a multiple bond or carbonyl system and then eliminates a leaving group to restore a π bond.

The names describe the sequence, not just the reactants. Identify the electron-rich site, the electrophile or nucleophile, and the group that leaves.

Nitration of benzene is electrophilic substitution: NO₂⁺ enters the ring and H⁺ is removed. Acyl substitution at an acid derivative follows addition of a nucleophile to C=O, then elimination of the leaving group.

Electrophilic substitution is not electrophile addition: the ring ends with its aromatic π system restored.

Each carbon in benzene is sp² hybridised and forms three σ bonds in a trigonal-planar arrangement. The remaining p orbital overlaps with neighbours to make one delocalised π system above and below the ring.

Delocalisation makes the ring planar and gives all six C–C bonds an intermediate character rather than three independent double bonds. The π electrons are spread around the ring.

Benzene has six equal C–C bond lengths, shorter than a single bond but longer than a local C=C double bond. This helps explain why addition would sacrifice aromatic stabilisation.

The circle in a benzene drawing is not an extra bond or atom, and the molecule is not a set of three isolated alkenes.

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.

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.

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.

A chiral drug may exist as enantiomers that have the same bulk physical properties but different biological effects because receptors, enzymes and transport proteins are chiral.

Synthetic preparation may therefore aim for one enantiomer, resolve a racemate, or test the mixture carefully. The useful enantiomer can be more active, while the other may be inactive or have a different effect.

A molecule designed to fit one enzyme binding site like a hand in a glove may have a mirror-image form that binds weakly or to another target.

“One enantiomer is always harmless” is not a chemical rule. Biological effects must be measured, and a racemate is not automatically equivalent to a pure enantiomer.