9701 Chemistry · Topics 13 & 27 · AS + A Level
Organic Chemistry Foundations Cheat Sheet — A Level Chemistry 9701
Get the foundations right and the rest of organic chemistry becomes pattern recognition. This sheet covers naming and functional groups, homolytic and heterolytic fission, the language of nucleophiles and electrophiles, curly arrow conventions, and both kinds of stereoisomerism — cis–trans and optical — including how to spot a chiral centre at a glance.
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What’s on this cheat sheet
Organic Chemistry — Foundations
01 · Functional groups
| Class | Group | Suffix |
|---|---|---|
| alkane | C–C | ‑ane |
| alkene | C=C | ‑ene |
| halogenoalkane | –X | halogeno‑ |
| alcohol | –OH | ‑ol |
| aldehyde | –CHO | ‑al |
| ketone | C=O | ‑one |
| carboxylic acid | –COOH | ‑oic acid |
| ester | –COO– | ‑oate |
| amine | –NH₂ | ‑amine |
| amide | –CONH₂ | ‑amide |
| nitrile | –C≡N | ‑nitrile |
02 · Naming rules
1 · Longest chain containing the group gives the stem. 2 · Number from the end giving the group the lowest locant. 3 · Substituents alphabetically, with di‑ and tri‑ as needed.
Commas between numbers, hyphens between numbers and letters: 2,3‑dimethylbutane.
03 · Homologous series
A family with the same general formula and functional group, each member differing by CH₂, showing a gradual change in physical properties and similar chemical properties.
General formulae: alkane CnH2n+2, alkene CnH2n, alcohol CnH2n+1OH, carboxylic acid CnH2n+1COOH.
04 · Primary, secondary, tertiary
Classified by how many carbon atoms are attached to the carbon carrying the functional group: 1° one, 2° two, 3° three.
This decides the products of alcohol oxidation and the mechanism of halogenoalkane substitution, so identify it before predicting anything.
05 · Structural isomerism
Same molecular formula, different structural formula. Three kinds:
Chain — branching differs: butane and 2‑methylpropane.
Position — the group sits on a different carbon: propan‑1‑ol and propan‑2‑ol.
Functional group — different group entirely: propanal and propanone, or ethanol and methoxymethane.
06 · Cis–trans isomerism
Needs a C=C (rotation is blocked by the π bond) and two different groups on each carbon of the double bond.
In the cis isomer the like groups are on the same side; in trans they are on opposite sides. But‑2‑ene shows it; but‑1‑ene does not.
07 · Optical isomerism
Arises at a chiral carbon — one bonded to four different groups. The two forms are non‑superimposable mirror images called enantiomers.
They are identical in every physical and chemical property except that they rotate plane‑polarised light in opposite directions and react differently with other chiral molecules. A 50:50 mixture is racemic and shows no rotation.
08 · Worked example — count the isomers
but‑1‑ene · but‑2‑ene · 2‑methylpropene · cyclobutane · methylcyclopropane
Only but‑2‑ene has two different groups on each C of the C=C, so only it exists as cis and trans forms.
09 · Types of formula
| Type | Propan‑1‑ol |
|---|---|
| empirical | C₃H₈O |
| molecular | C₃H₈O |
| structural | CH₃CH₂CH₂OH |
| displayed | every bond drawn |
| skeletal | lines for the carbon chain |
10 · Bond fission
Homolytic — each atom takes one electron, giving two radicals. Shown with single‑headed (fish‑hook) arrows. Typical of UV‑initiated reactions.
Heterolytic — one atom takes both electrons, giving a cation and an anion. Shown with double‑headed curly arrows, which always start at a lone pair or a bond and point at where the pair goes.
11 · Attacking species
Nucleophile — an electron‑pair donor, attracted to electron‑poor carbon: OH⁻, CN⁻, NH₃, H₂O.
Electrophile — an electron‑pair acceptor, attracted to electron‑rich sites: NO₂⁺, Br⁺, H⁺, the δ+ end of a polarised bond.
Radical — a species with an unpaired electron, formed by homolysis.
12 · Reaction types to name
Addition — two molecules become one (alkene + Br₂).
Substitution — one atom or group replaces another.
Elimination — a small molecule is lost, forming a double bond.
Hydrolysis — split by water or aqueous acid/alkali.
Oxidation / reduction — judged by gain or loss of oxygen or hydrogen; use [O] and [H] in equations.
13 · Worked example — name the compound
Longest chain containing the –OH: five carbons → pentan‑ol
Number from the end nearest the –OH → carbon 2
Methyl branch on carbon 4
→ 4‑methylpentan‑2‑ol, a secondary alcohol.
Marks lost here
— Numbering the chain from the wrong end, or missing the longest chain when it bends on the page.
— Claiming cis–trans isomerism when one carbon of the C=C carries two identical groups.
— Drawing a curly arrow from an atom instead of from a bond or lone pair.
— Calling a species a nucleophile without saying it donates an electron pair.
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Organic Chemistry Foundations — Frequently Asked Questions
What makes a carbon atom a chiral centre?
A carbon atom bonded to four different groups. Such a molecule has no plane of symmetry and exists as two non-superimposable mirror images that rotate plane-polarised light in opposite directions.
What is the difference between homolytic and heterolytic fission?
In homolytic fission each atom takes one electron from the bond, producing two radicals and drawn with half-headed arrows. In heterolytic fission one atom takes both electrons, producing a cation and an anion, and is drawn with full-headed curly arrows.
Which way does a curly arrow point?
Always from a region of electron density — a lone pair, a π bond or a σ bond — towards where those electrons end up. Starting an arrow at a positive charge is one of the fastest ways to lose a mechanism mark.
