What you'll learn
Organic chemistry is the chemistry of carbon compounds — the fuels, plastics, medicines, foods and fibres that shape modern life. Because carbon forms four bonds and can join to other carbon atoms in chains and rings, there are millions of organic compounds. A systematic way of naming them and understanding how the same atoms can be arranged differently (isomerism) is essential. This guide covers homologous series and functional groups, the different types of formulae, IUPAC nomenclature for alkanes, alkenes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters, amines and amides, structural isomerism (chain, positional and functional group), and stereoisomerism — geometric (cis–trans) and optical isomerism, including chiral centres and their importance in biological systems and medicines.
CAPE questions ask you to name compounds from structures and draw structures from names, identify and draw isomers, and explain why certain molecules show geometric or optical isomerism. Precision in naming and drawing is essential.
Key terms and definitions
Homologous series — A family of compounds with the same functional group and general formula, in which each member differs from the next by CH₂, with similar chemical properties and gradually changing physical properties.
Functional group — An atom or group of atoms responsible for the characteristic reactions of a compound.
Molecular formula — The actual number of atoms of each element in a molecule (e.g. C₂H₆O).
Structural formula — Shows the arrangement of atoms without drawing every bond (e.g. CH₃CH₂OH).
Displayed formula — Shows every atom and every bond.
Skeletal formula — Shows the carbon skeleton as lines, with carbon atoms at the corners and ends, hydrogen atoms on carbon omitted, and functional groups shown.
Structural isomers — Compounds with the same molecular formula but different structural formulae.
Stereoisomers — Compounds with the same structural formula but different spatial arrangements of atoms.
Geometric (cis–trans) isomerism — Stereoisomerism caused by restricted rotation about a C=C double bond, with two different groups on each carbon of the double bond.
Optical isomerism — Stereoisomerism in which molecules are non-superimposable mirror images (enantiomers).
Chiral centre — A carbon atom bonded to four different atoms or groups.
Enantiomers — A pair of optical isomers that are non-superimposable mirror images.
Racemic mixture — An equal mixture of two enantiomers, which shows no overall optical activity.
Core concepts
Homologous series and functional groups
| Series | Functional group | General formula | Example (name) |
|---|---|---|---|
| Alkanes | C–C single bonds only | CₙH₂ₙ₊₂ | CH₃CH₃ (ethane) |
| Alkenes | C=C | CₙH₂ₙ | CH₂=CH₂ (ethene) |
| Halogenoalkanes | C–X | CₙH₂ₙ₊₁X | CH₃CH₂Br (bromoethane) |
| Alcohols | –OH | CₙH₂ₙ₊₁OH | CH₃CH₂OH (ethanol) |
| Aldehydes | –CHO | — | CH₃CHO (ethanal) |
| Ketones | C=O within chain | — | CH₃COCH₃ (propanone) |
| Carboxylic acids | –COOH | CₙH₂ₙ₊₁COOH | CH₃COOH (ethanoic acid) |
| Esters | –COO– | — | CH₃COOCH₂CH₃ (ethyl ethanoate) |
| Amines | –NH₂ | — | CH₃NH₂ (methylamine) |
| Amides | –CONH₂ | — | CH₃CONH₂ (ethanamide) |
Physical properties change gradually along a series: boiling points increase with chain length because London forces increase.
IUPAC naming rules
- Find the longest carbon chain containing the functional group — this gives the stem: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
- Identify the functional group — this gives the suffix (-ane, -ene, -ol, -al, -one, -oic acid) or prefix (chloro-, bromo-, amino-).
- Number the chain so that the functional group gets the lowest possible number.
- Name and number branches (methyl-, ethyl-) and list substituents in alphabetical order.
- Use di-, tri-, tetra- for repeated groups, with commas between numbers and hyphens between numbers and letters.
Examples:
- CH₃CH(CH₃)CH₂CH₃ → 2-methylbutane
- CH₃CH=CHCH₃ → but-2-ene
- CH₃CH(OH)CH₃ → propan-2-ol
- CH₃CH₂CH₂CHO → butanal
- CH₃CH₂COCH₃ → butanone
- CH₃CHClCH₂Cl → 1,2-dichloropropane
Esters are named from the alcohol part first, then the acid part: ethanol + ethanoic acid → ethyl ethanoate; methanol + propanoic acid → methyl propanoate.
Classifying alcohols and halogenoalkanes
- Primary — the carbon bearing –OH (or –X) is attached to one other carbon (e.g. propan-1-ol).
- Secondary — attached to two other carbons (e.g. propan-2-ol).
- Tertiary — attached to three other carbons (e.g. 2-methylpropan-2-ol).
This classification determines their reactions, especially oxidation.
Structural isomerism
Chain isomerism — different arrangements of the carbon skeleton: butane and 2-methylpropane (both C₄H₁₀).
Positional isomerism — the functional group is in a different position: propan-1-ol and propan-2-ol; but-1-ene and but-2-ene.
Functional group isomerism — different functional groups: ethanol (CH₃CH₂OH) and methoxymethane (CH₃OCH₃), both C₂H₆O; propanal and propanone, both C₃H₆O; propanoic acid and methyl ethanoate, both C₃H₆O₂.
Geometric (cis–trans) isomerism
The C=C double bond contains a π bond that prevents rotation. If each carbon of the double bond carries two different groups, two isomers exist:
- cis (Z) — similar groups on the same side;
- trans (E) — similar groups on opposite sides.
But-2-ene has cis and trans isomers. But-1-ene does not, because one carbon of the double bond carries two hydrogen atoms.
Geometric isomers often have different physical properties — for example, different melting points, because trans isomers usually pack more closely.
Optical isomerism
A carbon atom with four different groups attached is a chiral centre. The molecule and its mirror image cannot be superimposed, like left and right hands. The two enantiomers:
- have identical physical and chemical properties in most respects;
- rotate plane-polarised light in opposite directions by equal amounts;
- may interact differently with other chiral molecules, such as enzymes and receptors in the body.
2-hydroxypropanoic acid (lactic acid), CH₃CH(OH)COOH, has a chiral centre at carbon 2 (attached to H, CH₃, OH and COOH). Amino acids (except glycine) are chiral, and natural proteins use only one enantiomer.
Biological importance: many drugs are chiral, and one enantiomer may be effective while the other is inactive or harmful. The thalidomide tragedy of the 1950s and 1960s highlighted the need to test enantiomers separately. Many modern medicines are produced as a single enantiomer.
A racemic mixture (50:50 of each enantiomer) shows no optical activity because the rotations cancel. Laboratory synthesis often produces racemic mixtures, whereas enzymes in living organisms usually produce a single enantiomer.
Worked examples
Example 1: Naming
Name CH₃CH₂CH(CH₃)CH₂OH.
Longest chain containing the –OH carbon: 4 carbons (butan-). –OH on carbon 1; methyl on carbon 2. Name: 2-methylbutan-1-ol.
Example 2: Drawing isomers
Draw and name the structural isomers of C₃H₇Cl.
CH₃CH₂CH₂Cl — 1-chloropropane; CH₃CHClCH₃ — 2-chloropropane (positional isomers).
Example 3: Geometric isomerism
Explain why pent-2-ene shows geometric isomerism but 2-methylbut-2-ene does not.
In pent-2-ene (CH₃CH=CHCH₂CH₃), each carbon of the double bond has two different groups (H and CH₃; H and CH₂CH₃), so cis and trans forms exist. In 2-methylbut-2-ene, (CH₃)₂C=CHCH₃, one carbon of the double bond carries two identical CH₃ groups, so no geometric isomers exist.
Example 4: Identifying a chiral centre
Does butan-2-ol show optical isomerism? Explain. (2 marks)
Yes (1): carbon 2 is bonded to four different groups — H, OH, CH₃ and CH₂CH₃ — so it is a chiral centre and the molecule exists as two non-superimposable mirror images (1).
Example 5: Naming from a skeletal formula
A skeletal formula shows a five-carbon zig-zag chain with a C=O at the second carbon from one end. Name the compound.
The chain has five carbons (pent-), the functional group is a ketone (-one), and the carbonyl is on carbon 2 when numbered from the nearer end. The compound is pentan-2-one, CH₃COCH₂CH₂CH₃.
Example 6: Counting the isomers of C₄H₈
Draw and name all the isomers of C₄H₈ that are alkenes, including stereoisomers.
- But-1-ene, CH₂=CHCH₂CH₃ (no geometric isomers — one carbon of C=C carries two H atoms).
- cis-But-2-ene and trans-but-2-ene, CH₃CH=CHCH₃ (each carbon of C=C carries H and CH₃).
- 2-Methylpropene, (CH₃)₂C=CH₂ (no geometric isomers).
That gives four alkene isomers. C₄H₈ also has cyclic isomers — cyclobutane and methylcyclopropane — which are functional group isomers of the alkenes.
Example 7: Isomers of C₄H₈O₂
Give two carboxylic acids and two esters with the formula C₄H₈O₂.
Acids: butanoic acid, CH₃CH₂CH₂COOH; 2-methylpropanoic acid, (CH₃)₂CHCOOH. Esters: ethyl ethanoate, CH₃COOCH₂CH₃; methyl propanoate, CH₃CH₂COOCH₃ (others include propyl methanoate). Acids and esters with the same molecular formula are functional group isomers.
Example 8: Optical isomers of an amino acid
Explain why alanine, CH₃CH(NH₂)COOH, is optically active but glycine, H₂NCH₂COOH, is not.
In alanine, the central carbon is bonded to four different groups — H, CH₃, NH₂ and COOH — so it is a chiral centre, and two non-superimposable mirror images exist. In glycine, the central carbon carries two hydrogen atoms, so it is not chiral and the molecule is superimposable on its mirror image.
Naming priorities at a glance
| Functional group | Suffix | Prefix (when not main group) |
|---|---|---|
| Carboxylic acid | -oic acid | carboxy- |
| Ester | -oate | — |
| Amide | -amide | — |
| Aldehyde | -al | oxo- / formyl- |
| Ketone | -one | oxo- |
| Alcohol | -ol | hydroxy- |
| Amine | -amine | amino- |
| Alkene | -ene | — |
| Halogen | — | fluoro-, chloro-, bromo-, iodo- |
When a molecule has more than one functional group, the group higher in the table usually becomes the suffix and the others become prefixes — for example, 2-hydroxypropanoic acid (lactic acid).
Common mistakes and how to avoid them
- Not choosing the longest chain, especially when it bends on a diagram.
- Numbering from the wrong end — give the functional group the lowest number.
- Forgetting alphabetical order for substituents (bromo before chloro before methyl).
- Claiming geometric isomerism when one alkene carbon has two identical groups.
- Looking for chiral centres on carbons with two identical groups (such as CH₂ or C with two methyls).
- Drawing enantiomers that are actually identical — check they are mirror images that cannot be superimposed.
Self-test questions
- Name (CH₃)₃CCH₂OH. Answer: 2,2-dimethylpropan-1-ol.
- Draw the structure of 3-chlorobut-1-ene and state whether it shows optical isomerism. Answer: CH₂=CH–CHCl–CH₃. Yes — carbon 3 is bonded to H, Cl, CH₃ and CH=CH₂, four different groups, so it is a chiral centre.
- Classify propan-2-ol, 2-methylpropan-2-ol and ethanol as primary, secondary or tertiary. Answer: secondary; tertiary; primary.
- What type of isomerism is shown by propanal and propanone? Answer: functional group isomerism (aldehyde and ketone, both C₃H₆O).
How this links to your Internal Assessment
Isomerism can be explored practically using molecular model kits to build structural isomers and enantiomers, and by investigating physical properties of isomers — for example, comparing boiling points of straight-chain and branched alcohols, or melting points of cis and trans isomers (such as maleic and fumaric acids). In organic SBAs, accurate naming and structural formulae in your write-up demonstrate understanding, and a polarimeter (where available) can show optical activity of sugar solutions.
Exam technique for Nomenclature and isomerism
- Draw displayed or skeletal formulae clearly, with every bond for displayed formulae.
- When naming, write the stem, numbers, substituents and suffix in the correct order with correct punctuation.
- For geometric isomers, draw both and label cis/trans (or Z/E).
- For optical isomers, draw 3D structures with wedges and dashes showing mirror images, and identify the chiral centre with an asterisk.
Quick revision summary
- Homologous series share a functional group and general formula; members differ by CH₂.
- Formulae: molecular, structural, displayed, skeletal.
- IUPAC: longest chain, lowest number for the functional group, alphabetical substituents.
- Primary, secondary and tertiary alcohols and halogenoalkanes.
- Structural isomers: chain, positional, functional group.
- Geometric isomers need restricted rotation (C=C) and two different groups on each carbon.
- Optical isomers have a chiral centre (four different groups); enantiomers rotate plane-polarised light oppositely; racemic mixtures are inactive; chirality matters for medicines.