Organic Chemistry Basics: Functional Groups and Naming
Organic chemistry is the study of compounds built around carbon atoms. It covers everything from the fuels in a car engine to the amino acids in your proteins. The field seems vast, but it is organised by a surprisingly compact set of rules about bonding and functional groups.
Why Carbon Is Special
Carbon sits in group 14 of the periodic table, giving it four electrons in its outer shell. It can form four covalent bonds, and crucially, those bonds can link carbon atoms to each other in chains, branches, and rings of almost unlimited length. No other element constructs such a rich variety of stable frameworks. Hydrogen, oxygen, nitrogen, sulfur, and the halogens attach to these carbon skeletons and give each molecule its chemical personality.
When carbon forms only single bonds to other carbons and hydrogens, the compound is described as saturated. When double or triple carbon-carbon bonds are present, the compound is unsaturated. Saturation matters for reactivity: unsaturated bonds are electron-rich sites that react with bromine water, for instance, decolourising it — a classic test used in the lab.
Hydrocarbons: The Simplest Organic Compounds
A hydrocarbon contains only carbon and hydrogen. Three families are worth knowing at this level:
- Alkanes — single bonds only; general formula CnH2n+2. Examples: methane (CH4), ethane (C2H6), propane (C3H8). Relatively unreactive; burn completely in excess oxygen to give CO2 and H2O.
- Alkenes — contain at least one C=C double bond; general formula CnH2n. The double bond is the site of addition reactions with H2, Br2, HCl, and water.
- Arenes (aromatic hydrocarbons) — contain a benzene ring (C6H6), a hexagonal ring of six carbons with delocalised electrons. Benzene undergoes substitution rather than addition, preserving its stable ring.
Functional Groups
A functional group is an atom or cluster of atoms that gives an organic molecule its characteristic reactions. Molecules with the same functional group belong to the same homologous series and share similar chemistry. The table below summarises the key functional groups at A-level standard:
| Family | Functional group | Example | Key reaction |
|---|---|---|---|
| Alkene | —C=C— | ethene | Addition (electrophilic) |
| Alcohol | —OH | ethanol | Oxidation; esterification |
| Aldehyde | —CHO | ethanal | Reduction; Tollens' test |
| Ketone | —CO— | propanone | Reduction; no Tollens' reaction |
| Carboxylic acid | —COOH | ethanoic acid | Esterification; neutralisation |
| Ester | —COO— | ethyl ethanoate | Hydrolysis (acid or base) |
| Amine | —NH2 | methylamine | Acts as a base; nucleophilic |
| Halogenoalkane | —X (X = F, Cl, Br, I) | chloroethane | Nucleophilic substitution |
IUPAC Naming: The Core Rules
The International Union of Pure and Applied Chemistry (IUPAC) provides a systematic naming system. Learning a handful of prefixes and suffixes unlocks the name of any straightforward organic molecule.
- Count the longest carbon chain and use the appropriate prefix: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
- Add a suffix that identifies the main functional group: -ane (alkane), -ene (alkene), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid), -oate (ester).
- Number the chain from the end nearest the principal functional group or the first point of difference, and indicate the position of any substituent or double bond with the lowest possible locant.
- Name substituents as prefixes in alphabetical order: methyl-, ethyl-, chloro-, bromo-, hydroxy-, and so on.
For example, CH3CH(OH)CH3 has a three-carbon chain with an —OH on carbon 2, so it is propan-2-ol. CH3COOCH2CH3 is an ester formed from ethanoic acid and ethanol, named ethyl ethanoate.
Isomerism
Molecules that share the same molecular formula but differ in how their atoms are arranged are called isomers. Structural isomers differ in which atoms are bonded to which: butan-1-ol and butan-2-ol are both C4H10O but the —OH sits on different carbons. Stereoisomers have the same connectivity but differ in the arrangement of atoms in space; cis-trans isomerism around a C=C bond is a common example at this level. Isomerism explains why two compounds with identical formulas can have very different boiling points, solubilities, and biological activities.
Summary
Carbon's four bonds and ability to chain to itself create an enormous variety of molecules organised by functional groups. Alkanes, alkenes, alcohols, carboxylic acids, esters, and amines each have characteristic reactions that follow directly from the nature of their functional group. IUPAC nomenclature gives every organic compound a unique, unambiguous name built from chain length, functional group suffix, and substituent prefixes. Mastering these patterns is the foundation for all further organic chemistry, from reaction mechanisms to biochemistry.