How to Name Almost Any Organic Compound Without Memorising It

The LPG in a kitchen cylinder, the ethanol in hand sanitiser, the sugar in chai and the DNA in every cell are all built on chains and rings of carbon. Few elements can form such an enormous variety of stable compounds.

To work with so many compounds, chemists need clear ways to draw them, group them and name them.

This part covers carbon's bonding, ways of representing structures, classification and functional groups, and IUPAC nomenclature.

**Carbon has four valence electrons and forms four covalent bonds (tetravalence), bonds strongly to other carbon atoms to form chains and rings (catenation), and uses sp${}^3$, sp${}^2$ or sp hybrid orbitals, giving tetrahedral, planar or linear shapes.

- sp${}^3$** — four single bonds, tetrahedral, $109.5°$ — methane, ethane
- **sp${}^2$** — one double bond, trigonal planar, $120°$ — ethene
- sp — one triple bond or two double bonds, linear, $180°$ — ethyne

Worked example 1 — assigning hybridisation. In CH${}_2$=CH–C$\equiv$CH, reading left to right, the carbons are **sp${}^2$, sp${}^2$, sp, sp.

Worked example 2 — bond lengths.** C–C $154$ pm, C=C $134$ pm, C$\equiv$C $120$ pm — more s-character gives shorter bonds.

Worked example 3 — counting bonds. Propene, CH${}_3$–CH=CH${}_2$, has $8$ sigma bonds and $1$ pi bond.

An everyday example. Butane in an LPG cylinder has four carbons in a chain, while candle wax has chains many times longer — catenation at work.

The substance. A higher s-character makes carbon more electronegative, which is why the hydrogen on ethyne is slightly acidic while that on ethane is not.

A complete structural formula shows every bond; a condensed formula groups atoms along the chain; a bond-line formula shows only the carbon skeleton as a zig-zag with heteroatoms and their hydrogens; and wedge-dash formulas show bonds pointing towards or away from the viewer.

Worked example 1 — butan-1-ol.

- Complete: all four carbons, every C–H, C–C, C–O and O–H bond drawn
- Condensed: CH${}_3$CH${}_2$CH${}_2$CH${}_2$OH, or CH${}_3$(CH${}_2$)${}_3$OH
- Bond-line: a zig-zag of four vertices with OH at one end

Reading bond-line formulas. Every end and every bend is a carbon; add enough hydrogens to give each carbon four bonds.

Worked example 2. A zig-zag with five points and an OH on the second point is CH${}_3$CH(OH)CH${}_2$CH${}_2$CH${}_3$ — pentan-2-ol.

Worked example 3 — three dimensions. For methane drawn in wedge-dash form, two C–H bonds lie in the plane of the page, a solid wedge points towards you and a dashed wedge points away.

An everyday example. A metro route map shows only the stations and turns, not every building — just as a bond-line formula shows only the carbon skeleton.

The substance. Heteroatoms such as O, N and Cl, and the hydrogens attached to them, are always drawn in a bond-line formula.

**Organic compounds are acyclic (open-chain) or cyclic — alicyclic, aromatic or heterocyclic; a homologous series is a family with the same functional group whose successive members differ by CH${}_2$; and a functional group is the atom or group that gives a compound its characteristic reactions.

Classification:

- Acyclic: butane, ethanol
- Alicyclic: cyclohexane
- Aromatic: benzene
- Heterocyclic: rings containing atoms other than carbon, such as pyridine

Common functional groups:** –OH (alcohol), –CHO (aldehyde), >C=O (ketone), –COOH (carboxylic acid), –COOR (ester), –NH${}_2$ (amine), –X (halide), –CN (nitrile).

Worked example 1 — a homologous series. Methanol CH${}_3$OH ($32$ u), ethanol C${}_2$H${}_5$OH ($46$ u), propan-1-ol C${}_3$H${}_7$OH ($60$ u) — each differs by CH${}_2$, or $14$ u.

Worked example 2 — spotting groups. Lactic acid, CH${}_3$CH(OH)COOH, contains an alcohol group and a carboxylic acid group.

An everyday example. Vinegar contains acetic acid (–COOH), and nail-polish remover contains acetone (>C=O) — different functional groups, very different behaviour.

The substance. Members of a homologous series react alike, but their physical properties, such as boiling point, change steadily with chain length.

Choose the longest carbon chain containing the principal functional group, number it to give that group the lowest locant, name substituents as prefixes in alphabetical order with their positions, and add the suffix of the principal group — which follows the priority acid > ester > aldehyde > ketone > alcohol > amine.

Worked example 1 — branched alkanes.

- CH${}_3$CH(CH${}_3$)CH${}_2$CH${}_3$ — 2-methylbutane
- CH${}_3$CH(C${}_2$H${}_5$)CH${}_2$CH(CH${}_3$)CH${}_3$ — the longest chain has six carbons, numbered to give locants $2$ and $4$: 2,4-dimethylhexane

Worked example 2 — one functional group.

- CH${}_3$CH${}_2$CH(OH)CH${}_3$ — butan-2-ol
- CH${}_2$=CHCH${}_2$OH — prop-2-en-1-ol; the OH takes the lowest number

Worked example 3 — polyfunctional and cyclic.

- OHC–CH${}_2$–CH${}_2$–COOH — the acid is principal, so the aldehyde becomes the prefix "oxo": 4-oxobutanoic acid
- A cyclohexane ring with one OH — cyclohexanol

Worked example 4 — name to structure. 2,2-dimethylpropane is a central carbon joined to four CH${}_3$ groups, C(CH${}_3$)${}_4$.

An everyday example. "Rubbing alcohol" and "nail-polish remover" are common names; propan-2-ol and propanone tell a chemist the exact structure.

The substance. "Di" and "tri" are ignored when alphabetising substituents, so ethyl comes before dimethyl.