4.1.1 Basic concepts of organic chemistry

When you name an organic molecule, start by finding the longest carbon chain that contains the principal functional group or multiple bond you need to name. Number from the end that gives the important feature the lowest possible number, then add substituents with their positions.

I find it safer to build the name in pieces: carbon-chain stem, positions and substituents, then the suffix. If you draw the structure from your finished name and get back to the original molecule, you have a useful check.

You need the first ten carbon-chain stems to become automatic: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec-. Add -ane for the alkane; remove one H and the corresponding substituent ends -yl.

So methane gives methyl, ethane gives ethyl and propane gives propyl. Once you know the stems, a lot of organic naming becomes pattern recognition rather than separate memorisation.

• 1 meth-
• 2 eth-
• 3 prop-
• 4 but-
• 5 pent-
• 6 hex-
• 7 hept-
• 8 oct-
• 9 non-
• 10 dec-

You should be comfortable moving between several ways of showing the same molecule. A displayed formula shows every atom and bond; a structural formula groups atoms together; a skeletal formula leaves out carbon symbols and the hydrogens attached to carbon.

When you read a skeletal formula, every line end and corner is a carbon unless another atom is written there. I always do a quick valency check: carbon needs four bonds, oxygen normally two and hydrogen one.

There is quite a lot of vocabulary on this line, but only homologous series needs the formal definition. Learn it as a family of organic compounds with the same functional group and general formula, similar chemical reactions, and successive members differing by CH₂.

For the others, make sure you can use the words correctly in context: a functional group is the reactive part of a molecule; an alkyl group comes from an alkane after losing H; saturated compounds have no C=C or C≡C, while unsaturated compounds do. Aliphatic means non-aromatic; alicyclic means a non-aromatic ring; aromatic compounds contain an aromatic ring system.

Once you know the general formula, substitute the number of carbon atoms for n. For an alkane, CₙH₂ₙ₊₂, a six-carbon member has n = 6, so its molecular formula is C₆H₁₄.

Do the substitution before trying to name the compound from memory. It is especially useful when a question gives you an unfamiliar member or asks you to compare two homologous series.

Structural isomers have the same molecular formula but different structural formulae. In practice, that means the atoms are connected in different ways.

When you are asked to find the isomers, work systematically. Change the carbon skeleton first, then consider different positions for the functional group or multiple bond where the formula allows it. Keep checking the molecular formula so you do not quietly add or lose a carbon or hydrogen.

When a covalent bond breaks, ask where the bonding electron pair goes. In homolytic fission, each atom takes one electron from the bond, so you form radicals. In heterolytic fission, one atom takes both bonding electrons, so ions form.

That distinction tells you what sort of mechanism you are dealing with. Alkane halogenation uses radicals; nucleophilic and electrophilic mechanisms use electron-pair movement.

For this one, I’d learn the definition exactly: a radical is a species containing an unpaired electron. When you write Cl• or CH₃•, the dot represents that unpaired electron.

Keep the dot with the species throughout a radical mechanism. If it disappears from a propagation step without another radical being formed, something has gone wrong with the chain.

A full curly arrow shows the movement of an electron pair. When you read a mechanism, ask where those two electrons are before the step and where they go afterwards.

For bond formation, the arrow normally starts at a lone pair, negative charge or existing bond and finishes where the new bond forms. For heterolytic bond breaking, it starts on the bond and finishes on the atom taking both electrons.

Before you draw any arrow, mark the useful polarity. The electron-rich species or bond supplies the pair; the electron-poor centre receives it. Then make each curly arrow start exactly where the electron pair begins and finish at the atom or bond where it ends up.

I would check the structure after every arrow: normal valencies, sensible charges and any leaving group carrying the electron pair it has taken. That catches many mechanism errors before they spread into the next step.