Carbon bonding, isomers and functional groups

Begin with carbon's bond count

Carbon commonly forms four covalent bonds in neutral organic molecules. A single bond counts once, a double bond twice and a triple bond three times when checking the valence of an atom. A carbon atom joined to three other carbons by single bonds therefore needs one further bond, often to hydrogen. Carbon's capacity to bond to itself supports chains, branches and rings. The geometry and bond types affect reactivity, so counting total atoms is only the beginning of understanding a structure.

A molecular formula records how many atoms of each element occur in a molecule. A structural formula adds information about which atoms connect and, depending on the drawing, how they are arranged. Condensed formulas compress repeated groups but retain useful connectivity clues. In skeletal drawings, unlabelled corners and line ends represent carbon atoms, and hydrogens bonded to those carbons are usually omitted. Heteroatoms such as oxygen or nitrogen are shown explicitly. Restore omitted hydrogens by making each ordinary neutral carbon's bond count reach four.

Separate hydrocarbon families

Hydrocarbons contain only carbon and hydrogen. An acyclic alkane has only carbon–carbon single bonds and, for a connected saturated chain, follows the formula CnH2n+2. An acyclic hydrocarbon with one carbon–carbon double bond and no other unsaturation has CnH2n. An acyclic hydrocarbon with one triple bond has CnH2n−2. The conditions matter. A ring also reduces the hydrogen count by two relative to the corresponding open saturated chain, so CnH2n alone does not prove that a compound contains a double bond.

Alkenes contain carbon–carbon double bonds and alkynes contain carbon–carbon triple bonds. Aromatic compounds such as benzene have a distinctive delocalised bonding pattern; they should not be treated as ordinary chains with interchangeable isolated double bonds. A compound can also contain several different functional groups. Rather than assigning one label from the formula alone, inspect the bonding features actually shown and say which family or group the question asks about.

Compare connectivity before declaring isomerism

Isomers have the same molecular formula but differ in structure. Structural isomers differ in connectivity. For example, a straight four-carbon chain and a branched four-carbon chain can share the same total atom counts while linking their carbon atoms differently. Rotating a page or bending the drawing of an unchanged chain does not create a new structural isomer. Follow the neighbours of each atom, not the direction in which a line points on paper.

Some molecules share connectivity but differ in spatial arrangement. Restricted rotation around a double bond can allow distinct arrangements when each double-bonded carbon has two different substituents. Other forms of stereoisomerism involve three-dimensional arrangements around suitable centres. Do not assume that every double bond produces such a pair: if one carbon has two identical substituents, that particular double bond cannot supply the required distinction. Keep formula, connectivity and spatial arrangement as three separate levels of information.

Recognise groups through their neighbours

An alcohol contains a hydroxyl group bonded to a saturated carbon. An ether has an oxygen linking two carbon-containing groups. A carbonyl is a carbon–oxygen double bond: its neighbouring groups distinguish families. An aldehyde has a carbonyl carbon bonded to at least one hydrogen, while a ketone has that carbon bonded to two carbon groups. A carboxylic acid contains a carboxyl group, combining a carbonyl and hydroxyl on the same carbon. An ester has a carbonyl carbon bonded to an oxygen that continues to a carbon group.

Amines contain nitrogen derived conceptually from ammonia by replacing hydrogens with carbon-containing groups. Amides contain nitrogen directly bonded to a carbonyl carbon, producing different behaviour from an ordinary amine. Functional groups influence polarity, hydrogen bonding and acid–base behaviour. Small alcohols can interact strongly with water, while a long non-polar carbon chain can reduce overall water solubility. The group is therefore a useful clue, but molecular size and the rest of the structure still matter. Always inspect the immediate neighbours of oxygen or nitrogen before naming the functional group.