Alcohols

• meth- means 1 carbon;
• eth- means 2 carbons;
• prop- means 3 carbons;
• but- means 4 carbons.

In propan-1-ol and butan-1-ol, the number 1 means that the –OH group is attached to carbon 1, at the end of the chain. These are the forms of propanol and butanol required here.

Formulae give different amounts of information

A molecular formula counts the atoms in one molecule. A structural formula also indicates their order. For example, ethanol has molecular formula $C_2H_6O$, while $CH_3CH_2OH$ shows that its two carbon atoms form a chain ending in –OH.

The required formulae are:

Neighbouring members gain one carbon atom and two hydrogen atoms: each step adds $CH_2$. The same pattern can be written as $C_nH_{2n+1}OH$, where $n$ is the number of carbon atoms.

Drawing every covalent bond

A covalent bond is a shared pair of electrons and is represented by a line. To show all covalent bonds, use a displayed formula rather than merely writing a condensed formula.

Three bonding rules let you build and check each structure:

• carbon forms four bonds;
• oxygen forms two bonds;
• hydrogen forms one bond.

Start with the required number of carbon atoms in a single chain. Join adjacent carbons with single bonds, attach an oxygen to the end carbon, and attach a hydrogen to that oxygen. Finally, add enough hydrogen atoms to give every carbon four bonds. For methanol there is only one carbon, so it bonds to three hydrogens and to oxygen; the oxygen also bonds to hydrogen.

Notice that the O–H bond must be drawn: writing an O beside an H without a bond does not show every covalent bond. Likewise, every C–H bond must appear in a fully displayed structure.

Recognising the pattern

Methanol begins with one carbon. Extending its carbon chain by one $CH_2$ unit produces ethanol; repeating this gives propan-1-ol and then butan-1-ol. Although the diagrams become longer, the bonding checks never change. Counting four bonds around each carbon, two around oxygen and one around each hydrogen is a reliable way to find missing or extra bonds.