Displayed formulae

A displayed formula shows every atom in a molecule and every bond between its atoms. Each atom is represented by its chemical symbol, and lines between the symbols show which atoms are bonded.

A single line represents a single bond. Two parallel lines represent a double bond. The lines show how atoms are joined; they do not necessarily show the molecule’s three-dimensional shape.

For example, the displayed formula of methane is:

The formula shows one carbon atom, $$\ce{C}$$, and four hydrogen atoms, $$\ce{H}$$. Each hydrogen atom is joined to the carbon atom by a single bond. The hydrogen atoms are not directly bonded to one another.

To interpret a displayed formula, identify the atoms from their symbols, then follow the lines to see which atoms are joined. To draw one, include every atom and show every bond between them.

General formulae of organic homologous series

A general formula describes the molecular formulae of members of a homologous series using a variable, $$n$$. For a chosen member, substituting a value for $$n$$ gives the number of atoms of each element in its formula.

The general formulae for four series are:

• Alkanes: $$\ce{C_{n}H_{2n+2}}$$
• Alkenes: $$\ce{C_{n}H_{2n}}$$
• Alcohols: $$\ce{C_{n}H_{2n+1}OH}$$
• Carboxylic acids: $$\ce{C_{n}H_{2n+1}COOH}$$

In the alkane and alkene formulae, $$n$$ is the total number of carbon atoms. In the alcohol and carboxylic acid formulae, $$n$$ counts the carbon atoms in the $$\ce{C_{n}H_{2n+1}}$$ portion. The $$\ce{OH}$$ group adds no carbon atoms, so an alcohol has $$n$$ carbon atoms in total. The $$\ce{COOH}$$ group adds one carbon atom, so a carboxylic acid has $$n+1$$ carbon atoms in total.

Substituting into general formulae

For an alkane with $$n=2$$, the number of hydrogen atoms is $$2(2)+2=6$$, giving $$\ce{C2H6}$$. For an alkene with $$n=2$$, it is $$2(2)=4$$, giving $$\ce{C2H4}$$.

For an alcohol with $$n=2$$, the formula is $$\ce{C2H5OH}$$. Counting all its atoms gives the molecular formula $$\ce{C2H6O}$$. For a carboxylic acid with $$n=2$$, the formula is $$\ce{C2H5COOH}$$. Its total molecular formula is $$\ce{C3H6O2}$$: two carbon atoms come from the first portion and one from the $$\ce{COOH}$$ group.

Recognising a general formula pattern

A molecular formula can be compared with a series’ general formula. For example, $$\ce{C4H8}$$ fits $$\ce{C_{n}H_{2n}}$$ when $$n=4$$, so it is consistent with an alkene. A match alone does not prove which series a substance belongs to: its structure is needed to confirm that.

Functional groups and homologous series

A functional group is an atom or group of atoms in a molecule that determines its characteristic chemical properties. Compounds containing the same functional group therefore tend to react in similar ways, even when other parts of their molecules differ.

A homologous series is a family of similar compounds whose members share the same characteristic functional group. For example, ethanol ($$\ce{CH3CH2OH}$$) and propanol ($$\ce{CH3CH2CH2OH}$$) have different carbon chains, but both contain the $$\ce{-OH}$$ functional group. They are both alcohols and have similar chemical properties.

Different functional groups characterise different families:

• Alcohols contain the $$\ce{-OH}$$ group.
• Carboxylic acids contain the $$\ce{-COOH}$$ group.
• Alkenes contain a carbon–carbon double bond, $$\ce{C=C}$$.

The shared functional group helps explain why members of the same series behave similarly, despite differences elsewhere in their molecules.

Saturated and unsaturated compounds

Saturation describes the bonds between carbon atoms in a compound’s molecules.

A saturated compound has only single carbon–carbon bonds. For example, ethane, $$\ce{CH3-CH3}$$, is saturated because its two carbon atoms are joined by a single bond.

An unsaturated compound has at least one carbon–carbon double or triple bond. For example, ethene, $$\ce{CH2=CH2}$$, is unsaturated because it contains a carbon–carbon double bond. Ethyne, $$\ce{CH#CH}$$, is also unsaturated because it contains a carbon–carbon triple bond.

The distinction depends on carbon–carbon bonds, not on bonds between carbon and other elements. For example, propanone, $$\ce{CH3-C(=O)-CH3}$$, contains a carbon–oxygen double bond, but its carbon–carbon bonds are both single. It is therefore saturated by this definition.