Hydrocarbons

The members of this group of hydrocarbons are distinguished by possessing the general molecular formula C$_n$H$_{2n+2}$, where n is 1, 2, 3, etc., for successive members of the group. The first member of the series (n = 1) is methane (CH$_4$) and the second (n = 2) is ethane (C$_2$H$_6$).

Both are gases at room temperature and pressure. This general formula can be used to work out the formula of any other alkane if we know the number of carbon atoms it consists of. The following table gives the molecular formula and name of the first few alkanes, plus an indication of some of their physical properties.

Table for Formulae and physical properties of some alkanes

Name	Molecular formula	Melting point (°C)	Boiling point (°C)	Density (g/cm³)
Methane	CH$_4$	–183	–162	gas
Ethane	C$_2$H$_6$	–172	–89	gas
Propane	C$_3$H$_8$	–188	–42	gas
Butane	C$_4$H$_{10}$	–135	–1	gas
Pentane	C$_5$H$_{12}$	–130	36	0.626
Hexane	C$_6$H$_{14}$	–95	69	0.659
Heptane	C$_7$H$_{16}$	–91	98	0.684
Octane	C$_8$H$_{18}$	–57	126	0.703
Nonane	C$_9$H$_{20}$	–54	151	0.718
Decane	C$_{10}$H$_{22}$	–30	174	0.730

Structure of alkanes

In the alkanes, all carbon atoms show a covalency of four. Table 2.3 shows the structures of the first five members of the alkanes.

Structure of alkanes

Explain the concept of isomerism

Isomers are organic compounds with the same molecular formula but different structural formulae.

Isomerism is the occurrence of two or more compounds with the same molecular formula but different molecular structures (structural formulae).

Isomers of the same molecular formula have different physical and chemical properties because of structural differences.

Alkenes are members of a homologous series of hydrocarbons with general molecular formula C$_n$H$_{2n}$ where n = 2, 3, 4, etc. The common structural feature is the presence of a carbon–carbon double bond:

Alkenes are characterized by possessing one or more carbon to carbon double bond(s) (C=C) in the carbon chain. Alkenes are unsaturated hydrocarbons because it is possible to break the double bond and add extra atoms to the parent molecule.

The table showing the structure of alkenes

Isomerism

Alkenes, like alkanes show branching isomerism, for example CH$_3$–CH=CH–CH$_3$ (pent-1-ene) can be branched into (3-methylbut-1-ene) as follows:

Other examples

These are homologous series of hydrocarbons with the general formula C$_n$H$_{2n-2}$. The common structural feature is the presence of a carbon-carbon triple bond (C≡C). There is no member for n = 1 because, as for the alkenes, there needs to be at least two carbon atoms present to have a triple bond.

Table below shows the first five members of the alkynes

Table below shows structure of alkynes

Nomenclature of alkynes

Isomerism

Alkynes show branching isomerism and positional isomerism. The alkyne with the molecular formula, C$_5$H$_8$, shows chain branching isomerism, for instance,