Alkanes are the saturated hydrocarbons. They are also known as paraffins (Latin: little affinity or reactivity). The general formula of an acyclic alkane is CnH2n+2. Alkanes having one cyclic ring have the general formula CnH2n (where n is more than 3). Cycloalkanes having two rings have the general formula CnH2n-2.
Alkanes have only single bonds (sigma-bonds) between carbon-carbon and carbon-hydrogen atoms. The carbon atoms in alkanes are sp3 hybridized. The four bonds of carbon in alkanes are directed towards the corners of a regular tetrahedron.
For example, carbon in methane (CH4) is located in the centre of a regular tetrahedron and four carbon-hydrogen bonds are directed towards the corner of the tetrahedron. Each face of the tetrahedron is a triangle and there is hydrogen at each corner of the triangle.
Structural Formulae and Shape of Alkanes
Structural formulae clearly indicate the mode of linkage of each atom with other atoms in a molecule. There are several ways of writing structural formulae. As an example, the electron and graphic formulae of ethane are given below.
- Electron-dot formulae are ones in which the bonding electrons are represented by dots.
- Graphic formulae are those in which a dash between the bonded atoms indicates the pair of shared electrons.
- Condensed structural formulae, such as CH3CH2CH3 for propane, CH3CH3 for ethane, etc. are also used. These are easy to write.
- Spatial formulae Structural formulae of alkanes indicate the mode of linking of various hydrogen and carbon atoms in the alkane molecule. Since the molecule is three dimensional, therefore, it is difficult to imagine it on a two dimensional surface like paper, etc. Spatial formulae are the attempts to help us imagine three-dimensional formulae on a two dimensional surface.