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Structure of Alkanes

The molecules of alkanes contain only single covalent bonds between carbon-carbon or carbon-hydrogen atoms. The formation of only single bonds requires that each carbon atom in an alkane be sp3-hybridis&d. These hybrid orbitals of each carbon atom are directed towards the comers of a regular tetrahedron, the carbon atom being at the centre of the tetrahedron.

For example, let us study the structure of methane molecule. ·

 

STRUCTURE OF METHANE

The carbon atom in methane molecule is sp3 hybridised. The four sp3 hybrid orbitals of carbon overlap with Is orbitals of four hydrogen atoms forming four C-H sigma bonds. Due to sp3 hybridisation methane is tetrahedral molecule. In methane molecule carbon is at the centre of the tetrahedron, whereas hydrogen atoms are at the comers. H-C-H bond angles are 109.50 and C-H bond length is 109 pm (Fig. 40.1).

The carbon atoms in the alkane molecule with more than two carbon atoms do not lie along a straight line but form a zig-zag pattern. This is due to the sp3-hybrid state of the carbon atoms. Due to sp3-hybrid state of carbon atoms the C-C-C bond angle in higher alkanes is expected to be around 109°. It is clear from the model of propane shown in Fig. 40.2

All the carbon atoms in cycloalkanes are also sp3-hybridised because each carbon atom is directly attached to four other atoms. Thus, alkanes have tetrahedral structure around each carbon. The average C-C and C-H bond distances are 154 pm and 110 pm respectively.

 

CLASSIFICAT10N OF CARBON TOMS

The carbon atom in a molecule may be classified as primary (1°), secondary (2°), tertiary (3°) or quaternary (4°) carbon atom depending upon whether it is directly linked to one, two, three or four carbon atoms respectively. Terminal carbon atoms are always primary. Primary, secondary, tertiary or quaternary carbon atoms in a molecule are denoted by the letters, p, s, t and q respectively. This is illustrated in the following formula:

There are six primary carbons, two secondary carbons, two tertiary carbons and one quaternary carbon atom in the above formula. It may be noted that hydrogen atoms in a molecule can also be termed as primary, secondary or tertiary hydrogens depending upon whether they are linked to primary, secondary, or tertiary carbon atom respectively.