Alkanes have the general formula CnH2n+1. Alkanes are non- polar molecules and only contain carbon and hydrogen. The bonding is covalent and there are two types of bonds C-C and C-H. All the bonds are single covalent bonds. The atoms are held together by the sigma (σ) orbitals. The bond is formed by the overlap of the two sigma orbitals. The pair of sharing electrons in the orbital attract both nuclei binding them together in a sigma bond. Alkanes form a tetrahedral shape with four sigma covalent bond around each carbon. All bond angles are 109.5o. As each bond is a single covalent bond the molecules can rotate freely around each C-C bond.
The volatility of the alkanes decreases with increasing the number of carbon atoms.
Definition “volatility is the ease a liquid turns into a vapour”. This trend is shown in the table below.
Alkane Boiling point (k)
Methane 109
Ethane 185
Propane 231
Butane 273
Pentane 309
As number of side chains and branches is increased on the carbon chain the boiling point decreases. This is because the long flexible chains are more easily packed closer together; the closer they are the stronger the intermolecular forces thus higher boiling points. This trend is shown below.
Alkane Boiling Point (k)
Pentane 309
2-methylbutane 301
2,2-dimethylpropane 283
Below is a 3D reresentation of methane.
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