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      Aliphatic Hydrocarbons

      Aliphatic hydrocarbon derivatives (also known as paraffins or paraffin derivatives) are straight chain or branched saturated organic compounds with composition C_nH_2n+2. Paraffin derivatives are present in large amounts in nature and low molecular weight paraffin derivatives are found in natural gas. Methane is the lowest member of the paraffin series of hydrocarbons. With the increase in size of molecule, several hydrocarbons may exist. Such hydrocarbons have different properties and are known as isomers. On the other hand, olefin hydrocarbon derivatives have carbon atoms joined by two bonds.

      Hydrocarbon derivatives are classified into chemical families according to their structure. All structures are members of the homologous series of even hydrogen numbers. The chemical formula is C_nH2n +2 or lower in hydrogen content C_nH_2n. The carbon-carbon molecule chains have different chemical bonding arrangements such as (i) saturated hydrocarbon derivatives that are linked by carbon-carbon single bonds and are given the suffix ane and (ii) unsaturated hydrocarbon derivatives that are linked by multiple bonds double bond C=C (suffix ene) or triple C≡C(suffix yne). However, the same molecule can contain several multiple bonds – if there are two low sets of the double bond, the suffix is diene.

Normal alkanes (straight-chain paraffins) consist of a chain of carbon atoms. Each carbon atom is linked to four atoms, which can be either carbon or hydrogen, their general formula C_nH_2n+2 . The carbon skeleton can be structured as straight chains as are the normal paraffin, CH₃(CH₂)_nCH₃. The boiling points increase with the number of carbon atoms. With the low carbon numbers, the addition of a carbon increases the boiling point to approximately 25°C. Further additions give smaller increase. At the same time, the density increases with the molecular weight 0.626 kg/L for pentane, and 0.791 kg/L for pentacosane; on the other hand, the density is always much lower than 1. The normal alkanes from C₁ to C₄ are colorless gases; C₅ to C₁₇ colorless liquids; and from C₁₈ onwards, colorless solids. Other physical properties, such as melting point, density, and viscosity, also increase in the same way as boiling point (Table A-13). There is a relationship between physical properties and chemical composition. The variation in boiling point of compounds is due to different intermolecular forces such as hydrogen bonding. The alkanes are insoluble in water.

Table A-13 Physical properties of n-paraffins.