James G. Speight - Encyclopedia of Renewable Energy

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, CH 3(CH 2) nCH 3. 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 C 1to C 4are colorless gases; C 5to C 17colorless liquids; and from C 18onwards, 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-13Physical properties of n-paraffins.

Isoparaffins ( Table A-14) are paraffins in which branching is present, usually at the number 2 carbon atom, although branching can take place at a different position in the chain, although such molecules are not strictly isoparaffins. Isoparaffins (branched paraffins) have a boiling point lower than normal paraffin with same number of carbon atoms, and generally, the greater the branching, the lower the boiling point ( Table A-14).

Table A-14Physical properties of selected branched paraffins.

Octane number is a measure of the ability of a fuel (gasoline) to avoid knocking. The test engine is adjusted to give knock from the fuel rated. Then, various mixtures of isooctane (2,2,4-trimethylpentane) and n-heptane are used to find the ratio of the two reference fuels that will give the same intensity knock as that from unknown fuel. Defining isooctane as having an octane number of 100 and n-heptane as 0, the octane number is the volumetric percentage of isooctane in heptane that matches knock from the unknown fuel is reported as octane number of the fuel.

Unsaturated aliphatic hydrocarbon derivatives (olefin derivatives) ( Table A-15) have one or more double bonds between carbon atoms. Also, olefin derivatives have different types of isomers – for example, butene (C 4H 8) isomers have many arrangements which include 1-butene (CH 3CH 2CH=CH 2), cis 2-butene (cis-CH 3CH=CHCH 3), trans 2-butene (trans-CH 3CH=CHCH 3), and isobutene [(CH 3) 2CH=CH2].

Table A-15Physical properties of selected olefins.

Owing to the presence of a double bond, the alkene undergoes a large number of addition reactions, but under special conditions, they also undergo substitution reactions. Alkene is readily hydrogenated under pressure in the presence of a catalyst. Platinum and palladium are effective at room temperature. Addition polymerization occurs between molecules containing double or triple bonds. The following are some reactions of aliphatic hydrocarbons to enhance its properties.