Chemical Properties of Alkanes

Alkanes are quite unreactive towards most reagents -- a box notes that 'paraffin', the older family name for alkanes, comes from the Latin for 'little activity' -- but under favourable conditions they undergo combustion, halogenation, aromatisation, reaction with steam, pyrolysis (cracking) and isomer …

A combustion reaction is the reaction of a substance with oxygen, releasing heat and (usually) light as a flame. With sufficient oxygen, alkanes burn completely to carbon dioxide and water, e.g. CH4 + 2O2 --> CO2 + 2H2O, deltaH-standard = -890.4 kJ (highly exothermic). With an insufficient oxygen supply, alkanes burn incompletely: 2CH4 + 3O2 --> 2CO + 4H2O (carbon monoxide) or CH4 + O2 --> …

A halogenation reaction substitutes one or more hydrogens of an alkane with halogen atoms. Chlorination and bromination are the two widely used halogenation reactions -- fluorination is too violent and iodination too slow to be practical. Methane reacts with chlorine in the presence of light or heat in a stepwise substitution: CH4 + Cl2 --> CH3Cl (methyl chloride) + HCl, then successively to CH2Cl2 (methylene chloride), CHCl3 (chloroform) and finally CCl4 (carbon tetrachloride), each step releasing HCl. The reaction proceeds by a FREE-RADICAL CHAIN MECHANISM in three stages. Chain initiation: UV light (or heat) causes homolytic fission of the weak Cl-Cl bond (chosen over the stronger C-C/C-H bonds) into two chlorine free radicals, Cl2 --hv--> 2 Cl-dot. Chain propagation: (a) a chlorine radical abstracts a hydrogen from methane, breaking the C-H bond and generating a methyl radical plus HCl, Cl-dot + CH4 --> CH3-dot + HCl; (b) the methyl radical attacks another Cl2 molecule to give chloromethane and regenerate a chlorine radical, CH3-dot + Cl2 --> CH3Cl + Cl-dot; this regenerated chlorine radical cycles back into step (a), setting up a self-sustaining chain. Chain termination: once reactants are largely consumed, the chain ends when two radicals combine -- Cl-dot + Cl-dot --> Cl2, CH3-d …

Alkanes with six to ten carbon atoms convert into homologues of benzene at high temperature over a catalyst, a process called aromatisation, proceeding by simultaneous cyclisation and dehydrogenation. For example, n-hexane passed over Cr2O3 supported on alumina at 873 K gives …

Methane reacts with steam at 1273 K over a nickel catalyst, decomposing to carbon monoxide and hydrogen gas: CH4(g) + H2O(g) --Ni, 1273K--> CO(g) + 3H2(g). Producing H2 gas from methane this way is called the steam-reforming process, a well-established indus …

Pyrolysis is the thermal decomposition of an organic compound into smaller fragments in the absence of air, under heat ('pyro' = fire, 'lysis' = separating); for alkanes this is also called cracking. Passing alkane vapours (in the absence of air) through a red-hot metal tube breaks them down into simpler hydrocarbons -- for example, at 773 K, a higher alkane can crack to give a mixture of a smaller alkane and an alkene, or two smaller alkanes and alkenes together (e.g. 2 CH3-CH3 --773K--> CH2=CH2 + 2CH4). The exact product mix depends on the alkane's nature, the temperature, the pressure and whether a catalyst is present; the ease of cracking increases with molecular weight and with branching. Cracking is central to t …

Isomerisation converts a compound into one of its isomeric forms. Normal (straight-chain) alkanes can be converted into their branched isomers in the presence of AlCl3 and HCl at 298 K (e.g. n-butane --AlCl3/HCl, 298K--> isobutane). This process is of great industrial importance, since the quality (octane rating) of gasoline is improved by …