Chemical Properties and Reactions of Alkanes

Author: Hans Lohninger

Alkanes generally show low reactivity, because their C-C bonds are stable and cannot be easily broken. As they are inert against ionic or other polar substances they are also called "paraffins" (Latin "para + affinis" = "lacking affinity").

Gaseous alkanes are explosive when mixed with air, the liquid alkanes are highly flammable. The most common reactions occuring with alkanes are reactions involving free radicals (combustion, substitution cracking, and reformation).

Reactions with oxygen

All alkanes react with oxygen in a combustion reaction. The general equation for complete combustion is:

2 C_nH_2n+2 + (3n+1) O₂ 2(n+1) H₂O + 2n CO₂

2 CH₄ + 3 O₂ 2 CO + 4 H₂O
CH₄ + O₂ C + 2 H₂O

	Higher Heating Value [MJ/kg]	Air/Fuel Ratio	Adiabatic Flame Temperature [°C]	Ignition Temperature [°C]	Lower Explosive Limit [%]	Upper Explosive Limit [%]
Methane	55.536	17.195	1920	537	5.0	15.0
Ethane	51.926	15.899	1950	515	3.0	12.5
Propane	50.404	15.246	1970	466	2.1	10.1
n-Butane	49.595	14.984	1970	384	1.9	8.4
iso-Butane	49.478	14.984	1970	462	1.8	8.4
n-Pentane	49.069	15.323	2230	309	1.4	7.8
iso-Pentane	48.957	15.323	2230	420	1.3	9.2
Neopentane	48.797	15.323	2240	450	1.4	7.2
n-Hexane	48.769	15.238	2220	248	1.3	7.0
Neohexane	48.688	15.238	2235	425	1.2	7.6
n-Heptane	48.508	14.141	2200	228	1.0	6.0
n-Octane	48.374	15.093		220	0.9	3.2
iso-Octane	48.313	15.093		447	0.8	5.9

Reactions with halogens

The halogenation reactions of alkanes are quite different, depending on the involved halogen. While flourine reacts explosively with alkanes and can hardly be controlled, chlorine and bromine react satisfactorily (bromine much slower than chlorine), and iodine is unreactive. The calculated heats of reaction for the halogenation of hydrocarbons are (kcal/mol):

fluorine	-116
chlorine	-27
bromine	-10
iodine	+13

Free halogen radicals are the reactive species and usually lead to a mixture of products. For chlorine and bromine the free radicals have to be created by light and UV radiation, respectively.

The fluorination is difficult to control; the only successful direct fluorination of liquid or solid alkanes is performed at low temperatures (on dry ice, -78°C) with highly diluted fluorine (in helium). This procedure yields completely fluorinated compounds.

The chlorination of alkanes is a three step process which leads to a mixtue of products. It is shown for the chlorination of methane as an example:

1. Initiation: splitting a chlorine molecule into two chlorine atoms with unpaired electrons (free radical). This step is initiated by ultraviolet radiation (thus chlorination of alkanes does not occur in the dark):

Cl₂ 2 Cl·

CH₄ + Cl· CH₃· + HCl
CH₃· + Cl₂ CH₃Cl + Cl·

3. Termination: the chain reaction stops if two free radicals recombine:

Cl· + Cl· Cl₂
CH3· + Cl· CH₃Cl
CH₃· + CH₃· C₂H₆

Cracking

Cracking, the most important process for the commercial production of gasoline, breaks up heavy alkane molecules into lighter ones by means of heat and/or pressure and/or catalysts. It yields gasoline and gases such as methane, ethane, ethylene, and propane.

The thermal cracking process follows a homolytic mechanism forming (symmetric) pairs of free radicals, whereas the catalytic cracking follows a heterolytic (assymetric) breakage of bonds, resulting in ions (carbocations and hydride ions). The catalysts involved are solid acids, such as silica-alumina and zeolites.

As free radicals and carbocations are highly unstable, they quickly undergo C-C cleavage, chain rearrangements and hydrogen transfer.

Reforming

Catalytic reforming is used in the petroleum industry to create alicyclic and aromatic compounds from the C₆-C₁₀ gasoline fraction. Reforming is based on the heating of alkanes with hydrogen in the presence of catalysts. This finally results in aromatic compounds such as benzene, toluene, and xylenes which form the basis of a whole chemical industry.