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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 CnH2n+2 + (3n+1) O2 2(n+1) H2O + 2n CO2

In the absence of sufficient oxygen, carbon monoxide and/or soot can be formed, as shown, for example, for methane:

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

In presence of sufficient oxygen alkanes burn with a non-luminous flame. The standard enthalpy change of combustion, ΔH0, for alkanes increases by about 650 kJ/mol per CH2 group. The combustion properties of selected alkanes in air are listed in the following table:

  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, -78C) 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

2. Propagation: a hydrogen atom is pulled off from methane resulting in a methyl radical. Then the methyl radical pulls a chlorine atom from the Cl2 molecule, leaving another chlorine radical.

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

This results in the chlorinated product. This created radical will then go on to take part in another propagation reaction causing a chain reaction.

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

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

Methane and ethane yield randomly distributed products since all hydrogen atoms are equivalent, having an equal chance of being replaced. In higher alkanes the hydrogen atoms of CH2 or CH groups are preferentially replaced.

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.



Last Update: 2011-02-21