Chemical Properties and Reactions of Alkanes

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):

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·

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.

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.

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.