The alkanes are the simplest of the hydrocarbons, consisting of carbon, hydrogen and single bonds only. Chemically, although they are fairly unreactive, they are flammable and used in many applications as a readily available source of heat and energy. They are obtained from the petrochemicals industry by fractional distillation of crude oil and cracking.
Syllabus reference R3.3.3Reactivity 3.3.3 - Radicals take part in substitution reactions with alkanes, producing a mixture of products.
- Explain, using equations, the propagation and termination steps in the reactions between alkanes and halogens.
Guidance
- Reference should be made to the stability of alkanes due to the strengths of the C–C and C–H bonds and their essentially non-polar nature.
Tools and links
- Reactivity 2.2 - Why are alkanes described as kinetically stable but thermodynamically unstable?
Saturated hydrocarbons
Alkanes are the simplest of the hydrocarbons, comprising carbon and hydrogen chains with only single bonds. The alkanes have no functional groups and only single bonds. This means that they are relatively unreactive, due to the strength of the carbon-carbon and the carbon-hydrogen bonds.
- carbon-carbon bond = 348 kJ mol-1
- carbon-hydrogen bond = 414 kJ mol-1
Another factor that contributes to the low reactivity of the alkanes is their lack of polarity. The electronegativity of carbon and hydrogen is similiar and the bonding electrons are shared evenly between them in single bonds. No dipoles are formed, meaning that there are no regions of partial positive, or negative charge on the hydrocarbon chain that can invite attack by nucleophiles or electrophiles.
Halogenation
In the presence of ultraviolet light, alkanes react with halogens via a free radical mechanism. Free radical processes are chain reactions that proceed in three stages:
- 1 Initiation
- 2 Propagation
- 3 Termination
In the initiation stage, chlorine molecules undergo homolytic fission and are broken into free radicals by UV light. The single electron on the free-radical is shown as a black dot.
Cl2 → 2Cl•
In the propagation stage the chlorine free radicals react with the alkane, making alkyl (methyl) free radicals and hydrogen chloride.
Cl• + CH4 → •CH3 + HCl
The methyl free radical can then react with another chlorine molecule, making a haloalkane and propagating the reaction further:
•CH3 + Cl2 → Cl• + CH3Cl
The chloride free radical can then react with another alkane molecule and continue the process.
The final stage in a free radical reaction is termination, when two free radicals collide to end the chain reaction:
Cl• + •CH3 → CH3Cl
Although this reaction proceeds easily, it is of limited use in synthesis, as a mixture of products is formed. Free radicals react with the first molecule that they encounter, meaning that several by-products are also formed. To try to limit the number of by-products, the gases are mixed in an appropriate ratio, according to the stoichiometry of the equation. In the case of the reaction between methane and chlorine in the presence of ultraviolet light, the equation is:
CH4 + Cl2 → CH3Cl + HCl
The stoichiometry of the reaction suggests that the ideal molar ratio of chlorine to methane is 1:1