IB Chemistry Sub-topic R3.3 Notes

Structure of alkanes

The third reaction type you need to know about is electron sharing, which typically takes the form of free radical substitution. This occurs in alkanes because they have no functional group and consist purely of C-C and C-H bonds. As a result, their general formula is C_nH_2n+2.

Due to their structure, alkanes are non-polar so they only have weak London dispersion forces, making them volatile and insoluble in water. However, the high strength of the C-C and C-H bonds makes them unreactive.

Thus, the only reactions they undergo are with very reactive species called radicals. Thus, during free radical substitution, alkanes reacts with halogens in the presence of UV light to form a halogenoalkane and hydrogen halide. For example:

C₂H­₆ + Cl₂ → C₂H₅Cl + HCl

Free Radical Substitution of Alkanes 

You need to know the process of free radical substitution in more detail. It occurs in three stages:

1. Initiation - during this, the halogen absorbs UV to form two halogen radicals, called homolytic fission. The movement of electrons is signified by curly half-headed arrows.

   X₂ → 2X^•

2. Propagation - during this, radicals react with the alkane, creating more radicals.

X^• + CH₄ → H₃C^• + HX

H₃C^• + X₂ → CH₃X + X^•

3. Termination - eventually, the radicals react with one another to cancel out.

2X^• → X₂

H₃C^•+ H₃C^• → C₂H₆

X^• + H₃C^• → CH₃X