Relative Rates of Bromination of Hydrocarbons/Free-Radical Substitution
Methods and Background
The purpose of the experiment was to determine the relative rates of bromination for different
-butylbenzene, isopropylbenzene, ethylbenzene, cyclohexane, and
methylcyclohexane) and the effect of temperature on the rate of bromination of hydrocarbons.
Bond-breaking can occur in two ways: heterolysis and homolysis.
In heterolysis, both electrons
of a bond are transferred to the more electronegative atom (out of a pair of bonded atoms),
resulting in a cation and anion.
In homolysis, however, the two atoms have a low
electronegativity difference (or no difference at all), so when the bond breaks, one electron is
transferred to each atom, resulting in free radicals, atoms with unpaired electrons.
In the first
step of bromination of alkanes, known as initiation, heat (200-400 °C) or ultraviolet light (hν)
applied to a mixture of the alkane and bromine, causing the σ-bond of the molecular bromine
) to undergo homolysis, generating bromine radicals, Br·; Br
undergoes homolysis as there is
no difference in electronegativity between the identical bromine atoms.
Br· are formed in low
concentration, resulting in a net increase in the concentration of free radicals within the system.
In the next step of bromination of alkanes, propagation, a hydrogen atom is abstracted from the
hydrocarbon by a Br· to produce a new free radical, R·, and hydrobromic acid, H-Br.
to yield the alkyl bromide, R-Br, and Br·, thus regenerating the free radical needed for
In propagation, there is no net change in the concentration of radicals.
In the last step of bromination of alkanes, termination, the free radicals formed in the reaction
(Br· and R·) combine with each other to give different molecules (Br
, R-Br, and R-R), resulting
in a net decrease in radical concentration and a decrease in the rate of the reaction.