When alkanes react with halogen (Cl2 or Br2), with heat or light, hydrogen atom of the alkane is replaced by halogen atom and alkyl halide is produced as product. This can be generally shown as:
The net reaction for halogenation seems straightforward, the mechanism is more complicated though, it go through multiple steps that include initiation, propagation and termination.
We will take the example of mono-chlorination of methane, for the discussion of reaction mechanism.
CH4 + Cl2 → CH3Cl + HCl
Mechanism for mono-chlorination of methane:
Initiation: Production of radical
With the energy provided from heat or light, chlorine molecule dissociates homolytically, each chlorine atom takes one of the bonding electrons, and two highly reactive chlorine radicals, Cl•, are produced.
Propagation: Formation of product and regeneration of radical
The propagation step involve two sub-steps. In the 1st step, the Cl• takes a hydrogen atom from the methane molecule (this is also called as hydrogen abstraction by Cl•), and C-H single bond breaks homolytically. A new σ bond is formed by Cl and H each donate one electron and HCl is produced as the side product. The CH3 radical, CH3•, the critical intermediate for the formation of product in next step, is formed as well.
In the 2nd step, the CH3• abstracts a chlorine atom to give final CH3Cl product, together with another Cl•. The regenerated Cl• can attack another methane molecule and cause the repetition of step 1, then step 2 is repeated, and so forth. Therefore the regeneration of the Cl• is particularly significant, it makes the propagation step self-repeat hundreds or thousands of time. The propagation step is therefore called the self-sustaining step, only small amount of Cl• is required at the beginning to promote the process.
Initiation and propagation are productive steps for the formation of product. This type of sequential, step-wise mechanism in which the earlier step generate the intermediate that cause the next step of the reaction to occur, is call the chain reaction.
The chain reaction will not continue forever though, because of the termination steps.
Termination: Consumption of radicals
The propagation steps are the core steps in halogenation. The energy level diagram helps to provide further understanding of the propagation process.
The 1st step in propagation is endothermic, while the energy absorbed can be offset by the 2nd exothermic step. Therefore the overall propagation is exothermic process and the products are in lower energy level the than reactants.
The reaction heat (enthalpy) for each of the propagation step can also be calculated by referring to the homolytic bond dissociation energies (Table 9.1). For such calculation, energy absorbed for bond-breaking step, so the bond energy was given “+” sign, and energy released for bond-forming step, and the “-” sign applied.
|A — B → A • + B •|
|F — F||159||H —Br||366||CH3 — I||240|
|Cl — Cl||243||H — I||298||CH3CH2 —H||421|
|Br — Br||193||CH3 — H||440||CH3CH2 —F||444|
|I — I||151||CH3 — F||461||CH3CH2 —Cl||353|
|H — F||570||CH3 — Cl||352||CH3CH2 — Br||295|
|H — Cl||432||CH3 — Br||293||CH3CH2 — I||233|
Table 9.1 Homolytic Bond Dissociation Energies for Some Single Bonds
Calculation reaction energy for the propagation step of mono-chlorination of methane (referring to the corresponding bond energies in Table 9.1.)
Step 1: H — CH3 + •Cl → CH3• + H — Cl
The H — CH3 bond broken, absorb energy, so +440 kJ
The H — Cl bond formed, release energy, so – 432 kJ
ΔH1 = +440 + (-432) = +8 kJ
Step 2: Cl — Cl + CH3• → CH3 — Cl + •Cl
The Cl — Cl bond broken, absorb energy, so +243 kJ
The CH3 — Cl formed, release energy, so -352kJ
ΔH2 = +243 + (-352) = – 109kJ
ΔHpropagation = ΔH1 + ΔH2 = +8 + ( – 109 ) = – 101kJ
The calculated data does match with the data from the energy diagram.
Reactivity Comparison of Halogenation
The energy changes for halogenation (substitution) with the other halogens can be calculated in the similar way, the results are summarized in Table 9.2.
H — CH3 + •X → CH3• + H — X
X — X + CH3• → CH3 — X + • X
H — CH3 + X — X → CH3 — X + HX
Table 9.2. Enthalpy of the Propagation Steps in Mono-halogenation of Methane (kJ/mol)
The data above indicate that the halogen radicals have different reactivity, fluorine is most reactive and iodine is least reactive. The iodine radical is very unreactive with overall “+” enthalpy, so iodine does not react with alkane at all. On the other side, the extreme high reactivity of fluorine is not a benefit either, the reaction for fluorine radical is so vigorous or even dangerous with lots heat released, and it is not practical to apply this reaction for any application because it is hard to control it. So Cl2 and Br2, with reactivity in the medium range, are used for halogen substitutions of alkanes. Apparently Cl2 is more reactive than Br2, and this leads to the different selectivity and application between the two halogens, more discussions in section 9.4.