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Alkyl Halides

Preparation of Alkyl Halides

The main reaction of alkanes is radical halogenation, which is the addition of Br2, Cl2, or NBS to an alkane in the presence of heat or light. In radical halogenation, a hydrogen is replaced with a halogen.

Radical reactions are reactions that have an initiation step, one or more propagation steps, and a termination step. During an initiation step, a covalent bond breaks and produces two radical species. Propagation step is a step in a radical chain reaction where a reactive free radical reacts with a stable molecule to form a new free radical. Termination step is a step in a radical reaction where two radicals combine to form a covalent bond and no new radicals are formed.

Breaking bonds requires energy, which is called bond dissociation energy. When bonds are broken so that the two bonding electrons divide equally between the two atoms, it is called homolytic cleavage. The resulting fragments have unpaired electrons called radicals. The unpaired electrons make the fragments very reactive and difficult to isolate. However, at low concentrations, radicals serve as intermediates to the reaction.

When chlorine is added to an alkane in the presence of light or heat (as a catalyst), a radical reaction occurs. In the initiation step, ultraviolet (UV) light fractures the chlorine molecule homolytically, creating two chlorine free radicals, atoms with an unpaired electron. In the propagation step, the free radical abstracts (or removes) a hydrogen to form hydrogen chloride and a methyl radical. In the termination step, the product (CH3Cl) forms when two radicals combine to form a covalent bond.

Radical Halogenation of Methane

In a free radical reaction, a hydrogen is replaced with a halogen.
Chlorine radicals form readily, so they are not always selective. Ideally, a chlorine radical should abstract a hydrogen from the most substituted carbon to form the most substituted (stable) carbon radical. When that radical is halogenated, the most substituted hydrogen has been substituted with a halogen. However, chlorine will often abstract the most accessible or most prevalent type of hydrogen, leading to a product where the most substituted hydrogen has not been replaced by the chlorine but, instead, a less substituted hydrogen has been replaced. Additionally, since chlorine is so reactive, it will often perform multiple substitutions on an alkane substrate.

Radical Halogenation of 2-methylbutane

A mixture of products is often formed when doing radical halogenation because of chlorine's reactivity. In addition to the product where the most substituted hydrogen has been replaced, additional monosubstituted products along with polysubstituted products are formed.
While chlorine is very reactive due to exothermic formation of HCl in the propagation step, the formation of HBr in the propagation step is endothermic. For radical bromination, there are two reagent combinations:
  • Br2, heat, or light (h𝜈)
  • NBS, heat, or light (h𝜈)
Since the bromine radical forms at a slower rate, the reaction is more of a thermodynamic reaction producing the most stable product, which is the substitution of the most substituted hydrogen. Therefore, the radical bromination reaction seems to be more selective and gives mostly a monosubstituted product where the most substituted hydrogen is replaced with bromine.

Radical Bromination of 2-methylbutane

Radical halogenation with bromine yields a more predictable product that shows higher selectivity for substitution of the most substituted hydrogen with a bromine.