Free Radical Mechanism: MDCAT Chemistry notes

Bond fission and reactive intermediates

• Homolytic fission: the bond breaks evenly and each atom keeps one electron. This produces free radicals, species with an unpaired electron such as $\mathrm{Cl^\bullet}$ and $\mathrm{CH_3^\bullet}$. Homolysis of a C–H bond in an alkane gives an alkyl free radical.
• Heterolytic fission: the bond breaks unevenly and one atom takes both electrons. This produces ions.

Intermediate	Description
Carbocation	Carbon bearing a positive charge, bonded to three atoms or groups ($sp^2$, planar)
Carbanion	Carbon bearing a negative charge and a lone pair
Free radical	Neutral species with one unpaired electron

Nucleophiles and electrophiles

• Nucleophiles ("nucleus-loving") are electron-pair donors. They are negative ions or neutral molecules with a lone pair, e.g. $\mathrm{OH^-}$, $\mathrm{CN^-}$, $\mathrm{NH_2^-}$, $\mathrm{CH_3^-}$, $\mathrm{H_2O}$, $\mathrm{NH_3}$.
• Electrophiles ("electron-loving") are electron-pair acceptors, e.g. $\mathrm{H^+}$, $\mathrm{NO_2^+}$, $\mathrm{Br^+}$, and electron-deficient $\mathrm{BF_3}$ and $\mathrm{AlCl_3}$.

Halogenation of alkanes

Alkanes react with chlorine or bromine in the presence of ultraviolet light (diffused sunlight) or heat. The reaction is a substitution: a hydrogen atom is replaced by a halogen atom. It goes by a free radical chain mechanism, called free radical substitution.

$$\mathrm{CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl}$$

1. Initiation

UV light splits the halogen molecule homolytically:

$$\mathrm{Cl_2 \xrightarrow{h\nu} 2Cl^\bullet}$$

Radicals are created and none are used up.

2. Propagation (chain steps)

$$\mathrm{Cl^\bullet + CH_4 \to CH_3^\bullet + HCl}$$ $$\mathrm{CH_3^\bullet + Cl_2 \to CH_3Cl + Cl^\bullet}$$

In each step one radical is consumed and another is produced, so the chain keeps going. The attack of $\mathrm{Cl^\bullet}$ on methane to give the methyl radical is a propagation step, not initiation.

3. Termination

Two radicals combine to form a stable molecule, which ends the chain:

$$\mathrm{Cl^\bullet + Cl^\bullet \to Cl_2}\qquad \mathrm{CH_3^\bullet + Cl^\bullet \to CH_3Cl}\qquad \mathrm{CH_3^\bullet + CH_3^\bullet \to C_2H_6}$$

Traces of ethane in the product show that methyl radicals are present.

With excess chlorine, substitution continues to give $\mathrm{CH_2Cl_2}$, $\mathrm{CHCl_3}$ and $\mathrm{CCl_4}$. The order of reactivity of the halogens is $\mathrm{F_2 > Cl_2 > Br_2 > I_2}$.

Common MDCAT traps

• Cl• + CH₄ is propagation. Initiation is only the splitting of $\mathrm{Cl_2}$.
• Termination produces molecules, not radicals or ions.
• The reaction is substitution, not addition. Addition is typical of alkenes.
• The condition is UV light or heat, not low temperature or high pressure.
• $\mathrm{BF_3}$ is an electrophile (it accepts a pair), not a nucleophile.

Quick revision

• Homolysis gives free radicals and heterolysis gives ions.
• The three steps are initiation, propagation and termination.
• Alkane halogenation is free radical substitution in UV light.
• Nucleophiles donate an electron pair and electrophiles accept one.
• A carbocation is a positive carbon with three groups attached.