Free Radical Substitution - Mechanism
- The free-radical substitution reaction consists of three steps:
Initiation step
- In the initiation step, the Cl-Cl or Br-Br is broken by energy from the UV light
- This produces two radicals in a homolytic fission reaction
The first step of the free-radical substitution reaction is the initiation step, in which two free radicals are formed by sunlight
Propagation step
- The propagation step refers to the progression (growing) of the substitution reaction in a chain reaction
- Free radicals are very reactive and will attack the unreactive alkanes
- A C-H bond breaks homolytically (each atom gets an electron from the covalent bond)
- An alkyl free radical is produced
- This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
- This free radical can then repeat the cycle
The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain reaction
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- This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed
- If there is enough chlorine/bromine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C2Cl6/C2Br6)
The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane
Termination step
- The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
- Multiple products are possible
The final step in the substitution reaction to form a single unreactive molecule
Limitations of the free radical mechanism
- In the termination step, there are a number of possibilities
- Remember that termination involves any free radical bonding with another free radical
- If we have two •CH3 radicals, they can bond to form ethane, CH3CH3
- •CH3 + •CH3 → CH3CH3
- If we are trying to form a chloroalkane, then ethane is an impurity
Further substitution
- Excess chlorine present when reacted with methane in the presence of UV light will promote further substitution and could produce CH2Cl2, CHCl3, CCl4
- Further substitution can occur as follows
- CH3Cl + •Cl → HCl + •CH2Cl
- •CH2Cl + Cl2 → CH2Cl2 + •Cl
- These reactions could occur
- CH3Cl + Cl2 → CH2Cl2 + HCl
- CH2Cl2 + Cl2 → CHCl3 + HCl
- CHCl3 + Cl2 → CCl4 + HCl
Substitution of different carbon atoms
- If we have an alkane with a middle carbon such as propane, substitution can occur here
- Propagation steps for the substitution of propane with excess bromine in the presence of UV light on the middle carbon are as follows:
- CH3CH2CH3 + •Br → CH3•CHCH3 + HBr
- CH3•CHCH3 + Br2 → CH3CH(Br)CH3 + •Br
- If the question asks for the halogen to be substituted onto a middle carbon, you must show the radical dot in the correct place, so on the electron-deficient carbon