Question

Iodination of alkanes using iodine (I₂)is usually an unfavorable reaction. (See problem 4-17, for example). Tetraiodomethane (Cl₄) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔHfor each of the steps in your proposed mechanism.

The following bond-dissociation energies may be helpful:

Short answer

Initiation steps

Propagation steps

Proposed Mechanism

Step-by-step solution

Free radicals

An atom or group of atoms containing odd or unpaired electron is known as the free radical. The unpaired electron is represented by a single unpaired dot in the formula. Free radicals are electrically neutral. They are highly reactive species formed by homolytic fission of a covalent bond.

 Step 2: Steps involved in free radical chain reaction

In a free-radical chain reaction, free radicals are generally created in the initiation steps. A free radical and a reactant is combined to yield a product and another free radical in the propagation steps. Lastly, the number of free radicals generally decrease in the termination steps.

Bond dissociation enthalpy (BDE)

It may be defined as the amount of enthalpy required to break a bond homolytically in such a way that each bonded atom retains one of the bond’s two electrons.

Mathematically, ΔH⁰= Σ(BDE of bonds broken) - Σ(BDE of bonds formed)

Mechanism and calculation of ΔH   for each step

The proposed mechanism consists of four parts which are initiation steps, propagation step I and propagation step II.

Initiation steps

Propagation steps

Proposed Mechanism