10-10-21 E - Organic Chemistry (Mcmurry)

Question

Alkyl halides can be reduced to alkanes by a radical reaction with tributyltinhydride, ${\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH}$ in the presence of light (hv). Propose a radical chain mechanism by which the reaction might occur. The initiation step is the light-induced homolytic cleavage of the Sn-H bond to yield a tributyltin radical.

$\mathrm{R}-\mathrm{X}+{\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH}\stackrel{\mathrm{hv}}{\to }\mathrm{R}-\mathrm{H}+{\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnX}$

Step-by-Step Solution

Answer

Radical chain mechanism of the reaction is: 

$$\begin{gathered} {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH} \stackrel{\mathrm{hv}}{\to }{\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{H}· \\ {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{RX} \to {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnX}+\mathrm{R}· \\ {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH}+\mathrm{R}·\to {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{RH} \end{gathered}$$

Free radical reaction

Free radicals are intermediates and these are highly reactive. Free- radical mechanism takes place in the presence of free radicals and this reaction is also known as chain reaction. This reaction takes place in three different steps i.e. chain- initiation, chain– propagation and chain- termination.

Example

This is an example halogenation reaction in which formation of carbon- carbon bond is taking place via free radical reaction mechanism.

Step-by-Step mechanism of above reaction

The mechanism is given below:

$$\begin{gathered} {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH} \stackrel{\mathrm{hv}}{\to }{\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{H}· \\ {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{RX} \to {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnX}+\mathrm{R}· \\ {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{SnH}+\mathrm{R}·\to {\left({\mathrm{C}}_4{\mathrm{H}}_9\right)}_3\mathrm{Sn}·+\mathrm{RH} \end{gathered}$$