91Ó°ÊÓ

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 aradical chain mechanism by which the reaction might occur. The initiationstep is the light-induced homolytic cleavage of the Sn-H bondto yield a tributyltin radical.

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

Draw the electron-pushing mechanism for the propagation steps of the allylic bromination reactions below. You may omit NBS in your mechanism, and use Br∙ and Br₂.

(b)

Question: Assume that you have carried out a radical chlorination reaction on

and have isolated (in low yield) . How many stereoisomers of the product are formed, and in what ratio? Are any of the isomers optically active? (See Problem 10-38.)

Question: Why do you suppose it’s not possible to prepare a Grignard reagentfrom a bromo alcohol such as $\mathbf{4}-\mathbf{bromo}-\mathbf{1}-\mathbf{pentanol}$? Give another exampleof a molecule that is unlikely to form a Grignard reagent.

Question: How might you replace a halogen substituent by a deuterium atom if you wanted to prepare a deuterated compound?

Draw structures corresponding to the following IUPAC names: (b) 4-Bromo-4-ethyl-2-methylhexane.

Draw structures corresponding to the following IUPAC names: (c) 3-Iodo-2,2,4,4-tetramethylpentane

Draw structures corresponding to the following IUPAC names: (d) cis-1-Bromo-2-ethylcyclopentane

How would you prepare the following compounds, starting with cyclopentene and any other reagents needed? (a) Chlorocyclopentane.

How would you prepare the following compounds, starting with cyclopentene and any other reagents needed? (b) Methylcyclopentane.