Question

$Trans-1-$bromo$-2-$chlorocyclopentane is synthesized in laboratory by treating cyclopentene with bromine and dry HCl. Write the mechanism of reaction showing proper stereo chemistry.

Answer 1

In the first step, the pi electrons of the double bond are donated to $B{r}^{+}$ ion. This forms cyclic bromonium ion. In the second step, the chloride ion attacks the carbon atom attached to Br, thereby displaces the bromine and opens the three member ring. The chlorine and bromine atoms have trans geometry.