Question 11.8: Identifying Cis–Trans Isomers Identify each of the following...
Identifying Cis–Trans Isomers
Identify each of the following as the cis or trans isomer and give its name:
a. \begin{array}{r c}\begin{matrix} \ Br\ {\diagdown }\quad\quad\quad {\diagup }Cl \\C=C\\ H\ ^{^\diagup }\quad \quad \ \ ^{^\diagdown }H \end{matrix} \end{array}
b. \begin{array}{r c}\begin{matrix} \ CH_{3}\ {\diagdown }\quad \quad {\diagup }H \\ \quad C=C\\ \quad \quad \quad \quad \quad H\ ^{^\diagup }\quad \quad \ \ ^{^\diagdown }CH_{2}-CH_{3} \end{matrix} \end{array}
a. This is a cis isomer because the two halogen atoms attached to the carbon atoms of the double bond are on the same side. The name of the two-carbon alkene, starting with the bromo group on carbon 1, is cis-1-bromo-2-chloroethene.
b. This is a trans isomer because the two alkyl groups attached to the carbon atoms of the double bond are on opposite sides of the double bond. This isomer of the five-carbon alkene, 2-pentene, is named trans-2-pentene.