a Both ethylene and acetylene are planar molecules. true false b Cis-trans isomerism is possible in both alkenes and alkynes. true false The orientation of the carbon atoms of the parent chain determines whether an alkene is cis or trans. true false Cis-2-butene and trans-2-butene can be interconverted by free rotation about the carbon-carbon double bond. true false e 1-Hexene does not have cis-trans isomers. true false