When alkenes are substituents,they are termed alkenyl groups,and may be named systematically CH2 多-Hc-&=cH methylene 2-propenyl group group (allyl group) Geometric isomers The rigidity of a it bond gives rise to geometric isomers. When similar groups(not H's)are bound to the same side of the double bond the alkene is said to be cis. When similar groups are bound to opposite side of the double bond it is said to be trans. H H H CH2CH3 CH2CH3 trans-pent-2-ene cis-pent-2-ene Cycloalkenes must have or more carbons before they are large enough to incorporate a trans double bond. cyclohexene trans-cyclodecene Therefore cycloalkenes are deemed to be cis unless specified otherwise. Ch07 Alkenes:Struct synth (landscape) Page 8Ch07 Alkenes; Struct + synth (landscape) Page 8 When alkenes are substituents, they are termed alkenyl groups, and may be named systematically. Geometric isomers The rigidity of a  bond gives rise to geometric isomers. When similar groups (not H’s) are bound to the same side of the double bond the alkene is said to be cis. When similar groups are bound to opposite side of the double bond it is said to be trans. Cycloalkenes must have 8 or more carbons before they are large enough to incorporate a trans double bond. Therefore cycloalkenes are deemed to be cis unless specified otherwise. CH2 H2C H C CH2 methylene group 2-propenyl group (allyl group) cyclohexene trans-cyclodecene