: σ bonds : , π bond : σ bonds : , π bond : . . Isomerism Alkenes show both structural isomerism and geometrical isomerism. Structural isomerism : As in alkanes, ethene (C H ) and propene (C H ) can have only one structure but alkenes higher than propene have different structures.
Alkenes possessing C H as molecular formula can be written in the following three ways: I. CH = CH – CH – CH But- -ene (C H ) II. CH – CH = CH – CH But- -ene (C H ) Fig. .
Orbital picture of ethene showing formation of (a) π -bond, (b) π -cloud and (c) bond angles and bond lengths III. CH = C – CH -Methylprop- -ene (C H ) Structures I and III, and II and III are the examples of chain isomerism whereas structures I and II are position isomers . Problem . Write structures and IUPAC names of different structural isomers of alkenes corresponding to C H .
(a) CH = CH – CH – CH – CH Pent- -ene (b) CH – CH=CH – CH – CH Pent- -ene (c) CH – C = CH – CH -Methylbut- -ene (d) CH – CH – CH = CH -Methylbut- -ene (e) CH = C – CH – CH -Methylbut- -ene Geometrical isomerism: Doubly bonded carbon atoms have to satisfy the remaining two valences by joining with two atoms or groups. If the two atoms or groups attached to each carbon atom are different, they can be represented by YX C = C XY like structure. YX C = C XY can be represented in space in the following two ways : In (a), the two identical atoms i.e., both the X or both the Y lie on the same side of the double bond but in (b) the two X or