Which of the following alkenes can exist as double-bond...

Which of the following alkenes can exist as double-bond stereoisomers? Identify the stereocenters in each. (a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ (b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}$ (c) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}$ (d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CCH}_{3}$

Which of the following alkenes can exist as double-bond stereoisomers? Identify the stereocenters in each.
(a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{2} \mathrm{CH}_{3}$
(c) $\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3}$
(d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CCH}_{3}$

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Key Concepts

Identification of Stereocenters in Alkenes

In the context of double bonds, the stereocenters are the sp2-hybridized carbons that meet the substituent diversity requirement. These carbons are considered stereogenic when each is attached to two different groups, thus playing a crucial role in determining whether the alkene can exist in more than one geometrical configuration. Identifying these centers involves carefully analyzing the groups attached to each double-bonded carbon.

Geometric Stereoisomerism

This concept refers to the existence of distinct isomers (commonly called cis/trans or E/Z isomers) arising from the restricted rotation about a carbon?carbon double bond. Because the double bond cannot rotate under normal conditions, the spatial arrangement of substituents on each of the two sp2?hybridized carbons is fixed, allowing for different configurations when the groups on each carbon are different.

Substituent Diversity Requirement

For a carbon–carbon double bond to exhibit geometric stereoisomerism, each carbon of the double bond must be bonded to two different substituents. If either carbon bears identical substituents (for example, a CH2 group having two hydrogens), then there is no possibility for cis/trans (or E/Z) differentiation. This is a fundamental requirement when predicting whether an alkene can have stereoisomers.

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