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Chapter 11: Problem 8

Why is it that alkanes and alkynes, unlike alkenes, have no geometric isomers?

Short Answer

Expert verified
Alkanes and alkynes do not exhibit geometric isomerism because they have single and triple carbon-carbon bonds respectively. These bonds allow free rotation and hence, do not lead to distinct spatial arrangements of the attached groups. Unlike them, alkenes contain a double bond where rotation is restricted, leading to different non-interconvertible spatial arrangements and hence, distinct geometric isomers.

Step by step solution

01

Understand Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, occurs due to restricted rotation around a bond. This usually arises in the presence of double bonds or ring structures. The different spatial arrangements of atoms or groups of atoms attached to the carbons forming the bond with restricted rotation lead to different isomers.
02

Consider Alkanes and Alkynes

In alkanes and alkynes, the carbon atoms are connected by single and triple bonds respectively. There is no restriction on the rotation around these types of bonds, thus atoms or groups can freely rotate around the bond. This free rotation allows the molecule to adopt all possible spatial arrangements and does not lead to the formation of different isomers with distinct spatial structures.
03

Compare to Alkenes

In contrast to alkanes and alkynes, alkenes contain a carbon-carbon double bond where rotation is restricted. This restriction leads to different spatial arrangements that are not interconvertible without breaking the double bond, resulting in distinct geometric isomers.

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