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Chapter 15: Problem 9

Among alkenes, alkynes, and aromatic hydrocarbons, only alkenes exhibit cis- trans isomerism. Why don't the others?

Short Answer

Expert verified
Cis-trans isomerism occurs in alkenes due to restricted rotation around the double bond. Alkynes and aromatic hydrocarbons lack such features due to their linear and planar structures, respectively.

Step by step solution

01

Understand Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in compounds that have restricted rotation around a double bond or ring structure. This restriction allows for the existence of two distinct configurations: 'cis' where similar groups are on the same side, and 'trans' where similar groups are on opposite sides.
02

Analysis of Alkenes

Alkenes have a carbon-carbon double bond that restricts rotation. If the carbon atoms involved in the double bond each have two different groups attached, the molecule can exist in two different geometric forms: cis and trans.
03

Analysis of Alkynes

Alkynes have a carbon-carbon triple bond. This bond creates a linear geometry around the bonded carbon atoms. Because of this linear arrangement, the substituents are always on opposite sides, making cis-trans isomerism impossible.
04

Analysis of Aromatic Hydrocarbons

Aromatic hydrocarbons have delocalized electrons in a ring structure. This delocalization results in a uniform distribution of electrons that prevents the existence of cis and trans configurations. The planar structure of aromatic rings does not allow for restricted rotation like in alkenes.
05

Conclusion

Alkenes exhibit cis-trans isomerism because of the restricted rotation around the double bond and the requirement of having different groups attached to the double-bonded carbons. Alkynes and aromatic hydrocarbons do not exhibit this isomerism due to their linear and planar structures, respectively, which do not allow for the necessary geometric distinctions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alkenes
Alkenes are hydrocarbons characterized by at least one carbon-carbon double bond (C=C). This double bond consists of one sigma and one pi bond, which restricts the rotation of the bonded carbons. Due to this restricted rotation, alkenes can demonstrate geometric isomerism, often referred to as cis-trans isomerism.
  • **Cis Isomers:** In cis isomers, the similar or identical groups are on the same side of the double bond.
  • **Trans Isomers:** In trans isomers, the similar or identical groups are on opposite sides of the double bond.
To exhibit cis-trans isomerism, each carbon of the double bond must have two different groups attached.
Alkynes
Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond (C≡C). This triple bond consists of one sigma and two pi bonds. The triple bond gives the carbon atoms a linear geometry, meaning all substituents are arranged in a straight line.
Because of this linear configuration, cis-trans isomerism is not possible in alkynes. The substituents attached to the carbon atoms of the triple bond will always be on opposite sides, thus they do not form geometric isomers.
Aromatic Hydrocarbons
Aromatic hydrocarbons, such as benzene, have a ring structure with delocalized electrons. These delocalized electrons result in a stable structure known as aromaticity. The electrons are shared across all the carbon atoms in the ring, creating a uniform distribution.
The planar nature of aromatic rings and the delocalization of electrons prevent restricted rotation like that seen in alkenes. Hence, cis-trans isomerism does not occur in aromatic hydrocarbons because the concept of 'cis' and 'trans' configurations is not applicable to their structure.
Geometric Isomerism
Geometric isomerism, also known as cis-trans isomerism, arises due to the restricted rotation around double bonds or certain ring structures. This form of isomerism requires the presence of distinct substituents on each carbon of the double bond.
  • **Cis Isomers:** Similar substituents are on the same side.
  • **Trans Isomers:** Similar substituents are on opposite sides.
This type of isomerism is crucial for understanding molecular geometry and properties, especially in organic chemistry.
Double Bond Restriction
The restriction of rotation around a double bond is a key factor in the formation of geometric isomers. A double bond is composed of a sigma bond and a pi bond, where the pi bond's electron cloud above and below the plane of the atoms prevents free rotation.
This restriction is not present in single bonds where atoms can rotate freely, nor in triple bonds or aromatic structures due to their specific geometries.
Consequently, molecules with double bonds, like alkenes, are often prime candidates for demonstrating cis-trans isomerism.

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Most popular questions from this chapter

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