Question

Which of the following compounds will exhibit geometrical isomerism? (a) 1-phenyl-2-butene (b) 3-phenyl-1-butene (c) 2-phenyl-1-butene (d) 1,1-diphenyl-1-propene

Short answer

1-phenyl-2-butene (Option a) exhibits geometrical isomerism.

Step-by-step solution

Understanding Geometrical Isomerism

Geometrical isomerism arises in alkenes when each carbon atom in the double bond has two different groups attached to it. This allows for different spatial arrangements of the attached groups, leading to cis-trans or E-Z isomers.

Analyzing 1-phenyl-2-butene (Option a)

For 1-phenyl-2-butene, the double bond is between C2 and C3. The groups attached to these carbons are different (C2 has a phenyl group and a hydrogen; C3 has two different alkyl groups), so geometrical isomerism is possible.

Analyzing 3-phenyl-1-butene (Option b)

In 3-phenyl-1-butene, the double bond is between C1 and C2. The phenyl group is attached to C3, which is not part of the double bond, making the possibility for different groups on C1 and C2 not distinct, hence no geometrical isomerism.

Analyzing 2-phenyl-1-butene (Option c)

In 2-phenyl-1-butene, the double bond is between C1 and C2. Since C1 does not have two distinct substituents besides the hydrogen, it cannot exhibit geometrical isomerism.

Analyzing 1,1-diphenyl-1-propene (Option d)

For 1,1-diphenyl-1-propene, the double bond is between C1 and C2, but C1 has two identical phenyl groups attached, which does not allow for geometrical isomerism because there are no distinct substituents on C1.

Key concepts

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. This double bond is a defining feature of alkenes, distinguishing them from alkanes, which have only single carbon-carbon bonds. Alkenes have the general formula \( C_nH_{2n} \), showcasing that they have fewer hydrogen atoms compared to their alkane counterparts due to the presence of a double bond.
In the context of geometrical isomerism:

• The double bond restricts rotation, providing the structural foundation for isomers.
• To have isomerism in alkenes, each carbon atom involved in the double bond should have two different groups attached.

Overall, the unique structure of alkenes not only affects physical properties like boiling points but is crucial for understanding concepts like geometrical isomerism.

Cis-Trans Isomers

Cis-trans isomers are a type of geometrical isomerism seen mainly in alkenes. They occur due to the restricted rotation around the carbon-carbon double bond, which causes the atoms or groups connected to these carbons to have different orientations in space. The terms "cis" and "trans" denote this spatial arrangement.
Here's a bit more detail:

• "Cis" is used when similar or identical groups are on the same side of the double bond.
• "Trans" is used when the similar or identical groups are on opposite sides.

For example, if an alkene has two CH₃ groups attached, and they are on the same side, it would be called "cis"; if they are on opposite sides, it would be "trans".
However, it's important to note that this classification is effective when there are simple identical groups on each side of the double bond. When there is more complexity, we often look into the E-Z nomenclature for clarity.

E-Z Isomers

E-Z isomerism is a more advanced and detailed classification of geometrical isomers, primarily used when the simple naming of cis-trans is not sufficient. This system does not rely on identical groups but rather on the priority of the substituents attached to the double bonded carbons.
Here's how it works:

• "Z" (from the German "zusammen") is when the higher-priority groups are on the same side of the double bond.
• "E" (from the German "entgegen") occurs when these groups are on opposite sides.

Priority is determined based on the atomic number of the atoms attached directly to the double-bonded carbons; the higher atomic number gains a higher priority. If the first atoms are identical, the priority is determined by considering the next set of atoms along the substituent chain.
This way, the E-Z nomenclature provides a clear and precise method to differentiate complex isomers, ensuring accurate chemical communication.