Question

Which of the following does not show geometrical isomerism? (a) 1,1-dichloro-1-pentene (b) 1,2 -dichloro-1-pentene (c) 1,3 -dichloro-1-pentene (d) 1,4 -dichlroro-2-pentene

Short answer

1,1-dichloro-1-pentene does not show geometric isomerism.

Step-by-step solution

Understanding Geometric Isomerism

Geometric isomerism occurs in compounds with restricted rotation, typically around a double bond. The presence of different substituents on each of the carbon atoms involved in the double bond is necessary for geometric isomers to exist.

Analyzing Option (a)

In 1,1-dichloro-1-pentene, both chlorine atoms are attached to the same carbon atom (carbon 1) in the form of R2C=CR2, which does not allow for geometric isomerism as there is no restricted rotation regarding different substituents on each side of the double bond.

Analyzing Option (b)

In 1,2-dichloro-1-pentene, each chlorine atom is on a different carbon atom that is part of the double bond. This setup allows for restricted rotation with different substituents on each carbon, thus supporting geometric isomerism.

Analyzing Option (c)

In 1,3-dichloro-1-pentene, the double bond is between carbon atoms 1 and 2, while the chlorine atoms are on carbons 1 and 3. This allows for restricted rotation with different substituents on each carbon involved in the double bond, supporting geometric isomerism.

Analyzing Option (d)

In 1,4-dichloro-2-pentene, the locations of the chlorine atoms do not affect the arrangement about the double bond between carbons 2 and 3, allowing for restricted rotation if diverse substituents are present, thus supporting geometric isomerism.

Identify the Non-Isomeric Compound

Among the given options, only 1,1-dichloro-1-pentene has its chlorines on the same carbon atom, failing to meet the necessary conditions for geometric isomerism.

Key concepts

Geometric Isomers

Geometric isomers, also known as cis-trans isomers, are a type of stereoisomer where the same molecular formula has different spatial arrangements around a double bond or a ring structure. The key factor that allows for geometric isomerism is the restricted rotation around these bonds. This results in distinct configurations that are not easily interconvertible. One common example is the configuration around a carbon-carbon double bond.
For a molecule to be capable of geometric isomerism, each carbon atom involved in the double bond must have two different substituents. This means that if both substituents are the same on one carbon, such as in 1,1-dichloro-1-pentene, geometric isomerism cannot occur, as there's no distinction in possible spatial arrangement.

• Trans isomer: Substituents are on opposite sides.
• Cis isomer: Substituents are on the same side.

These different spatial arrangements can lead to variations in physical and chemical properties, highlighting the importance of geometric isomerism in organic chemistry.

Double Bond Rotation

The concept of restricted rotation in a double bond is key to understanding geometric isomerism. Unlike single bonds, which allow free rotation around the bond axis, double bonds and certain ring structures prevent such movement.
This restriction is due to the nature of a double bond, which consists of one sigma bond and one pi bond. The pi bond is formed by the side-to-side overlap of p orbitals, creating an electron cloud that spans the bonded atoms, hence preventing rotation.

• Rotating a double bond would break the pi bond, requiring significant energy input, thus demonstrating the stability of the bond.
• Due to this stability, once a double bond is formed, the configuration of substituents around it is locked, unless a chemical reaction occurs to change it.

Consequently, this locked configuration creates the basis for geometric isomerism, allowing different isomers to form based on the arrangement of substituents.

Organic Chemistry

Organic chemistry is the branch of chemistry that focuses on carbon-containing compounds, which includes the study of their properties, structures, and reactions. It is a vast field that explores the endless variety of compounds carbon can form, mainly due to its ability to create stable bonds with other elements, including itself.
Within organic chemistry, the study of isomerism is crucial for understanding how different compounds can have the same molecular formula but distinct structures and properties. Geometric isomerism is one form of stereoisomerism often encountered in organic chemistry, especially with alkenes (molecules with carbon-carbon double bonds) and cyclic compounds.

• Understanding isomerism helps chemists determine reactivity and functionality of organic molecules.
• It's also fundamental for applications in pharmaceuticals, agriculture, and materials science, where the spatial arrangement of atoms can significantly impact efficacy and safety.
• Through the study of geometric isomers, organic chemists can better predict the behavior of molecules under various conditions.

The study of geometric isomers specifically assists in the development and understanding of complex organic reactions and synthetic applications.