Question

The number of geometrical isomers of \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}=\mathrm{CHCl}\) are (a) 4 (b) 6 (c) 8 (d) 10

Short answer

The number of geometric isomers is 8 (option c).

Step-by-step solution

Understanding Geometric Isomerism

In geometric isomerism, particularly in alkenes, the different spatial arrangements of groups attached to carbon atoms connected by a double bond result in different isomers. Each double bond allows for two possible configurations, often referred to as cis and trans isomers, due to restricted rotation about the double bonds.

Identify the Relevant Double Bonds

The molecule given is \(\text{CH}_3\text{CH} = \text{CH} - \text{CH} = \text{CH} - \text{CH} = \text{CHCl}\). It has three double bonds, including \(\text{CH} = \text{CH}\), \(\text{CH} = \text{CH}\), and \(\text{CH} = \text{CHCl}\). Each of these can potentially be a site for geometric isomerism.

Analyze Each Double Bond

1. \(\text{CH}_3\text{CH} = \text{CH}\): This bond can have either a cis or a trans configuration. 2. \(\text{CH} - \text{CH} = \text{CH}\): This bond can also exist as cis or trans. 3. \(\text{CH} - \text{CH} = \text{CHCl}\): This bond can have a cis or trans configuration. As each double bond can independently be either cis or trans, each one contributes two configurations.

Calculate Total Isomers

The total number of geometric isomers is given by \(2^n\), where \(n\) is the number of double bonds capable of exhibiting cis-trans isomerism. For this molecule, \(n = 3\). Therefore, the number of geometric isomers is \(2^3 = 8\).

Conclusion

Based on the calculation, the number of geometric isomers possible for the given compound is 8. Therefore, the correct answer is option (c).

Key concepts

Cis-Trans Isomerism

Cis-trans isomerism is a specific form of geometric isomerism, where molecules with the same structural formula have different spatial configurations. This occurs due to the presence of double bonds. Double bonds restrict the rotation of the connected atoms, unlike single bonds, leading to different positions for the substituents attached to the carbons. These substituents can either be on the same side, known as "cis," or on opposite sides, known as "trans." This distinction is crucial in organic chemistry since it significantly influences the properties and reactivities of the molecules.

• In a cis isomer, the more significant groups attached to each double-bonded carbon are on the same side.
• In a trans isomer, the major groups are on opposite sides.

The terms 'cis' and 'trans' origin from Latin, meaning 'on this side' and 'across', respectively. Recognizing whether a molecule is a cis or trans isomer is essential for understanding its behavior in chemical reactions.

Double Bonds in Alkenes

Double bonds in alkenes are fundamental in defining the structure and reactivity of these organic molecules. An alkene contains at least one carbon-carbon double bond, which defines its unsaturated nature. This double bond involves the sharing of two pairs of electrons between two carbon atoms, represented by a sigma bond and a pi bond.

• The sigma bond forms the axis around which the atoms in the molecule rotate.
• The pi bond restricts this rotation, leading to the formation of distinct isomers.

A molecule's ability to exist in different geometric forms, such as cis and trans, directly results from this restricted rotation. Double bonds make alkenes more reactive compared to their alkane counterparts as the pi bond is weaker than the sigma bond, making it more accessible to react with other molecules or ions.

Stereochemistry in Organic Chemistry

Stereochemistry is an area in organic chemistry focused on the study of the spatial arrangements of atoms within molecules. Understanding stereochemistry is fundamental since the three-dimensional configuration can significantly affect a molecule’s chemical properties and biological interactions. In the context of organic chemistry, particularly with alkenes, this involves the delineation of isomers such as cis and trans.

• Stereochemistry impacts how molecules interact with enzymes, receptors, and other biological structures.
• It is important in the design of pharmaceuticals, as the efficacy and safety of a drug can depend on its stereochemistry.

Moreover, stereochemistry encompasses more than just geometric isomerism; it also includes topics like optical isomerism, which relates to how isomers can vary in their interaction with polarized light. Understanding these concepts is vital for predicting how molecules behave in real-world chemical reactions and applications.