Question

Which of the following can show geometrical isomerism ? (a) But - 1 - ene (b) But \(-2\) - ene (c) Hex \(-3\) - ene (d) 2 - Chlorobut \(-2\) - ene

Short answer

(b) But-2-ene and (c) Hex-3-ene can show geometrical isomerism.

Step-by-step solution

Understand Geometrical Isomerism

Geometrical isomerism occurs in compounds that have restricted rotation around a bond, usually a double bond, and different groups attached to the atoms forming this double bond. For alkenes, geometrical isomerism can occur if each carbon of the double bond has two different groups attached.

Analyze But-1-ene

The structure of But-1-ene is CH3-CH2-CH=CH2. The double bond is between the first and second carbon atom. The terminal CH2 group only has two hydrogen atoms, which are not different groups. Therefore, But-1-ene cannot show geometrical isomerism.

Analyze But-2-ene

The structure of But-2-ene is CH3-CH=CH-CH3. The double bond is between the second and third carbon atom. Each of these carbons has two distinct groups attached (CH3 and H), thus But-2-ene can show geometrical isomerism, resulting in cis and trans forms.

Analyze Hex-3-ene

The structure of Hex-3-ene is CH3-CH2-CH=CH-CH2-CH3. The double bond is between the third and fourth carbon atom. Both carbon atoms have different groups attached, enabling Hex-3-ene to exhibit geometrical isomerism.

Analyze 2-Chlorobut-2-ene

The structure of 2-Chlorobut-2-ene is CH3-C(Cl)=C(CH3)-CH3. The double bond between the second and third carbon atom has identical groups (carbon atoms with CH3 groups) on one of the double-bonded carbons, so despite the chlorine substitution, it cannot show geometrical isomerism due to these identical groups.

Key concepts

Alkenes

Alkenes are a group of hydrocarbons characterized by having at least one carbon-carbon double bond in their molecular structure. This double bond is pivotal in defining the chemical properties of alkenes. The presence of a double bond implies that these molecules are unsaturated, meaning they have fewer hydrogen atoms compared to alkanes with the same number of carbon atoms.

The general formula for alkenes is \(C_nH_{2n}\). This formula showcases the proportion of carbon to hydrogen and helps differentiate alkenes from other hydrocarbon classes. Alkenes are versatile and can be found in many natural and synthetic substances. They are colorless and usually non-polar, making them soluble in organic solvents rather than water.

• Common examples include: Ethene, propene, and butene, each differing by the number of carbons and position of their double bonds.
• Key Property: The double bond makes alkenes reactive, especially in addition reactions where atoms or groups are added to the carbon atoms in the double bond.

Understanding these characteristics of alkenes sets the stage for exploring concepts like geometrical isomerism, which stems from the unique features of their double bonds.

Geometrical Isomerism Criteria

Geometrical isomerism is a type of stereoisomerism that occurs due to the restricted rotation about the carbon-carbon double bond in alkenes. To have geometrical isomerism, an alkene must meet specific criteria.

• Each carbon involved in the double bond must have two different groups attached. This difference in groups leads to different spatial arrangements, known as "cis" and "trans" configurations.
• In the "cis" isomer, the similar or identical groups are on the same side of the double bond, while in the "trans" isomer, they are on opposite sides.
• The inability to rotate around the double bond is crucial for preserving these distinct arrangements over time.

Not all alkenes exhibit geometrical isomerism; it is contingent upon the diversity of groups attached to the double-bonded carbons. For instance, in the case of but-2-ene, both ends of the double bond have different groups attached, allowing for two distinct isomers: cis-but-2-ene and trans-but-2-ene.

Structural Analysis of Alkenes

Performing a structural analysis of alkenes involves examining the arrangement and connectivity of atoms within the molecule, particularly focusing on the double bond locations.

This step is integral in determining whether an alkene can exhibit geometrical isomerism. Let's consider a few cases:

• But-1-ene: Its structure is CH3-CH2-CH=CH2, where one end of the double bond has two identical hydrogen atoms, hence lacking the variability needed for geometrical isomerism.
• But-2-ene: The structure CH3-CH=CH-CH3 contains a double bond between carbons two and three, each having different groups attached (methyl and hydrogen), making it capable of geometrical isomerism.
• Hex-3-ene: With CH3-CH2-CH=CH-CH2-CH3, the double bond is flanked by two distinct groups on each carbon, allowing for the existence of cis and trans forms.

This analysis highlights that structural features, such as group diversity around the double bond, are key in identifying potential isomerism.