Question

ndicate whether each statement is true or false. (a) Two geometric isomers of pentane are \(n\)-pentane and neopentane. (b) Alkenes can have cis and trans isomers around the CC double bond. (c) Alkynes can have cis and trans isomers around the \(\mathrm{CC}\) triple bond.

Short answer

(a) False: \(n\)-pentane and neopentane are structural isomers, not geometric isomers. (b) True: Alkenes can have cis and trans isomers around the C=C double bond. (c) False: Alkynes cannot have cis and trans isomers around the \(\mathrm{C} \equiv \mathrm{C}\) triple bond.

Step-by-step solution

Statement (a) Explanation

First, let's understand the difference between geometric isomers and structural isomers. Geometric isomers are molecules with the same structural formula, but the arrangement of atoms or groups in space differs. In contrast, structural isomers are molecules with the same molecular formula but different connectivity of atoms. In this statement, we have \(n\)-pentane and neopentane, which are two different structures of pentane (C5H12). The molecular formula remains the same, but the arrangement of the carbon atoms in these compounds is different. Since these are two different structures, they are structural isomers, not geometric isomers. Therefore, statement (a) is false.

Statement (b) Explanation

Alkenes have a carbon-carbon double bond between two carbon atoms, written as C=C. Geometric isomerism (cis and trans isomers) usually occurs in alkenes when there is restricted rotation around a carbon-carbon double bond. The cis isomer has the large substituents on the same side of the double bond, and the trans isomer has them on opposite sides. Statement (b) is true because alkenes can indeed have cis and trans isomers around the C=C double bond.

Statement (c) Explanation

Alkynes have a carbon-carbon triple bond between two carbon atoms, symbolized as \(\mathrm{C} \equiv \mathrm{C}\). However, geometrical isomers (cis and trans) don't exist around the carbon-carbon triple bond because of the bond's linear nature. The carbon atoms involved in the triple bond are already at their maximum allowed valency, and there is no possibility of having different spatial arrangements of atoms or groups around the triple bond to create geometrical isomers. Hence, statement (c) is false. In summary: (a) False (b) True (c) False

Key concepts

Structural Isomers

Structural isomers are compounds with the same molecular formula but different arrangements of atoms. Imagine being given a set of building blocks with the same shapes and sizes but told to create different structures each time; this is essentially what occurs with structural isomers. Their connectivity varies, leading to different physical and chemical properties.

For example, with pentane (C5H12), we can arrange the carbon atoms in a straight chain to create -n-pentane, or arrange the carbons such that there's branching to form neopentane. These rearrangements create compounds that are distinct from each other in structure, despite having the same number of carbons and hydrogens.

• Chain Isomers: Differ in the arrangement of the carbon skeleton.
• Position Isomers: Functional groups attached to different positions on the carbon chain.
• Functional Isomers: Different functional groups altogether.

Understanding structural isomerism is crucial in organic chemistry as it explains the existence of numerous compounds with the same molecular formula but vastly different characteristics.

Alkenes Cis and Trans Isomerism

The cis-trans isomerism in alkenes arises due to the restricted rotation about the carbon-carbon double bond. Each carbon in the double bond has two substituents. When substituents on both carbons are the same, they can either be on the same side (cis) or opposite sides (trans) of the double bond.

For instance, 2-butene can occur in two forms:

• The cis form, where the methyl groups (CH3) are on the same side of the double bond.
• The trans form, where the methyl groups are on opposite sides.

These two isomers have different physical properties and reactivities because their spatial arrangements influence the molecular interactions they can participate in. Cis isomers often have higher boiling points due to stronger dipole-dipole interactions, while trans isomers are generally more stable due to less steric strain. This concept is essential for understanding how subtle changes in molecular structure impact the properties and functions of molecules in chemistry and biochemistry.

Alkynes Chemical Properties

Alkynes, characterized by their carbon-carbon triple bonds, are more reactive than alkenes and alkanes due to the high energy of the triple bond. Since the triple bond introduces a linear geometry, there is no possibility of cis-trans isomerism within alkynes - there's simply no 'space' for the atoms to arrange on different sides of the bond.

Some key chemical properties of alkynes include:

• They can undergo addition reactions, where the triple bond is broken, leading to the formation of alkenes or alkanes.
• Alkynes can participate in polymerization reactions to create synthetic materials.
• Acetylene, the simplest alkyne, is a precursor for many organic syntheses and is commonly used in welding due to its high-temperature flame.

The reactivity of alkynes makes them valuable in various industrial and synthetic processes, but their incompatibility with cis-trans isomerism limits their isomeric diversity when compared to alkenes.