Question

Indicate 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 CC triple bond.

Short answer

(a) The statement about pentane is false, as geometric isomerism is not possible for alkanes with single bonds. (b) The statement about alkenes is true, as they can have cis and trans isomers around the C=C double bond. (c) The statement about alkynes is false, as they cannot exhibit geometric isomerism around the C≡C triple bond.

Step-by-step solution

Statement 1: Geometric isomers of pentane are n-pentane and neopentane.

Geometric isomerism occurs due to the difference in spatial arrangement of substituents around a bond or a ring structure. In the case of pentane, it is an alkane, with the molecular formula C5H12. Alkanes have single bonds, and spatial arrangements are not restricted around single bonds, which means geometric isomerism is not possible for alkanes. Therefore, this statement is false.

Statement 2: Alkenes can have cis and trans isomers around the CC double bond.

Alkenes have a carbon-carbon double bond (C=C), which restricts the rotation around the bond. This leads to different spatial arrangements of the substituents around the C=C bond. The terms cis and trans describe the relative positions of these substituents. In cis configuration, the similar substituents are on the same side of the C=C bond, while in trans configuration, they are on the opposite sides. Therefore, alkenes can have cis and trans geometric isomers, and this statement is true.

Statement 3: Alkynes can have cis and trans isomers around the CC triple bond.

Alkynes have a carbon-carbon triple bond (C≡C), allowing no rotation around the bond. However, each carbon in the triple bond is only bonded to one other substituent, which means that there are no relative positions to compare the substituents. As a result, alkynes cannot exhibit geometric isomerism (cis and trans isomers) around the carbon-carbon triple bond. Therefore, this statement is false. To summarize: (a) False (b) True (c) False

Key concepts

Alkanes

Alkanes are the simplest family of hydrocarbons, compounds made up of carbon and hydrogen atoms. They are characterized by having only single bonds between the carbon atoms, which gives them a general formula of C_nH_2n+2. The quintessential feature of alkanes is their lack of reactive functional groups, which is why they are often referred to as saturated hydrocarbons.

Due to the nature of single bonds, alkanes can freely rotate around these bonds. This rotation prevents the existence of geometric isomerism since any such isomerism requires restricted rotation—a feature absent in alkanes. Therefore, variations of pentane, like n-pentane and neopentane, are examples of structural isomerism rather than geometric isomerism. Structural isomers have the same molecular formula but differ in the connectivity of the atoms.

Alkenes Cis-Trans Isomerism

Alkenes are hydrocarbons with at least one carbon-carbon double bond, signified by the formula C_nH_2n. The double bond is key to alkenes' chemical behavior, particularly when exploring geometric isomerism. Unlike alkanes, the double bonds in alkenes create a rigidity that restricts rotation, setting the stage for the existence of cis and trans isomers.

In the cis configuration, substituents on the same side of the double bond create a polar effect and closer molecular interactions. The trans isomer has substituents on opposite sides, leading to a more symmetrical and generally less polar structure. Cis-trans isomerism can affect the physical and chemical properties of these molecules, including boiling points, solubility, and reactivity. Understanding the difference between these isomers is critical in fields such as organic synthesis and pharmaceuticals, where the effects of molecular shape on function are paramount.

Importance of Cis-Trans Isomerism

• Influence on physical properties: Cis and trans isomers have distinct boiling points and solubility, which are important for the separation and identification process.
• Impact on chemical reactivity: The spatial arrangement can significantly affect how these molecules interact with other chemicals, enzymes, or receptors.

Alkynes

Alkynes represent another class of hydrocarbons distinguished by having at least one carbon-carbon triple bond, expressed by the formula C_nH_2n-2. This triple bond incorporates two pi bonds on top of a sigma bond, essentially 'locking' the involved carbons in a linear array and precluding any rotation around the axis. Unlike alkenes, alkynes cannot display geometric isomerism because each carbon in the triple bond can only be bonded to one other group or atom.

This limitation on bonding sites means there is no opportunity for two different groups to exist on the same carbon and thus no possibility for isomers that differ in spatial arrangement. Alkynes, therefore, can be thought of as unsaturated hydrocarbons with a 'fixed' linear geometry. They do have other types of isomerism, such as chain isomerism and positional isomerism, but not geometric isomerism, highlighting the unique structural characteristics of triple-bonded carbon compounds.