a. Which of the following compounds can exist as cis–trans isomers?

Verified step by step guidance

1. Step 1: Understand the concept of cis-trans isomerism. Cis-trans isomerism occurs in compounds with restricted rotation around a double bond, typically due to the presence of a π-bond. For cis-trans isomerism to exist, the two groups attached to each carbon of the double bond must be different.
2. Step 2: Analyze compound 1 (CH3CH=CHCH2CH2CH3). Check the groups attached to each carbon of the double bond. The first carbon has CH3 and H, and the second carbon has CH2CH2CH3 and H. Since both carbons have two different groups, this compound can exist as cis-trans isomers.
3. Step 3: Analyze compound 2 (CH3CH=CHCH3). Check the groups attached to each carbon of the double bond. The first carbon has CH3 and H, and the second carbon also has CH3 and H. Since the groups attached to the second carbon are identical, this compound cannot exist as cis-trans isomers.
4. Step 4: Analyze compound 3 (CH3CH2C(CH2CH3)=CHCH3). Check the groups attached to each carbon of the double bond. The first carbon has CH3CH2 and CH2CH3, and the second carbon has CH3 and H. Since both carbons have two different groups, this compound can exist as cis-trans isomers.
5. Step 5: Analyze compound 4 (CH3CH2CH=CH2). Check the groups attached to each carbon of the double bond. The first carbon has CH3CH2 and H, and the second carbon has H and H. Since the second carbon has identical groups, this compound cannot exist as cis-trans isomers.

Key Concepts

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in alkenes where the arrangement of substituents around a double bond can differ. In cis isomers, similar groups are on the same side of the double bond, while in trans isomers, they are on opposite sides. This difference in spatial arrangement can lead to distinct physical and chemical properties.

Alkenes and Double Bonds

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). The presence of this double bond restricts rotation, allowing for the possibility of cis-trans isomerism. The ability to form isomers depends on having two different substituents on each carbon of the double bond.

Substituent Diversity

For a compound to exhibit cis-trans isomerism, each carbon atom involved in the double bond must have two different substituents. If both substituents on a carbon are the same, the compound cannot have cis-trans isomers. Evaluating the substituents on the double bond is crucial for determining the existence of these isomers.