For each of the following compounds, indicate whether or not it can exist as cis–trans stereoisomers. If it can exist as the two isomers, draw both as a condensed structure.
(a) H₂C=CHCH₂CH₃

Verified step by step guidance

1. Step 1: Understand the concept of cis-trans stereoisomerism. Cis-trans isomerism occurs in alkenes (compounds with a carbon-carbon double bond) when each carbon in the double bond is attached to two different groups. This restriction in rotation around the double bond allows for the existence of two distinct isomers: cis (same side) and trans (opposite sides).
2. Step 2: Analyze the given compound, H₂C=CHCH₂CH₃. The structure contains a carbon-carbon double bond (C=C), which is a prerequisite for cis-trans isomerism.
3. Step 3: Examine the groups attached to each carbon in the double bond. For the first carbon in the double bond (H₂C=), the two groups are hydrogen (H) and another hydrogen (H). For the second carbon in the double bond (=CH), the two groups are hydrogen (H) and a propyl group (CH₂CH₃).
4. Step 4: Determine if the groups on each carbon are different. For cis-trans isomerism to occur, each carbon in the double bond must have two different groups attached. In this case, the first carbon has two identical groups (H and H), so it does not meet the requirement for cis-trans isomerism.
5. Step 5: Conclude that the compound H₂C=CHCH₂CH₃ cannot exist as cis-trans stereoisomers because one of the carbons in the double bond does not have two different groups attached.

Key Concepts

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in compounds with restricted rotation around a double bond or a ring structure. In this type of isomerism, the spatial arrangement of substituents differs, leading to distinct physical and chemical properties. For alkenes, 'cis' refers to substituents on the same side of the double bond, while 'trans' indicates they are on opposite sides.

Alkenes and Double Bonds

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). The presence of this double bond is crucial for the possibility of cis-trans isomerism, as it restricts the rotation of the carbon atoms involved. The structure of alkenes can significantly influence their reactivity and physical properties, making the understanding of their geometry essential in organic chemistry.

Condensed Structural Formula

A condensed structural formula is a way of representing a chemical compound that shows the arrangement of atoms without depicting all the bonds explicitly. It provides a simplified view of the molecule, indicating how atoms are connected. For example, in the case of alkenes, the condensed formula can help visualize the positioning of substituents around the double bond, which is critical for identifying cis and trans isomers.