A Level Chemistry OCR Practice Exam

What is cis-trans isomerism?

1. A type of isomerism that only involves carbon chains

2. A special type of E/Z isomerism involving non-hydrogen groups

3. An isomerism with identical molecular formulas

4. A type of reaction mechanism in organic chemistry

The correct answer is: A special type of E/Z isomerism involving non-hydrogen groups

Cis-trans isomerism, also known as geometric isomerism, occurs in compounds that have restricted rotation around a double bond, particularly in alkenes. In this type of isomerism, the spatial arrangement of groups around the double bond differs, leading to different isomers that can have distinct physical or chemical properties. The correct answer indicates that this form of isomerism specifically relates to the presence of non-hydrogen groups attached to the same carbon atoms in a molecule, distinguishing cis isomers (where similar groups are on the same side) from trans isomers (where similar groups are on opposite sides). This explanation fits because cis-trans isomerism is often categorized as a subtype of E/Z isomerism based on the priority of the groups attached to the double bond. The other choices do not provide an accurate or complete description of cis-trans isomerism. For example, it does not only involve carbon chains (as other types of compounds can exhibit this phenomenon), nor does it pertain to reaction mechanisms in organic chemistry. The presence of identical molecular formulas is common to all isomers, not just those involved in cis-trans isomerism.