Organic Chemistry MCAT Practice Exam

Geometric isomers require what type of restriction?

1. Restriction in rotation about a double bond or ring

2. The presence of chiral centers

3. Symmetrical arrangement of molecular structure

4. Non-rotatable single bonds

The correct answer is: Restriction in rotation about a double bond or ring

Geometric isomers, also known as cis-trans isomers, arise from restricted rotation around a double bond or within a ring structure. This restriction is crucial because it creates distinct spatial arrangements of the substituents attached to the carbon atoms involved in the double bond or the atoms in the ring. In the case of double bonds, the pi bond formed prevents free rotation; thus, different attachments can occupy different spatial orientations, leading to the formation of isomers where the same atoms are connected but arranged differently in space. For instance, in a cis arrangement, similar groups are on the same side of the double bond, whereas in a trans arrangement, they are on opposite sides. This restriction is not dependent on the presence of chiral centers, even though chiral centers can lead to stereoisomerism as well. However, the defining feature of geometric isomers is the inability to freely rotate around specific bonds. In other cases, symmetrical arrangements of molecular structures or non-rotatable single bonds are not requirements for the existence of geometric isomers. Thus, the correct answer highlights that the key factor enabling the formation of geometric isomers is the restriction in rotation about the double bond or in a ring structure.