Question

Consider (E)-2-butene and (Z)-2-butene. This is a pair of what type(s) of isomers? I. Cis-trans isomers II. Diastereomers III. Enantiomers (A) \(\quad\) I only (B) \(\quad \)II only (C) \(\quad\) I and II only (D) \(\quad\) I and III only

Short answer

Option (C). I and II only.

Step-by-step solution

- Identify (E)-2-butene and (Z)-2-butene

(E)-2-butene and (Z)-2-butene are geometric isomers of each other. These compounds have the same molecular formula, but different spatial arrangements of atoms around the double bond.

- Understand Cis-trans Isomers

Cis-trans isomers, also known as geometric isomers, are compounds that differ in the positions of substituents around a double bond. (E)-2-butene has the higher-priority groups on opposite sides (trans), while (Z)-2-butene has the higher-priority groups on the same side (cis).

- Examine Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other and are not enantiomers. Cis-trans isomers fall under the category of diastereomers because they include compounds that are not mirror images but have different spatial arrangements.

- Consider Enantiomers

Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. (E)-2-butene and (Z)-2-butene are not mirror images and thus cannot be enantiomers.

- Conclude the Correct Answers

Based on the above analysis, (E)-2-butene and (Z)-2-butene are both cis-trans isomers and diastereomers. Hence, the correct answer is option (C) I and II only.

Key concepts

Cis-trans isomers

Cis-trans isomers, also known as geometric isomers, are a type of isomer where molecules have the same molecular formula but differ in the arrangement of atoms around a double bond or ring structure. The terms 'cis' and 'trans' describe the relative positions of substituent groups. In cis isomers, substituents are on the same side of the double bond, making the molecule appear more 'bent'. An example is (Z)-2-butene. In trans isomers, substituents are on opposite sides, giving the molecule a more 'linear' shape. An example is (E)-2-butene. The distinction arises due to restricted rotation around the double bond, which forces a fixed spatial arrangement. This spatial restriction leads to unique physical and chemical properties. For instance:

• Cis-2-butene has a higher boiling point than trans-2-butene because the bent shape causes stronger intermolecular forces.
• Trans-2-butene is more stable due to lesser steric hindrance.

Understanding cis-trans isomers helps in predicting the properties and reactivity of organic molecules.

Diastereomers

Diastereomers are a type of stereoisomer that are not mirror images of each other. This class includes cis-trans isomers. More broadly, diastereomers arise in molecules with two or more chiral centers. These isomers have different configurations at one or more (but not all) of the chiral centers.
It is crucial to note that diastereomers have different physical and chemical properties. Examples include melting points, boiling points, and reactivity. For instance:

• They may have different solubilities in various solvents.
• Their interaction with other chiral environments (like other molecules or catalysts) can vary.

(E)-2-butene and (Z)-2-butene are diastereomers because they are not mirror images. The different spatial arrangements lead to diverse behaviors in chemical reactions, which can be very useful in fields like pharmacology and materials science.

Enantiomers

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. This means that they have chiral centers – typically a carbon with four different groups attached – leading to two configurations that are “left” and “right” handed. These are denoted as 'R' and 'S' enantiomers.
What makes enantiomers especially interesting is their interaction with polarized light. Each enantiomer will rotate plane-polarized light in opposite directions, a property known as optical activity. Here's what you need to know:

• One enantiomer will rotate light clockwise (dextrorotatory, +).
• The other will rotate light counterclockwise (levorotatory, -).

It's also important to mention how enantiomers interact in biological systems since many biomolecules are chiral. This can lead to: Different pharmacological effects: One enantiomer might be therapeutic, while the other could be inactive or even harmful.
Given this specificity, (E)-2-butene and (Z)-2-butene are not enantiomers since they do not exhibit non-superimposable mirror image relationships. They don't have chiral centers but rather different spatial arrangements around a double bond.