Question

Indicate which of the following compounds show geometric (cis-trans) isomerism, draw the isomeric structures, and specify each as \(Z\) or \(E\). (a) 1-butane (g) 2 -pentene (b) 2-butane (h) 1-chloropropene (c) 1,1 -dichloroethene (i) 1 -chloro-2-methy1-2-butene (d) 1,2 -dichloroethene (j) 3 -methy \(1-4\) -ethy \(1-3\) -hexane (e) 2 -methy \(1-2\) -butane (k) 2,4 -hexadiene (f) 1-pentene \(\quad\left(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}=\mathrm{CHCH}_{3}\right)\)

Short answer

In summary, the compounds that show geometric (cis-trans) isomerism are 2-butene, 1,2-dichloroethene, 1-pentene, 2-pentene, 1-chloropropene, 1-chloro-2-methyl-2-butene, and 2,4-hexadiene. Their respective isomeric structures and Z/E labels are: (b) 2-butene: Z-isomer: H-\(\bold{C=C}\)-H\( \) E-isomer: H-\(\bold{C=C}\)-CH3\) with H and CH3 trans locations. (d) 1,2-dichloroethene: Z-isomer: Cl-\(\bold{C=C}\)-H\( \) E-isomer: Cl-\(\bold{C=C}\)-Cl\) with Cl and Cl trans locations. (f) 1-pentene: Z-isomer: H-\(\bold{C=C}\)-CH2-CH3\( \) E-isomer: H-\(\bold{C=C}\)-CH3\) with H and CH3 trans locations. (g) 2-pentene: Z-isomer: CH3-\(\bold{C=C}\)-CH2-CH3\( \) E-isomer: CH3-\(\bold{C=C}\)-CH3\) with CH3 and CH3 trans locations. (h) 1-chloropropene: Z-isomer: Cl-\(\bold{C=C}\)-H\( \) E-isomer: Cl-\(\bold{C=C}\)-CH3\) with Cl and CH3 trans locations. (i) 1-chloro-2-methyl-2-butene: Z-isomer: CH3-\(\bold{C=C}\)-CH2-CH3\( \) E-isomer: CH3-\(\bold{C=C}\)-CH3\) with CH3 and CH3 trans locations. (k) 2,4-hexadiene: (multiple isomers) Z,Z-isomer (both double bonds are in Z configuration) E,E-isomer (both double bonds are in E configuration) Z,E-isomer (one double bond is in Z configuration and the other in E configuration) E,Z-isomer (one double bond is in E configuration and the other in Z configuration)

Step-by-step solution

Isomeric Structures and Z/E Labels

For compounds that have geometric isomers, we draw and label their structures as Z or E: (b) 2-butene: Z-isomer: H-\(\bold{C=C}\)-H\(\ E-isomer: H-\(\bold{C=C}\)-CH3\) with H and CH3 trans locations. (d) 1,2-dichloroethene: Z-isomer: Cl-\(\bold{C=C}\)-H\(\ E-isomer: Cl-\(\bold{C=C}\)-Cl\) with Cl and Cl trans locations. (f) 1-pentene: Z-isomer: H-\(\bold{C=C}\)-CH2-CH3\(\ E-isomer: H-\(\bold{C=C}\)-CH3\) with H and CH3 trans locations. (g) 2-pentene: Z-isomer: CH3-\(\bold{C=C}\)-CH2-CH3\(\ E-isomer: CH3-\(\bold{C=C}\)-CH3\) with CH3 and CH3 trans locations. (h) 1-chloropropene: Z-isomer: Cl-\(\bold{C=C}\)-H\(\ E-isomer: Cl-\(\bold{C=C}\)-CH3\) with Cl and CH3 trans locations. (i) 1-chloro-2-methyl-2-butene: Z-isomer: CH3-\(\bold{C=C}\)-CH2-CH3\(\ E-isomer: CH3-\(\bold{C=C}\)-CH3\) with CH3 and CH3 trans locations. (k) 2,4-hexadiene: It has multiple isomers (we will only show the most relevant ones) Z,Z-isomer (both double bonds are in Z configuration) E,E-isomer (both double bonds are in E configuration) Z,E-isomer (one double bond is in Z configuration and the other in E configuration) E,Z-isomer (one double bond is in E configuration and the other in Z configuration)

Key concepts

Cis-Trans Isomerism

Geometric isomerism, particularly cis-trans isomerism, is an important aspect of chemistry that explains how compounds with the same molecular formula can have different physical properties. Ever wondered why some compounds with identical molecular formulas can smell different or have varying boiling points? It's the subtle differences in how the atoms are arranged in space.

Cis-trans isomerism occurs in molecules with restricted rotation around a bond, commonly a double bond or a ring structure. In such cases, atoms or groups attached to the double-bonded carbons can be situated differently in space leading to the 'cis' (meaning 'on the same side') or 'trans' (meaning 'across' or 'on the opposite side') arrangements.

For example, in 2-butene, which shows cis-trans isomerism, the two methyl groups (CH3) can either be on the same side (cis-2-butene) or on opposite sides (trans-2-butene) of the double bond. These configurations are not interconvertible without breaking the double bond, making them distinct molecular entities with unique properties.

Z-E Isomerism

Closely related to cis-trans isomerism is Z-E isomerism, a more encompassing system that looks at the priority of substituents attached to the double-bonded carbons based on their atomic numbers or substituent mass (the Cahn-Ingold-Prelog priority rules). 'Z' stands for 'zusammen', German for 'together', implying that the higher priority groups are on the same side. Conversely, 'E' (from 'entgegen', meaning 'opposite') denotes that the higher priority groups are on opposite sides of the double bond.

This distinction is particularly useful when dealing with complex molecules where the simple cis-trans terminology falls short. For instance, in 1-chloro-2-methyl-2-butene, saying 'cis' or 'trans' isn't sufficient because there are more than just hydrogen and one type of group attached to the double bonded carbons. Using 'Z' and 'E' gives a clearer description of which substituents are on which side of the molecule, making it a universally applicable system.

Structural Isomers

Beyond geometric isomerism, structural isomers are another major category of isomers, also known as constitutional isomers. Structural isomerism arises when molecules with the same molecular formula have different bonding patterns and atomic organization. This can lead to significant diversity in chemical properties and biological activities.

These isomers vary widely in complexity, from the simplest form where they differ in the order of attachment of atoms (chain isomerism) to more complex forms like functional isomerism (same atoms arranged into different functional groups) and position isomerism (functional groups attached at different positions on the same carbon skeleton).

A classic example would be pentane and methylbutane, which share the molecular formula C5H12. Pentane has a straight-chain structure, whereas methylbutane has a branching chain, but both exhibit distinct physical properties due to their structural differences. Understanding these variations is key in fields ranging from pharmaceuticals where slight modifications can lead to different therapeutic outcomes, to petrochemical industries where different structural isomers fuel our vehicles in varied manners.