Question

The trans alkenes are generally more stable than the cis alkenes. Give two examples of unsaturated systems where you would expect the cis form to be more stable and explain the reason for your choice.

Short answer

The cis-1,2-dimethylcyclopropane and cis-9,10-dimethylbicyclo[4.2.0]octane are two examples of unsaturated systems where the cis-form is more stable than the trans-form. In the case of cis-1,2-dimethylcyclopropane, the stability is due to the reduced steric hindrance and ring strain in the small cyclopropane ring. For cis-9,10-dimethylbicyclo[4.2.0]octane, the stability arises from the locked conformation of the bicyclic compound, which leads to reduced strain in the molecule.

Step-by-step solution

Example 1: cis-1,2-dimethylcyclopropane

The first example is cis-1,2-dimethylcyclopropane. In this case, the cis-isomer is more stable than the trans-isomer due to the small ring size of the cyclopropane. The trans-1,2-dimethylcyclopropane is not stable because it would require the two methyl groups to be on opposite sides of the small, strained ring, leading to significant steric hindrance and increased ring strain. In the cis-isomer, the methyl groups are on the same side of the ring, and the steric hindrance is reduced, making it more stable than the trans-isomer.

Example 2: Disubstituted bicyclic compounds

The second example is disubstituted bicyclic compounds, such as cis-9,10-dimethylbicyclo[4.2.0]octane. In this case, the cis-isomer is more stable due to the locked conformation of the bicyclic compound. In the cis-isomer, the two methyl groups are on the same side of the bicycle and do not significantly hinder each other. In the trans-isomer, with the methyl groups facing in opposite directions, the bicyclic structure would need to be twisted in a non-planar conformation, which would lead to increased strain on the molecule. This strain makes the trans-isomer less stable compared to the cis-isomer. In both examples, the factors contributing to the stability of the cis-isomers over the trans-isomers are reduced steric hindrance and decreased strain in the molecules' overall conformation.

Key concepts

Steric Hindrance

Steric hindrance occurs when atoms within a molecule are so close that their electron clouds experience repulsion, leading to increased energy and decreased stability of the molecule. Simply put, when atoms or groups of atoms are too bulky, they can get into each other's way, creating a kind of molecular traffic jam. This often happens in organic compounds where substituents, like methyl groups, are large relative to the rest of the molecule.

For instance, in cis-1,2-dimethylcyclopropane, the two methyl groups are on the same side of the small ring, and despite being close, the molecule's constrained size means the groups cannot avoid each other entirely. However, this closeness is still preferable to the extreme discomfort the groups would experience in a trans configuration, where the ring size does not allow enough space for them to sit on opposite sides without extreme repulsion. Thus, the steric hindrance significantly influences stability and is a crucial concept in understanding why certain cis-trans isomers like cis-1,2-dimethylcyclopropane have a preferred, more stable configuration.

Ring Strain

Ring strain is a concept in chemistry that arises primarily from the geometric constraints of cyclic molecules. It is a measure of the instability caused by the compound's atoms being forced into unfavorable bonding angles and distances. This strain can result from bond angle distortion, torsional strain from eclipsed interactions, and non-bonded strain due to atoms being forced closer than their optimal Van der Waals radii.

As an example, cyclopropane rings are known for their high ring strain because the bond angles are forced to be 60 degrees, much smaller than the preferred 109.5 degrees in a tetrahedral carbon. When substituents are added, like in cis-1,2-dimethylcyclopropane, the ring strain can be amplified by the steric hindrance. In this situation, the cis isomer is preferred over the trans isomer because, with the substituents on the same side, the molecule can spread out the strain more evenly, reducing the overall energy and increasing the stability of the molecule.

Conformational Analysis

Conformational analysis involves studying the different shapes (conformations) that a molecule can adopt due to rotations around its single bonds, and predicting which conformations will be most energetically favorable. For cyclic compounds and unsaturated hydrocarbons, the process is particularly fascinating because pi bonds and ring structures limit the molecule's flexibility, making certain conformations more favorable than others.

In the case of disubstituted bicyclic compounds, like cis-9,10-dimethylbicyclo[4.2.0]octane, conformational analysis shows that the cis form limits the steric interactions between the methyl groups. The trans form, on the other hand, would force the molecule into a twisted conformation introducing additional strain, which makes it less stable. The conformational analysis predicts that the cis isomer's locked conformation maintains a lower energy state and hence is more stable.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons are organic compounds that contain at least one carbon-carbon double bond or triple bond. The presence of these multiple bonds introduces rigidity and limits the motion around the bond, affecting the molecule's reactivity, physical properties, and stability. Alkenes, which have double bonds, can exist in different isomeric forms, including cis and trans isomers characterized by the relative positioning of substituents around the double bond.

Unsaturated hydrocarbons like alkenes are key to understanding why, generally, trans alkenes are more stable than cis alkenes due to the reduced steric hindrance and overall lower energy conformation. However, as evident with cis-1,2-dimethylcyclopropane and cis-9,10-dimethylbicyclo[4.2.0]octane, the context of the molecular framework can lead to scenarios where the cis isomer is indeed the more stable form. Thus, analyzing unsaturated hydrocarbons, especially in terms of their conformation and steric interactions, is crucial for predicting their stability and behavior.