Question

Evaluate A certain reaction yields 80% trans-2-pentene and 20% cis-2-pentene. Draw the structures of these two geometric isomers, and develop a hypothesis to explain why the isomers form in the proportions cited.

Short answer

The structures of trans-2-pentene and cis-2-pentene are as follows: Trans-2-pentene: ``` H H \ / C==C | | H C | ... ``` Cis-2-pentene: ``` H H \ / C==C | | H H | | C ... ``` A possible hypothesis to explain the formation of these geometric isomers in an 80%-20% proportion could be related to steric hindrance. In the cis-2-pentene isomer, there is a higher degree of steric hindrance, making it less favorable compared to trans-2-pentene, which has reduced steric hindrance and is more stable. Therefore, trans-2-pentene forms 80% of the product, while cis-2-pentene forms only 20%.

Step-by-step solution

Draw the structures of trans-2-pentene and cis-2-pentene

To draw the structures of these geometric isomers, we need to remember that "cis" isomers have the same groups on the same side of the double bond, while "trans" isomers have the same groups on opposite sides of the double bond. Trans-2-pentene: ``` H H \ / C==C | | H C | ... ``` Cis-2-pentene: ``` H H \ / C==C | | H H | | C ... ```

Develop a hypothesis to explain the isomers' proportions

One possible hypothesis to explain the formation of these isomers in the given proportion could be based on steric hindrance. Steric hindrance refers to the spatial repulsion that occurs between atoms or groups of atoms due to their close proximity. In the case of cis-2-pentene, both alkyl groups at the second carbon are on the same side of the double bond, which can cause greater steric hindrance between the two groups. This can make the cis formation less favorable compared to the trans-2-pentene, where the steric hindrance is reduced because the alkyl groups are on opposite sides of the double bond. Therefore, the trans-2-pentene is likely to be more stable, and it forms the majority (80%) of the product due to the reduced steric hindrance, while the cis-2-pentene, which experiences more steric hindrance, forms only 20% of the product.

Key concepts

Trans-2-Pentene

In organic chemistry, the structure of molecules plays a crucial role in their properties and reactivity. Trans-2-pentene is a geometric isomer of pentene where the substituents adjacent to the double bond (in this case, hydrogen and a longer carbon chain) are situated opposite each other. Imagine the double bond as a page in a book; the substituents are like hands holding the book open on opposite sides.

The name 'trans' derives from Latin, meaning 'across', which aptly describes the positions of the important groups across the double bond. Trans isomers generally tend to exhibit lower boiling points compared to their cis counterparts due to their more linear structure, which results in less efficient packing and weaker intermolecular forces.

Trans-2-pentene's linearity also means there is less steric strain between the substituents, contributing to its prevalence in certain reactions, as is highlighted by the 80% yield in the given exercise.

Cis-2-Pentene

Cis-2-pentene stands in contrast to its trans counterpart, featuring both substituents on the same side of the double bond. The prefix 'cis' comes from Latin, meaning 'on this side of'. If we return to the book analogy, the substituents are like two hands holding the book open on the same side.

This configuration leads to a bent shape, making it less symmetrical and more compact. As a result, molecules of cis-2-pentene can pack more efficiently, leading to higher boiling points. Additionally, its shape introduces steric strain, as the adjacent substituents are closer to each other and can interfere, causing what is known as steric hindrance.

Steric Hindrance

Steric hindrance is a term used to describe the physical constriction of chemical reactions due to the spatial arrangement of atoms or groups within a molecule. This phenomenon is like a crowded room; just as people have a harder time moving around when there is less space, atoms and functional groups can hinder each other's movement or ability to react when they are too close together.

Steric hindrance is particularly relevant in reactions involving bulky groups or in cyclic systems where the rigidity of the ring limits the flexibility of the molecule. In the context of our exercise, the steric hindrance can explain why cis-2-pentene is formed less often than trans-2-pentene. The close proximity of the larger substituents on the same side of the molecule in the cis isomer creates a less favorable, more energetically strained, condition.

Chemical Stability

Chemical stability refers to how resistant a molecule is to changes or reactions under specific conditions. A molecule is considered stable if it does not react easily with other substances or convert to a different structure. Stability can be influenced by a variety of factors, including electronic effects, steric hindrance, and molecular strain.

In terms of geometric isomers, trans-2-pentene is more stable due to reduced steric hindrance, making it less reactive compared to cis-2-pentene. This enhanced stability is why the trans isomer is more abundant in the reaction outcome. Ultimately, the desire for molecules to adopt the lowest energy and most stable state is a key principle that drives the formation of certain products over others in chemical reactions.