Question

Trans arrangements of vicinal diols are more stable at higher \(\mathrm{pH}\).

Short answer

Based on the analysis, the trans arrangements of vicinal diols are more stable at higher pH values primarily because of reduced steric hindrance compared to cis arrangements. Additionally, under basic conditions with fewer available protons for hydrogen bonding, the stability of the trans arrangement increases.

Step-by-step solution

Understanding Vicinal Diols

Vicinal diols are compounds that have two hydroxyl groups (-OH) attached to adjacent carbon atoms. There are two possible arrangements for the hydroxyl groups in vicinal diols: cis and trans. In cis arrangements, the hydroxyl groups are on the same side of the molecule, while in trans arrangements, they are on opposite sides.

Analyzing Steric Hindrance

Steric hindrance refers to the restriction of a molecule's reactivity due to large groups preventing access to reactive sites. In the case of cis and trans arrangements of vicinal diols, the cis arrangement has more steric hindrance due to the hydroxyl groups being closer together. This leads to increased repulsion between the electron lone pairs on the oxygen atoms, making it less stable and more reactive compared to its trans counterpart.

Considering Intramolecular Hydrogen Bonding

In the case of cis arrangements, the proximity of the hydroxyl groups allows for intramolecular hydrogen bonding, which stabilizes the molecule to some extent. However, this stabilization is not sufficient to outweigh the destabilizing effect of steric hindrance. In the trans arrangement, intramolecular hydrogen bonding is less likely to occur due to the greater distance between the hydroxyl groups. This means that stability is primarily dictated by steric factors.

Understanding the Influence of pH on Stability

pH is a measure of the acidity or basicity of a solution, with low pH values indicating acidic conditions and high pH values indicating basic conditions. Under acidic conditions, the presence of protons (\mathrm{H}^+) can facilitate the formation of intermolecular or intramolecular hydrogen bonds between the hydroxyl groups and protons from the acidic medium, temporarily stabilizing the cis arrangement. However, in high pH environments where there are fewer protons available, these hydrogen bonds are less likely to form, and the trans arrangement becomes more stable due to its reduced steric hindrance.

Conclusion

Trans arrangements of vicinal diols are more stable at higher pH values primarily due to their reduced steric hindrance compared to cis arrangements. Under basic conditions, the lack of available protons for hydrogen bonding also contributes to the increased stability of the trans arrangement.