Question

Which of the following solvents would be LEAST useful for a nucleophile- electrophile reaction? A \(\mathrm{H}_{2} \mathrm{O}\) B \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) C \(\mathrm{CH}_{3} \mathrm{SOCH}_{3}\) D \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

D \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\)

Step-by-step solution

Understand the role of solvents in nucleophile-electrophile reactions

In nucleophile-electrophile reactions, the solvent can play a crucial role in determining the reaction's success. Solvents can either stabilize or destabilize the nucleophile and the electrophile, affecting the reaction rate and mechanism.

Identify polar and nonpolar solvents

Identify which of the given solvents are polar and which are nonpolar. Polar solvents (water, ethanol, dimethyl sulfoxide) can stabilize ions through solvation, whereas nonpolar solvents (hexane) do not.

Determine solvent properties

Analyze the properties of each solvent: - A: \(\text{H}_2\text{O}\) is a polar protic solvent- B: \(\text{CH}_3\text{CH}_2\text{OH}\) is a polar protic solvent- C: \(\text{CH}_3\text{SOCH}_3\) is a polar aprotic solvent- D: \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3\) (hexane) is a nonpolar solvent

Select the least useful solvent for nucleophile-electrophile reactions

Nonpolar solvents are least effective in facilitating nucleophile-electrophile reactions due to their inability to stabilize charged transition states or intermediates. Therefore, hexane (option D) would be the least useful solvent.

Key concepts

Polar Protic Solvent

Polar protic solvents are solvents that have a hydrogen atom attached to an electronegative atom like oxygen or nitrogen. This hydrogen becomes available for hydrogen bonding. Examples include water (H₂O) and ethanol (CH₃CH₂OH).
These solvents can stabilize both cations (positive ions) and anions (negative ions). They do so by hydrogen bonding with the anions and through dipole interactions with the cations. This makes polar protic solvents particularly helpful in reactions where ions are formed, such as sn1 reactions.

• Water (H₂O) is a classic polar protic solvent. It is excellent for reactions that involve ions.
• Ethanol (CH₃CH₂OH) is another example. Its properties are similar to water, but it is less polar.

While these solvents can be beneficial, they can also sometimes stabilize nucleophiles too much, reducing their reactivity. This makes them less ideal for nucleophile-electrophile reactions compared to other solvents.

Nonpolar Solvent

Nonpolar solvents, as the name suggests, do not have a significant dipole moment. They have low dielectric constants and do not solvate ions well. Examples include hexane and benzene.
Nonpolar solvents are not effective at stabilizing ions. This means that any reaction involving charged intermediates or transition states will proceed very poorly in these solvents.

• Hexane (C₆H₁₄) is a common nonpolar solvent. It's generally used in reactions that do not involve ions, such as certain organic syntheses.
• Benzene (C₆H₆) also falls into this category, though it has some pi-electron interactions that can influence some reactions.

Since nucleophile-electrophile reactions often involve charged intermediates, nonpolar solvents like hexane are the least useful for these types of reactions, making them a poor choice.

Polar Aprotic Solvent

Polar aprotic solvents are unique because they are polar enough to dissolve salts but do not have hydrogen atoms bonded to electronegative atoms like oxygen or nitrogen. They can solvate cations very well due to their polar nature, but they do not solvate anions effectively.
Common examples of polar aprotic solvents include dimethyl sulfoxide (DMSO) and acetone.

• Dimethyl sulfoxide (CH₃SOCH₃) is a highly polar aprotic solvent. It’s very effective for nucleophile-electrophile reactions because it can stabilize the cations without stabilizing the anions too much.
• Acetone (CH₃COCH₃) is another well-known polar aprotic solvent used in many reactions, particularly in substitution and elimination reactions.

These solvents are highly effective in nucleophile-electrophile reactions, as they can stabilize the cationic intermediates or transition states, while the nucleophile remains relatively reactive.