Question

The iodide ion cannot displace the \(-\mathrm{OH}\) group in ethanol, but excess HI will react to produce ethyl iodide. Explain.

Short answer

The weak nucleophilic nature of iodide ions prevents them from displacing the OH group in ethanol. However, excess hydroiodic acid (HI) provides a more concentrated source of iodide ions, hence overcoming the steric hindrance to the reaction, and leads to the formation of ethyl iodide.

Step-by-step solution

Understand the Reactivity of Iodide and Hydroxyl Ions

The iodide ion is a weak nucleophile because it carries a negative charge. Negative charges repel the electron cloud of the organic substrate, causing reduced reactivity. On the other hand, the hydroxyl (OH) group in ethanol is a strong base due to its ability to donate an electron pair. Therefore, it is less likely for the iodide ion to displace the hydroxyl group.

Consider the Effect of Excess HI

When there is excess HI present, the reaction can go forward because the increased concentration of iodide ions helps to overcome the steric hindrance to its reaction with the ethanol. Hence, the reaction with excess HI ends up forming ethyl iodide.

Formulate a Comprehensive Explanation

The iodide ion cannot displace the OH group in ethanol due to its weak nucleophilic nature as opposed to the strong basic nature of OH. However, in the presence of excess HI, the concentration of iodide ions is increased, which enables the reaction to go forward, leading to the formation of ethyl iodide.

Key concepts

Nucleophilic Displacement

Nucleophilic displacement is a type of reaction commonly seen in organic chemistry where a nucleophile replaces a leaving group in a compound. In this context, a nucleophile is a species that donates an electron pair to form a chemical bond in reaction. The ability of a nucleophile to displace a group depends on its strength, steric factors, and the nature of the leaving group.
A strong nucleophile is usually a species with a high electron density and greater reactivity, making it more likely to attack an electrophilic center. However, in the exercise case, iodide ions (I-) are considered weak nucleophiles. This weakness stems from their large size and relatively diffused charge, making them less effective at forming bonds with the carbon atoms of the ethanol molecule.
IV Includes the fact that the hydroxyl group (\(-\text{OH}\)) in ethanol is a poor leaving group because of its strong tendency to hold onto electrons. As a result, the iodide ion alone is not able to displace the \(-\text{OH}\) in ethanol unless specific reaction conditions are met.

Reactivity of Ions

In chemical reactions, the reactivity of ions significantly influences the outcome. Reactivity is determined by various factors such as charge, size, and the ability to donate or accept electron pairs.
The iodide ion, while carrying a negative charge, is a weak nucleophile compared to other halide ions. It exhibits less reactivity due to its larger atomic radius and lesser tendency to participate actively in reactions. This results in weaker interactions with other substances, hence less effective nucleophilic displacement.
Conversely, the hydroxyl group is known for its strong basic character, due to its tendency to maintain the electron pair it possesses, decreasing its inclination to be displaced. In the reaction of ethanol and iodide ion, the iodide's inability to displace hydroxyl can be attributed to both its weak nucleophilicity and the strong binding of the \(-\text{OH}\) group. Thus, an excess of HI is necessary to shift the reaction equilibrium.

Ethanol and Halide Reactions

Ethanol’s reaction with halides is an insightful example in organic chemistry, primarily involving the interplay between nucleophiles and leaving groups. In the context of ethanol and iodide ion interactions, the lack of reaction under normal conditions can be attributed to the properties of both the alcohol and the nucleophile.
In general, halide ions like I- are potent nucleophiles, but their nucleophilicity can be inadequate against strong bases like hydroxide. The \(-\text{OH}\) group in ethanol resists being displaced due to its basic nature, making it cling closely to the carbon atom.
Excess HI provides a high concentration of H+ ions, which can protonate the \(-\text{OH}\) group, temporarily converting it into a better leaving group, the water molecule (H2O). This enables the iodide ion to finally displace it, leading to the formation of ethyl iodide, an alkyl halide. Thus, through specific conditions, like the presence of excess HI, the reaction proceeds, showcasing the delicate balance of nucleophilicity and leaving group potential.