Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

The relative order of reactivity of \(\mathrm{F}^{-}, \mathrm{Cl}^{-} \mathrm{Br}\) and \(\mathrm{I}^{-}\)is/ are (a) \(\mathrm{F}^{-}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}<\mathrm{I}^{-}\)in polar solvent (b) \(\mathrm{F}^{-}>\mathrm{Cl}^{-}>\mathrm{Br}>\mathrm{I}^{-}\)in non polar solvent (c) \(\mathrm{F}^{-}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}<\mathrm{I}^{-}\)in non polar solvent (d) \(\mathrm{F}^{-}>\mathrm{Cl}^{-}>\mathrm{Br}^{-}>\mathrm{I}^{-}\)in polar solvent

Step-by-Step Solution

Answer

Both options (a) and (b) are correct.

Step 1: Understand the Solvent Effect on Reactivity

The reactivity of halide ions (39;45;, 46;, 25;, 31;) largely depends on the solvent used. In a polar solvent, the ions form hydrogen bonds and are more solvated. This reduces their reactivity as nucleophiles. Conversely, in a non-polar solvent, the lack of solvation makes them more reactive.

Step 2: Reactivity Order in Polar Solvent

In polar solvents, fluoride ion (39;) forms strong hydrogen bonds and is strongly solvated, reducing its reactivity compared to other halides. Hence, the relative order of reactivity is 39;<46;<25;<31;.

Step 3: Reactivity Order in Non-polar Solvent

In non-polar solvents, halide ions are not strongly solvated. The order of reactivity is then determined by the basicity of the ions. Fluoride ion (39;) being the most basic, is most reactive, and the order is 39;>46;>25;>31;.

Step 4: Compare Given Options

Let's evaluate the given options based on our analysis: - Option (a) 39;<46;<25;<31; in polar solvent - matches Step 2 - Option (b) 39;>46;>25;>31; in non-polar solvent - matches Step 3 - Option (c) 39;<46;<25;<31; in non-polar solvent - does not match - Option (d) 39;>46;>25;>31; in polar solvent - does not match

Key Concepts

Solvent Effect

The solvent effect is a crucial factor when examining the reactivity of halide ions, which include fluoride (\(\mathrm{F}^{-}\)), chloride (\(\mathrm{Cl}^{-}\)), bromide (\(\mathrm{Br}^{-}\)), and iodide (\(\mathrm{I}^{-}\)). A solvent can either be polar or non-polar, impacting the reactivity behavior of these ions in different ways. Understanding this effect allows us to predict how ions will react under various conditions, shaping both their solubility and reactivity. In general, polar solvents tend to stabilize ions through solvation. They can effectively shield ions from each other due to their ability to form hydrogen bonds or dipole interactions. This has the potential to decrease an ion's reactivity by mitigating the ion's natural electromagnetic tendencies. Non-polar solvents lack such properties, leading to different outcomes that influence reactivity in ways that can enhance it.

Polar Solvent Reactivity

In a polar solvent, such as water or alcohols, halide ions become solubilized through the formation of hydrogen bonds and other dipole interactions.

• Fluoride ions (\(\mathrm{F}^{-}\)) are particularly affected as they form strong hydrogen bonds due to both their small size and high charge density. This means they become heavily solvated.
• This strong solvation reduces their ability to participate in reactions, making them the least reactive. Conversely, iodide ions (\(\mathrm{I}^{-}\)) have larger atomic radii and are less solvated, allowing them to remain more reactive among halides.
• The reactivity order in polar solvents therefore follows: \(\mathrm{F}^{-}<\mathrm{Cl}^{-}<\mathrm{Br}^{-}<\mathrm{I}^{-}\).

This knowledge is important for predicting outcomes in organic reactions where polar solvents are employed, ensuring effective control over the process.

Non-Polar Solvent Reactivity

In non-polar solvents, such as hexane or carbon tetrachloride, the situation changes markedly. Non-polar solvents are unable to solvate ions effectively because they do not have the necessary dipole-dipole interactions.

• This lack of solvation allows the intrinsic characteristics of the ions to dictate their reactivity. Here, the inherent basicity of the ion becomes a significant factor.
• Fluoride ions, being the most basic of the halides due to their high electronegativity and small size, exhibit the highest reactivity.
• The reactivity order in non-polar solvents transitions to:\(\mathrm{F}^{-}>\mathrm{Cl}^{-}>\mathrm{Br}^{-}>\mathrm{I}^{-}\).

This has critical implications for certain chemical processes where non-polar environments are sought to enhance the reactivity of specific halide ions.