Problem 65 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Arrange the following in: Increasing reactivity in nucleophilic substitution reactions \(\mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{I}, \mathrm{CH}_{3} \mathrm{Br}, \mathrm{CH}_{3} \mathrm{Cl}\)

Step-by-Step Solution

Answer

Increasing reactivity: \(\text{CH}_3\text{F} < \text{CH}_3\text{Cl} < \text{CH}_3\text{Br} < \text{CH}_3\text{I}\).

Step 1: Understand Nucleophilic Substitution

Nucleophilic substitution is a reaction where a nucleophile replaces a leaving group attached to a carbon atom. The ease of this reaction depends on the nature of the leaving group; generally, weaker bases make better leaving groups.

Step 2: Analyze Halide Leaving Groups

The halides \( ext{F}^-\), \( ext{Cl}^-\), \( ext{Br}^-\), and \( ext{I}^-\) are the leaving groups for \( ext{CH}_3 ext{F}\), \( ext{CH}_3 ext{Cl}\), \( ext{CH}_3 ext{Br}\), and \( ext{CH}_3 ext{I}\), respectively. The strength as leaving groups is inversely related to their basicity: \( ext{F}^- > ext{Cl}^- > ext{Br}^- > ext{I}^-\) as bases, meaning \( ext{I}^-\) is the best leaving group and \( ext{F}^-\) is the worst.

Step 3: Determine Increasing Order of Reactivity

Since weaker bases (better leaving groups) enhance nucleophilic substitution reactions, the order from least to most reactive is based on the leaving group: \( ext{CH}_3 ext{F}\) (worst) < \( ext{CH}_3 ext{Cl}\) < \( ext{CH}_3 ext{Br}\) < \( ext{CH}_3 ext{I}\) (best).

Key Concepts

Leaving Groups

In nucleophilic substitution reactions, the term 'leaving group' refers to the atom or group of atoms that detaches from the main molecule when a nucleophile attacks. The effectiveness of a leaving group is crucial in determining the rate of the reaction. The better the leaving group, the faster the substitution will occur.
Leaving groups generally come from weaker bases. This is because weaker bases are more stable when they leave the molecule and exist independently. They do not readily reassume an electron pair to form a bond, making them good candidates for this process.
Some common characteristics of good leaving groups include:

• Low basicity: Weaker bases tend to be better leaving groups.
• Ability to stabilize negative charge: The more stable a species can be on its own, the better it will be as a leaving group.
• High electronegativity: Atoms with higher electronegativity tend to hold electrons tightly, which can help stabilize them as ions.

In our example, iodide ( I^- ) is the best leaving group among the halides ( text{F}^- , text{Cl}^- , text{Br}^- , text{I}^- ), and fluoride ( text{F}^- ) is the worst, due to its base strength.

Halide Reactivity

Halide reactivity is an essential factor in determining the order of reactivity in nucleophilic substitution reactions. Different halides ( text{F}^- , text{Cl}^- , text{Br}^- , text{I}^- ) vary in their ability to act as leaving groups. This variance is mainly due to differences in their atomic size and electronegativity.
As you move down the periodic table in the halogen group, atoms get larger, and their ability to stabilize the negative charge improves. This means that larger halides are generally better leaving groups.
Focusing on our specific case:

• Fluoride ( text{F}^- ): It has high electronegativity but is not a good leaving group due to its small size and high basicity.
• Chloride ( text{Cl}^- ): Slightly larger and less basic than fluoride, making it a better leaving group.
• Bromide ( text{Br}^- ): Larger and less basic than chloride, thus even better.
• Iodide ( text{I}^- ): Large size and low basicity make it the best leaving group in this series.

This results in the increasing order of reactivity with fluoride being the least reactive and iodide the most.

Base Strength

The strength of a base plays a crucial role in determining its effectiveness as a leaving group. In general, stronger bases hold onto electrons more tightly and are less willing to dissociate, which makes them poorer leaving groups in nucleophilic substitutions.
On the other hand, weaker bases can more readily accept their lone pairs back or exist stably on their own, enhancing their ability to leave. Thus, the base strength is inversely proportional to leaving group ability.
To sum up, consider the halide ions:

• Fluoride ( text{F}^- ): It is the strongest base among the halides mentioned, which makes it a poor leaving group.
• Chloride ( text{Cl}^- ), Bromide ( text{Br}^- ), and Iodide ( text{I}^- ): These are progressively weaker bases, with iodide being the weakest. Hence, they are better leaving groups in that order.

Understanding this concept assists in predicting the outcome of nucleophilic substitution reactions, leading to clearer insights into the chemical behavior of molecules.