Problem 126 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following haloalkanes: 1\. \(\mathrm{CH}_{3} \mathrm{~F}\) 2\. \(\mathrm{CH}_{3} \mathrm{Cl}\) 3\. \(\mathrm{CH}_{3} \mathrm{Br}\) 4\. \(\mathrm{CH}_{3} \mathrm{I}\) The increasing order of reactivity in nucleophilic substitution reaction is (a) \(4<3<2<1\) (b) \(1<3<2<4\) (c) \(1<2<3<4\) (d) \(1<2<4<3\)

Step-by-Step Solution

Answer

(c) \(1<2<3<4\)

Step 1: Understanding Reactivity in Nucleophilic Substitution

The reactivity of haloalkanes in nucleophilic substitution reactions depends on the leaving group ability. A good leaving group is one that can depart as a relatively stable, weakly basic species.

Step 2: Identifying Leaving Groups

The leaving groups in the given haloalkanes are F, Cl, Br, and I. The quality of a leaving group generally increases with the size of the halogen anion. Thus, in order of increasing leaving group ability, they are F < Cl < Br < I.

Step 3: Applying to Haloalkanes

For the haloalkanes: 1. CH₃F has fluorine as the leaving group. 2. CH₃Cl has chlorine as the leaving group. 3. CH₃Br has bromine as the leaving group. 4. CH₃I has iodine as the leaving group. Iodine is the best leaving group, making CH₃I the most reactive in nucleophilic substitution. Conversely, fluorine is the poorest leaving group, making CH₃F the least reactive.

Step 4: Arranging in Increasing Order of Reactivity

The increasing order of reactivity in nucleophilic substitution based on the leaving group ability is: CH₃F < CH₃Cl < CH₃Br < CH₃I. This corresponds to the option (c) in the multiple-choice question.

Key Concepts

Haloalkanes Reactivity

Haloalkanes, also known as alkyl halides, are compounds in which a halogen atom is bonded to an alkyl group. Their reactivity in nucleophilic substitution reactions varies significantly. In these reactions, a nucleophile replaces the halogen atom, known as the leaving group. These reactions are common and essential in organic chemistry, leading to the formation of new compounds.
The reactivity of haloalkanes largely depends on two main factors: the stability of the leaving group and the structure of the carbon chain to which it is attached. In our context, the ability of the leaving group plays a more pivotal role in determining reactivity. The larger the halogen atom, the more reactive the haloalkane tends to be. This is because larger halogen atoms form more stable anions when they leave the molecule, making the substitution process more favorable. For example, in our set of haloalkanes, CH₃F, CH₃Cl, CH₃Br, and CH₃I, the reactivity increases with the size of the halogen: F < Cl < Br < I. Therefore, CH₃I is the most reactive while CH₃F is the least.

Leaving Group Ability

The concept of a leaving group is central to understanding nucleophilic substitution reactions. A leaving group is an atom or group of atoms that detaches from the parent molecule during the reaction. The effectiveness of a leaving group is determined by its ability to stabilize the negative charge after departure.
In general, a good leaving group is a weak base and a stable anion. This characteristic allows it to leave without causing the parent molecule to become overly unstable or reactive. As we analyze the halogens — fluoride, chloride, bromide, and iodide — iodide is observed to be the best leaving group. This is due to its larger size which enables it to disperse the negative charge over a larger volume, contributing to its stability. Consequently, when comparing haloalkanes, those with better leaving groups (like iodine in CH₃I) will undergo nucleophilic substitution more readily compared to those with poorer leaving groups (like fluorine in CH₃F). Understanding this allows chemists to predict reaction speeds and choose suitable conditions for desired chemical processes.

Halogen Anions

Halogen anions are the negatively charged ions formed when halogens gain an electron. They play a crucial role in determining the leaving group ability of halogenated compounds. Each halogen has a distinct anionic form: fluoride (F⁻), chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻).
Let's delve into their characteristics. **Fluoride (F⁻)**, due to its small size, holds its negative charge tightly, making it a poor leaving group. **Chloride (Cl⁻)** is slightly larger and, therefore, a somewhat better leaving group. **Bromide (Br⁻)**, being larger still, is even more suitable. Finally, **iodide (I⁻)** is the largest and most stable due to its ability to effectively diffuse the negative charge, making it the best leaving group among the halogens.

This hierarchy of leaving group quality directly translates into the order of reactivity for haloalkanes in nucleophilic substitution: **CH₃F < CH₃Cl < CH₃Br < CH₃I**. As such, knowledge of halogen anions not only aids in understanding their reactive nature but also plays a decisive role in predicting and controlling organic reactions.