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: Understand Nucleophilic Substitution

Nucleophilic substitution reactions occur when a nucleophile, which is a species with a pair of electrons, attacks an electrophilic center, typically a carbon atom bonded to a halogen. The more polarizable the halogen, the easier it is for the nucleophile to replace it.

Step 2: Evaluate Halogen Bond Strength

The bond strength between the carbon atom and the halogen is key in determining reactivity. The bond strength decreases as the size of the halogen increases: C-F > C-Cl > C-Br > C-I. This is because larger halogens like iodine form weaker bonds with carbon.

Step 3: Determine Reactivity Based on Bond Strength

In nucleophilic substitutions, the weaker the C-X bond (where X is a halogen), the more reactive the compound. Thus, haloalkanes with iodine will react faster than those with bromine, chlorine, or fluorine.

Step 4: Arrange Haloalkanes by Reactivity

Based on bond strength and reactivity in nucleophilic substitution reactions, the order from least to most reactive is: - 1: CH₃F - 2: CH₃Cl - 3: CH₃Br - 4: CH₃I

Step 5: Match the Order to Given Choices

Look for the option that matches the reactivity order determined: - (a) 4<3<2<1 - (b) 1<3<2<4 - (c) 1<2<3<4 - (d) 1<2<4<3 The correct choice is (c) 1<2<3<4.

Key Concepts

Haloalkanes

Haloalkanes, also known as alkyl halides, are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms – typically fluorine, chlorine, bromine, or iodine. These compounds play a significant role in organic chemistry because of their reactivity and functionality.
Haloalkanes are characterized by their carbon-halogen bond. This bond is polar because halogens are more electronegative than carbon, creating a polarized molecule with a partial positive charge on the carbon atom. This positive charge makes the carbon susceptible to attack by nucleophiles, leading to nucleophilic substitution reactions.
Understanding the nature of haloalkanes is crucial in organic synthesis and various chemical transformations. The different types of halogens attached to the carbon can influence the compound's reactions, dictating how it behaves during chemical processes.

Bond Strength in Haloalkanes

The bond strength in haloalkanes is a fundamental concept when discussing their reactivity. Here, the bond strength refers to the energy required to break the bond between the carbon atom and the halogen atom. This bond strength is influenced by the size and electronegativity of the halogen.
The trend for bond strength in haloalkanes is as follows:

• Carbon-Fluorine (C-F) bonds are the strongest because fluorine is the smallest halogen and highly electronegative.
• Carbon-Chlorine (C-Cl) bonds are slightly weaker than C-F bonds.
• Carbon-Bromine (C-Br) bonds are weaker than both C-F and C-Cl bonds.
• Carbon-Iodine (C-I) bonds are the weakest due to iodine's large size and lesser electronegativity.

The decreased bond strength with heavier halogens occurs because larger atoms have more substantial atomic radii, leading to longer and weaker bonds.
This hierarchy in bond strength plays a critical role in governing the overall reactivity of haloalkanes in nucleophilic substitution reactions.

Reactivity Order

In nucleophilic substitution reactions, the reactivity of haloalkanes is directly linked to the bond strength between the carbon and halogen atoms. The weaker the C-X bond (where X is the halogen), the more readily the haloalkane will undergo substitution.
The order of reactivity increases as the bond strength decreases. Therefore, in nucleophilic substitution reactions, the reactivity of haloalkanes follows the order:

• \( \text{CH}_3\text{F} \) is the least reactive because it has the strongest C-F bond.
• \( \text{CH}_3\text{Cl} \) comes next due to its weaker C-Cl bond compared to C-F.
• \( \text{CH}_3\text{Br} \) is more reactive as the C-Br bond is weaker compared to C-F and C-Cl bonds.
• \( \text{CH}_3\text{I} \) is the most reactive, owing to the weakest C-I bond.

This reactivity order can be easily remembered by noting that larger halogen atoms like iodine form weaker bonds, which makes them more reactive in nucleophilic substitution reactions. Hence, when facing a substitution reaction, haloalkanes containing iodine will generally react faster than those containing other halogens.