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) \(4<3 \leq 2<1\) (b) \(1<3<2<4\) (c) \(1<2<3<4\) (d) \(1<2<4<3\)

Step-by-Step Solution

Answer

The increasing order of reactivity is option (c) \(1<2<3<4\).

Step 1: Understand the Reactivity Trend

In nucleophilic substitution reactions, the reactivity of haloalkanes is influenced by the bond strength between carbon and the halogen atom. Weaker bonds are easier to break, making the haloalkane more reactive.

Step 2: Bond Strength Analysis

Evaluate the bond strength of carbon with each halogen in the molecules provided. The bond strength order typically follows the pattern of C-F > C-Cl > C-Br > C-I. This means that the stronger the bond, the less reactive the compound is in nucleophilic substitution reactions.

Step 3: Determine Reactivity Order

Based on the bond strengths: - \((\mathrm{CH}_{3} \mathrm{~F})\) has the strongest bond, thus least reactive.- \((\mathrm{CH}_{3} \mathrm{Cl})\) follows due to a slightly weaker bond.- \((\mathrm{CH}_{3} \mathrm{Br})\) has a weaker bond than chlorine.- \((\mathrm{CH}_{3} \mathrm{I})\) has the weakest carbon-halogen bond, hence it is the most reactive.

Step 4: Arrange in Increasing Order of Reactivity

Arranging these from least to most reactive based on bond strength, we have: \(\mathrm{CH}_{3} \mathrm{~F} < \mathrm{CH}_{3} \mathrm{Cl} < \mathrm{CH}_{3} \mathrm{Br} < \mathrm{CH}_{3} \mathrm{I}\)

Key Concepts

Haloalkanes

Haloalkanes are a fascinating group of organic compounds where one or more hydrogen atoms in an alkane have been replaced by a halogen atom. These compounds are also known as alkyl halides. Common halogens include fluorine, chlorine, bromine, and iodine—giving rise to different types of haloalkanes like fluoroalkanes, chloroalkanes, bromoalkanes, and iodoalkanes.

Haloalkanes are significant because they participate in several important chemical reactions, most notably nucleophilic substitution reactions. In these reactions, a nucleophile, which is a negatively charged ion or molecule with a lone pair of electrons, replaces the halogen in the haloalkane, forming a new compound. The nature of the halogen affects the compound's reactivity in these reactions.

• The size and electronegativity of the halogen influence the properties of the haloalkane.
• Haloalkanes with larger halogens generally have weaker carbon-halogen bonds, which makes them more reactive.
• These reactions are important in both synthetic and biochemical contexts, making understanding haloalkanes essential in organic chemistry.

Understanding haloalkanes helps explain various organic synthesis processes and is crucial for comprehending how certain biochemical processes occur.

Bond Strength

Bond strength is a key factor in determining the reactivity of haloalkanes in nucleophilic substitution reactions. The bond strength between carbon and the halogen influences how easily the reaction can occur. Essentially, the weaker the bond, the easier it is for a nucleophile to break it and replace the halogen.

In terms of bond strength among haloalkanes:

• Fluorine forms the strongest bond with carbon due to its small size and high electronegativity, resulting in a challenging bond to break.
• As we move down the group to chlorine, bromine, and iodine, the carbon-halogen bond becomes progressively weaker.
• The weaker bonds in lower halogens like bromine and iodine allow nucleophiles more easily to attack and substitute the halogen atom.
• This is why iodoalkanes are the most reactive, followed by bromoalkanes and chloroalkanes, with fluoroalkanes being the least reactive.

Understanding the correlation between bond strength and reactivity helps predict how quickly and efficiently a haloalkane will undergo nucleophilic substitution.

Reactivity Series

The reactivity series of haloalkanes is a sequence that ranks these compounds based on their tendency to react in nucleophilic substitution reactions. This series is determined by the carbon-halogen bond strength, as previously mentioned.

Here's how the reactivity series unfolds:

• Iodoalkanes, with the weakest carbon-iodine bond, react the fastest, making them the most reactive.
• Bromoalkanes come next due to a slightly stronger bond than iodoalkanes but still weak enough for effective substitution.
• Chloroalkanes follow, as they have an even stronger carbon-chlorine bond.
• Lastly, fluoroalkanes are considered the least reactive because the carbon-fluorine bond is very strong, making it resistant to nucleophilic attack.

This series helps chemists and students predict the outcome of reactions involving various haloalkanes. By analyzing the bond strength and placing haloalkanes in a reactivity order, we can anticipate which compound will preferentially react under nucleophilic conditions. This knowledge is instrumental in designing synthesis pathways in organic chemistry.