Problem 186 - A Complete Resource Book in Chemistry for JEE Main

Question

In \(\mathrm{S}_{\mathrm{N}}^{2}\) reactions, the correct order of reactivity for the following compounds: \(\mathrm{CH}_{3} \mathrm{Cl}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\), \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) and \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) is: \([2014]\) (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (c) \(\mathrm{CH}_{3} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) (d) \(\mathrm{CH}_{3} \mathrm{Cl}>\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}>\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}>\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > \left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\)

Step 1: Understanding SN2 Reactions

The S_N2 reaction is a bimolecular nucleophilic substitution reaction where the rate of reaction depends on both the substrate and the nucleophile. S_N2 reactions are faster with less sterically hindered substrates because steric hindrance can slow down or even inhibit the reaction. Thus, primary alkyl halides react faster than secondary, and tertiary ones react the slowest or not at all.

Step 2: Evaluating Steric Hindrance

Let's evaluate the steric hindrance of each given compound: \(\mathrm{CH}_{3} \mathrm{Cl}\) is a primary alkyl halide, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) is also a primary alkyl halide, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) is a secondary alkyl halide, and \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\) is a tertiary alkyl halide. The order of steric hindrance, from least to most hindered, is: \(\mathrm{CH}_{3} \mathrm{Cl}\) < \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}\) < \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\) < \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\).

Step 3: Determining Reactivity Order

Due to the steric hindrance levels determined earlier, the order of reactivity for these compounds in S_N2 reactions should be from the least hindered to the most hindered: \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > \left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\).

Step 4: Selecting the Correct Answer

From the provided multiple-choice options, option (d) matches our predicted reactivity order for S_N2 reactions: \(\mathrm{CH}_{3} \mathrm{Cl} > \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} > \left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}\).

Key Concepts

Steric Hindrance

Steric hindrance refers to the obstruction that results from the physical presence of large groups or atoms within a molecule. Imagine trying to fit a big block through a narrow door; that's similar to how steric hindrance works in chemistry. In the case of a chemical reaction, particularly nucleophilic substitution reactions like the S_N2 reaction, steric hindrance can slow down or prevent a nucleophile from approaching the reactive center of the molecule.
Let's consider the compounds mentioned in our problem. Looking at the bulky groups attached to the carbon atom bonded to the chlorine:

• **Primary Alkyl Halides:** Compounds like \( \mathrm{CH}_{3}\ \mathrm{Cl} \) and \( \mathrm{CH}_{3}\ \mathrm{CH}_{2}\ \mathrm{Cl} \) have minimal steric hindrance since only one or no additional carbon is attached.
• **Secondary Alkyl Halides:** In \( (\mathrm{CH}_{3})_{2} \mathrm{CHCl} \) , the central carbon is bonded to two other carbon atoms adding significant steric hindrance.
• **Tertiary Alkyl Halides:** Complexes such as \( (\mathrm{CH}_{3})_{3} \mathrm{CCl} \) exhibit the highest steric hindrance with three carbon atoms crowding the reactive center, slowing the reaction the most.

Understanding steric hindrance helps in predicting which molecules will react faster in S_N2 reactions.

Nucleophilic Substitution

Nucleophilic substitution reactions involve the replacement of a leaving group (often a halogen) with a nucleophile. In S_N2 reactions, this substitution is said to be bimolecular, meaning that the reaction rate depends on both the concentration of the substrate and the nucleophile. This dependency marks a distinctive feature of S_N2 reactions.
A nucleophile is an electron-rich species that seeks out positive or electron-deficient areas in molecules to form bonds:

• In S_N2 reactions, the nucleophile must directly attack the carbon atom bonded to the leaving group from the backside (opposite side of the leaving group).
• This backside attack leads to an inversion of configuration, sometimes known as the "Walden Inversion," reversing the stereochemistry of the molecule.
• The simultaneous bond-breaking and bond-forming make S_N2 reactions a single-step mechanism without intermediates.

Nucleophilic substitution, particularly through the S_N2 mechanism, is essential in organic chemistry for its utility in transforming alkyl halides into various functional groups by changing the nature of the nucleophile.

Alkyl Halides

Alkyl halides, also known as haloalkanes, are a group of compounds containing a halogen atom (such as chlorine, bromine, or iodine) bonded to an alkyl group. These compounds are key players in nucleophilic substitution reactions due to their polarized carbon-halogen bonds.
The carbon-halogen bond is polarized because the halogen is more electronegative than carbon:

• This polarization creates a partial positive charge on the carbon, making it an ideal site for nucleophiles to attack in substitution reactions.
• Different types of alkyl halides (primary, secondary, tertiary) have different reactivity in S_N2 reactions based on their structural steric hindrance.
• The leaving group ability, primarily dictated by the halogen, also influences reactivity. Better leaving groups aid faster reaction rates.

In S_N2 reactions, primary alkyl halides like \(\mathrm{CH}_{3}\ \mathrm{Cl}\) generally display higher reactivity due to lower steric hindrance and effective nucleophile attack, unlike tertiary alkyl halides, which exhibit drastically reduced reactivity.