Question

Correct order of rate of substitution reaction for alkyl halides is (A) \(R-I>R-B r>R-C l>R-F\) (B) \(\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{l}\) (C) \(\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{I}\) (D) \(R-C l>R-B r>R-F>R-I\)

Short answer

The correct order of rate of substitution reaction for alkyl halides is (A) \(R-I > R-Br > R-Cl > R-F\).

Step-by-step solution

Understand the role of halogens in the reaction

In a nucleophilic substitution reaction, the halogen atom acts as a leaving group. The stability of a halogen atom as a leaving group depends on its size, electronegativity, and polarizability. As the size of the halogen increases, the bond length between the halogen and the alkyl group increases, which weakens the bond, making it easier for the larger halogen to leave the molecule.

Compare the sizes of the halogens

We should compare the size of the halogens given in the exercise: F, Cl, Br, and I. The atomic size increases down the group, so the order of size is F < Cl < Br < I. Larger halogens are better leaving groups because they can better stabilize the negative charge when they leave. Therefore, the order of reactivity should depend on the size of the halogens, with larger halogens reacting faster in substitution reactions.

Determine the correct order of reactivity

Based on the size and stability of the halogen leaving groups, we can predict the order of reactivity for alkyl halides in nucleophilic substitution reactions. The order of reactivity should be R-I > R-Br > R-Cl > R-F, as iodine is the largest halogen and the best leaving group, while fluorine is the smallest halogen and the worst leaving group.

Match the correct order with the given options

We predicted the reactivity order of alkyl halides as R-I > R-Br > R-Cl > R-F. Comparing this order with the given options, we find that it matches with option (A). Thus, the correct order of rate of substitution reaction for alkyl halides is (A) \(R-I > R-Br > R-Cl > R-F\).

Key concepts

Alkyl Halides

Alkyl halides, also known as haloalkanes, are a group of chemical compounds where a halogen atom is bonded to an alkyl group. These compounds are pivotal in organic chemistry, particularly in nucleophilic substitution reactions. Here's why they are important:

• A halogen (such as fluorine, chlorine, bromine, or iodine) replaces a hydrogen atom in an alkane, creating an alkyl halide.
• Alkyl halides serve as excellent starting materials and intermediates in synthesis due to their reactivity.
• The type of halogen affects the compound's reactivity, paving the path for multiple product variations in reactions.

Understanding alkyl halides contributes to grasping broader concepts like reaction mechanisms, functional groups, and the formation of functional derivatives.

Leaving Group Stability

In nucleophilic substitution reactions, the stability of a leaving group is crucial. The leaving group is the atom or group that gets 'kicked out' during the reaction. Let's unpack why certain leaving groups are more stable than others:

• The **halo** group in alkyl halides acts as the leaving group.
• A stable leaving group will readily dissociate from the carbon atom, facilitating the substitution process.
• Larger halogens like iodine are more stable because they can disperse the charge over more area, making them better at leaving.
• The bond between the alkyl group and the leaving halogen is weaker when the halogen is larger, which facilitates the leaving process.

For a halogen like fluorine, the bond is strong due to its small size and high electronegativity, making it a poor leaving group. Thus, the overall stability affects how quickly and efficiently a substitution reaction can proceed.

Halogen Reactivity Order

Halogens show varying reactivity in nucleophilic substitution reactions, which is mainly influenced by their physical properties. Here's the general order:

• Iodine (**I**) is the most reactive due to its large size and weak bond strength with the alkyl group.
• Bromine (**Br**) follows, being slightly smaller and forming bonds that are weaker than Cl but stronger than I.
• Chlorine (**Cl**) has an intermediate bond strength, leading to moderate reactivity.
• Fluorine (**F**) is the least reactive, with strong bonds and high electronegativity, making it a poor leaving group.

This order helps predict reaction outcomes, making it fundamental for chemists when designing and implementing synthetic pathways in organic chemistry.