Problem 21 - A Complete Resource Book in Chemistry for JEE Main

Question

The reactivity order of halides for dehydrohalogenation is (a) \(\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{I}\) (b) \(\mathrm{R}-\mathrm{I}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{F}\) (c) \(\mathrm{R}-\mathrm{I}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{F}\) (d) \(\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{l}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{Cl}\)

Step-by-Step Solution

Answer

Option (b) is the correct order: \(\mathrm{R}-\mathrm{I} > \mathrm{R}-\mathrm{Br} > \mathrm{R}-\mathrm{Cl} > \mathrm{R}-\mathrm{F}\).

Step 1: Understand the Concept

Dehydrohalogenation is the elimination of a hydrogen halide from an alkyl halide to form an alkene. The reactivity depends on how easily the halide can leave. This is influenced by the bond strength between the carbon and the halide.

Step 2: Analyze Bond Strength

The strength of the carbon-halogen bond varies. The C-F bond is the strongest due to the high electronegativity and small size of fluorine, followed by C-Cl, C-Br, and C-I. The weaker the bond, the better the leaving group the halogen becomes.

Step 3: Determine Reactivity Order

Since a stronger bond is harder to break, and a good leaving group makes the reaction proceed more readily, the order of reactivity for dehydrohalogenation, from most reactive to least, is influenced by the weakest C-X bond: C-I < C-Br < C-Cl < C-F.

Step 4: Match with Options

Option (b) (\(\mathrm{R}-\mathrm{I}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{Cl}>\mathrm{R}-\mathrm{F}\)) correctly represents the order from most reactive (easiest to leave) to least reactive.

Key Concepts

Reactivity order of halides

In dehydrohalogenation reactions, halides exhibit different levels of reactivity. This refers to how readily a halide can be eliminated from an alkyl halide to form an alkene. The ease of this reaction largely depends on the halide's ability to leave, which in turn is related to the strength of the carbon-halogen bond. Generally, the reactivity order for such reactions can be ranked based on how tightly each halide is bound to the carbon. Since a weaker bond is easier to break, halides with weaker carbon-halogen bonds tend to be more reactive.

For dehydrohalogenation, the order of reactivity is typically:

• Iodide (R-I)
• Bromide (R-Br)
• Chloride (R-Cl)
• Fluoride (R-F)

This means iodide is the most reactive, and fluoride is the least reactive in dehydrohalogenation reactions. This order reflects the decreasing bond strength from iodide to fluoride.

Carbon-halogen bond strength

The bond strength between carbon and halogen atoms plays a crucial role in determining how easily a halogen can be removed in chemical reactions, such as dehydrohalogenation. Bond strength is influenced by factors like electronegativity and atomic size.

The C-F bond is notoriously strong. This strength is due to fluorine's high electronegativity and the small size of its atoms, which allows for a strong and stable bond. As we move from fluorine to iodine in the halogen group, the bonds become progressively weaker.

• The C-F bond is the strongest.
• Followed by the C-Cl bond.
• C-Br bonds are weaker than C-Cl bonds.
• The C-I bond is the weakest.

Consequently, in dehydrohalogenation, a weaker C-X bond makes a halide a better leaving group, facilitating the reaction. This is why the reactivity order follows the trend: C-I < C-Br < C-Cl < C-F.

Halide leaving group ability

A halide's ability to act as a leaving group is essential for many chemical reactions, including dehydrohalogenation. The leaving group's strength greatly affects the reaction's overall mechanism and speed. Good leaving groups are those that can detach easily from the original molecule, carrying away a pair of electrons.

The most effective leaving groups are typically associated with the weakest carbon-halogen bonds as these bonds are easier to break. Therefore, in order of good leaving group ability, we have:

• Iodide, due to the weakest C-I bond.
• Bromide, with a slightly stronger C-Br bond.
• Chloride, which has a stronger C-Cl bond.
• Fluoride, known for its strong C-F bond, is typically a poor leaving group.

This order means that iodide leaves the molecule most readily, making it the preferred leaving group in most dehydrohalogenation reactions.