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) \(\mathrm{R}-\mathrm{F}>\mathrm{R}-\mathrm{I}>\mathrm{R}-\mathrm{Br}>\mathrm{R}-\mathrm{Cl}\)

Step-by-Step Solution

Answer

The correct answer is (b): R-I > R-Br > R-Cl > R-F.

Step 1: Understand Dehydrohalogenation

Dehydrohalogenation is a chemical reaction that involves the removal of a hydrogen halide (HX) from an alkyl halide. It is an elimination reaction commonly used to form alkenes. The reactivity of the halides depends on the ability to release the leaving group, which is usually the halogen.

Step 2: Analyze Leaving Group Ability

The reactivity order in dehydrohalogenation relates closely to the leaving group ability of the halides. A good leaving group is one that leaves easily from the compound, often forming a stable ion. Iodide (I⁻) is known to be a much better leaving group than bromide (Br⁻), chloride (Cl⁻), and especially fluoride (F⁻). This is because iodide is larger, more polarizable, and thus stabilizes the negative charge better.

Step 3: Evaluate Reactivity Order Based on Leaving Group Ability

Based on the effectiveness of halides as leaving groups, iodide will be the most reactive in dehydrohalogenation reactions, followed by bromide, chloride, and lastly fluoride. This is because as the halides move down the periodic table (from F to I), the atomic size and leaving group ability increase.

Step 4: Identify Correct Reactivity Order

Considering the leaving group abilities, the order should be: R-I > R-Br > R-Cl > R-F, making option (b) the correct answer.

Key Concepts

Halogen Reactivity

When talking about halogen reactivity, particularly in organic chemistry, it's important to differentiate between the tendencies of halogens as they appear in various compounds. In the context of dehydrohalogenation, we compare which halogen helps facilitate the reaction most effectively. Halogens can behave differently based on their bond strength and how easily they can be removed as a leaving group or participate in reactions.
Fluorine, although extremely reactive in other reactions, forms very strong bonds with carbon due to its high electronegativity. This makes it less effective in dehydrohalogenation reactions.
On the other hand, iodine has a weaker bond with carbon, which makes it more prone to engaging in reactions like dehydrohalogenation. Thus, when thinking about reactivity for this specific reaction,

• iodine stands out as the most reactive
• followed by bromine,
  
• then chlorine,
• and finally fluorine.

Leaving Group Ability

The leaving group ability is essential in understanding dehydrohalogenation and other elimination reactions. A good leaving group is one that departs from the parent molecule easily, forming a stable entity.
Halide ions are common leaving groups, and their effectiveness varies across the periodic table. For a halogen to function well as a leaving group, it should break away smoothly and stabilize its negative charge after departure.
Iodide (I⁻) excels as a leaving group due to its large size and polarizability, which allows it to distribute negative charge effectively.
Cheaper costs of breaking its bond make it leave the substrate faster than other halides.
Consequently, the order of leaving group ability for halides is:

• iodide (I⁻),
• followed by bromide (Br⁻),
• then chloride (Cl⁻),
• and lastly fluoride (F⁻).

Elimination Reaction

Elimination reactions are a fundamental part of organic chemistry. They involve removing elements from a molecule, forming a double bond (commonly seen forming alkenes from alkyl halides). One key feature of elimination reactions is the creation of unsaturated hydrocarbons.
Dehydrohalogenation is a specific type where a hydrogen and halide leave the molecule. The effectiveness of the reaction depends on factors such as the quality of the leaving group and other molecular conditions.
For an elimination reaction to proceed efficiently, the leaving group should be readily detachable (e.g., a good leaving group like iodide). Additionally, the substrate's structure and the conditions (like temperature) can affect the reaction.
We see the most successful dehydrohalogenation occur with:

• tertiary substrates, where carbocations are stabilized,
• and with strong bases that promote the elimination.

Alkyl Halides

Alkyl halides are the starting materials for many organic reactions, including dehydrohalogenation. They consist of an alkyl group attached to a halogen. Their properties and reactivity are influenced by both the alkyl group type and the halide.
The type of alkyl group can alter the ease of the reaction, with tertiary alkyl groups typically being more reactive than primary. The bond between the carbon of the alkyl group and the halogen can vary in strength, impacting how easily the halogen is released.
Understanding the reactivity of alkyl halides involves knowing both the stability of forming intermediates and the nature of the halogen:
Alkyl iodides (

• because they contain iodine, a good leaving group,

) are often more reactive compared to other halides. In preparing alkenes, choosing the right alkyl halide and reaction conditions is crucial for successful synthesis.