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 order is (b)  ext{R-I} > ext{R-Br} > ext{R-Cl} > ext{R-F}.

Step 1: Understand Dehydrohalogenation

Dehydrohalogenation is an elimination reaction where a hydrogen halide (HX) is removed from an alkyl halide, leading to the formation of an alkene. The ease of this reaction depends on the leaving ability of the halide ion, with better leaving groups resulting in more favorable reactions.

Step 2: Evaluate Halide Leaving Ability

The leaving group ability of halides generally increases down the group in the periodic table due to increasing atomic size and decreasing bond strength of C-X with larger halides. This means that iodide ( ext{I}^-) departs more readily than bromide ( ext{Br}^-), followed by chloride ( ext{Cl}^-), and lastly fluoride ( ext{F}^-).

Step 3: Order Halides for Dehydrohalogenation

Based on their leaving group ability, the expected order of reactivity for dehydrohalogenation is  ext{R-I} > ext{R-Br} > ext{R-Cl} > ext{R-F}. This follows from the fact that iodide is the best leaving group among the given halides.

Step 4: Compare to Given Options

Comparing our evaluated order with the given options: a)  ext{R-F} > ext{R-Cl} > ext{R-Br} > ext{R-I} - incorrect b)  ext{R-I} > ext{R-Br} > ext{R-Cl} > ext{R-F} - correct c)  ext{R-I} > ext{R-Cl} > ext{R-Br} > ext{R-F} - incorrect d)  ext{R-F} > ext{R-I} > ext{R-Br} > ext{R-Cl} - incorrect

Key Concepts

Elimination reactions

Elimination reactions are fascinating chemical processes where atoms are removed from a molecule, usually resulting in the formation of a multiple bond. In the case of dehydrohalogenation, a hydrogen halide (HX) is removed from an alkyl halide. This reaction leads to the creation of an alkene—a carbon-carbon double bond is formed.

During this type of reaction, both a hydrogen atom and a halogen atom are eliminated from their respective positions on the molecule.

• One of the most common mechanisms for elimination reactions is the E2 mechanism, where elimination occurs in a single concerted step.
• There is also the E1 mechanism where the leaving group departs first, forming a carbocation, which is then followed by the removal of a proton.

Both mechanisms are favored under certain conditions and can result in different outcomes. Understanding these processes is crucial for predicting the behavior and products of these reactions in synthetic chemistry.

Leaving group ability

The concept of a leaving group is vital when considering elimination reactions. A leaving group's ability determines how easily it can detach from the rest of the molecule.

For elimination reactions like dehydrohalogenation, a better leaving group will facilitate a more efficient reaction. Essentially, the weaker the bond between the carbon atom and the leaving group, the easier it becomes for the group to leave.

• In the context of halides:
  • Iodide (I^-) is a larger ion with a weaker bond to carbon, making it the best leaving group.
  • Fluoride (F^-), conversely, has a strong bond due to its smaller size, making it the least effective leaving group.
  • Chloride (Cl^-) and bromide (Br^-) fall in between, with bromide being a better leaving group than chloride.

Having a good leaving group is critical for the occurrence and speed of the reaction, and it principally affects the overall mechanism.

Reactivity order of halides

The reactivity order of halides in dehydrohalogenation is determined by their ability to leave the molecule during the reaction.

This order is generally based on the size and bond strength of the halides bonded to carbon. The larger the halide ion, the weaker the bond to the carbon, thus enhancing its leaving group ability and increasing the overall reactivity of the process.

• In dehydrohalogenation, the order of reactivity from most reactive to least is typically:
  • R-I (Iodide)
  • R-Br (Bromide)
  • R-Cl (Chloride)
  • R-F (Fluoride)
• This pattern is consistent across most elimination reactions involving halides.
• This ordering constitutes a crucial understanding for chemists aiming to carry out specific eliminations to obtain desired alkene products.

Comprehending this order is not just about mastering reaction predictions but also about reasoning how these reactions will proceed under different conditions, ensuring the accurate design of synthetic pathways.