Question

Choose the correct statements: (A) Generally chlorination is more selective but less reactive (B) Generally bromination is more selective but less reactive (C) Generally chlorination is more reactive but less selective (D) Both (B) and (C)

Short answer

The correct statements are (B), (C), and (D). Bromination is more selective but less reactive, while chlorination is more reactive but less selective.

Step-by-step solution

Statement A - Chlorination is more selective but less reactive

\(Chlorination\) is a type of halogenation reaction involving chlorine (\(Cl_2\)). The term "selective" means that the process can specifically target one part of a compound while leaving other parts unaffected. However, chlorination is generally less selective because it's difficult to control the extent of the reaction - it tends to react with multiple sites of a compound. Also, chlorine is more reactive due to its higher reactivity compared to bromine. Therefore, statement A is incorrect.

Statement B - Bromination is more selective but less reactive

\(Bromination\) is a halogenation involving bromine (\(Br_2\)). Bromination is indeed more selective than chlorination, as it is better at targeting a specific part of a compound because bromine radicals are large and less reactive compared to chlorine radicals. This means that bromination reactions are generally easier to control and result in less byproduct formation. Thus, statement B is correct.

Statement C - Chlorination is more reactive but less selective

This statement reiterates our analysis from statement A. Chlorination is indeed more reactive due to the higher reactivity of chlorine atoms, but this also leads to less selectivity since the reaction often proceeds through multiple pathways, resulting in a mixture of products. Therefore, statement C is also correct.

Statement D - Both (B) and (C)

Since statement B and statement C are both correct, as we've shown in our previous analysis, statement D is also correct. In conclusion: (A) is incorrect, (B) is correct, (C) is correct, (D) is correct. The correct statements are (B), (C), and (D).

Key concepts

Selectivity in Chlorination

When delving into the topic of halogenation in organic chemistry, the selectivity of reactions plays a vital role, especially for students preparing for the Joint Entrance Examination (JEE).

Selectivity in chlorination refers to the ability of chlorine to react with certain positions on an organic molecule preferentially. Despite chlorine's high reactivity due to its electronegativity and ability to form free radicals, chlorination can often be less selective than one would hope for. This is because chlorine radicals are relatively small and can access various positions within a molecule, leading to multiple possible reaction pathways and a mix of different chlorinated products.

For JEE aspirants, understanding the concept of selectivity helps in predicting the major product in a given chlorination reaction. Mastery of this concept requires a good grasp of the factors that influence selectivity, such as steric hindrance, the nature of the substrate, and the reaction conditions.

Reactivity in Bromination

Bromination reactions stand out in organic chemistry due to their higher selectivity compared to chlorination. This comes from the larger size of bromine atoms, which makes bromine radicals more cumbersome and less likely to access all available positions on an organic molecule.

As a result, bromine tends to add to the most accessible carbon atoms, usually the ones that are most substituted, as they provide the most stable radical intermediates. This specificity allows for better predictability of the end products and is particularly advantageous when seeking to synthesize complex molecules with high purity.

Such control over the reaction is crucial for students preparing for the JEE, as it often forms the basis of understanding reaction mechanisms and synthetic strategies.

Organic Chemistry for JEE

Organic chemistry is a significant portion of the JEE syllabus and requires a strong foundation in understanding reaction mechanisms, including halogenation. In the context of JEE preparation, students are expected to not only memorize reactions but also to apply the principles of organic chemistry to predict the outcomes of reactions accurately.

Halogenation reactions are a prime example where JEE aspirants must know how reactivity and selectivity impact the products of a reaction. Beyond memorizing facts, strong problem-solving skills are developed by applying these concepts to complex synthetic puzzles posed in the exam. Students should practice with a variety of problems, focusing on how different conditions affect the reactivity of halogens and the selectivity of the reactions.

Mastery of halogenation and other fundamental reactions is essential for success in the JEE and builds the groundwork for future studies in organic chemistry and related fields.