Problem 152 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following (a) \(\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}\) (p) Order of reactivity \(>\) RF \(\quad\) towards \(S_{N^{2}}\) process (b) \(\mathrm{RF}>\mathrm{RCl}>\mathrm{RBr}\) (q) Decreasing order \(>\) RI \(\quad\) of B.pt. (c) \(3^{\circ}>2^{\circ}>1^{\circ}\) (r) Decreasing order of (hydrocarbon) density (d) \(1^{\circ}>2^{\circ}>3^{\circ}\) (s) stability order (alkyl halide) (t) Order of replacement of \(\mathrm{H}\) atom

Step-by-Step Solution

Answer

(a) matches (p); (b) matches (q); (c) matches (s); (d) matches (t).

Step 1: Analyze Assertion (a) RI > RBr > RCl

This assertion reflects the order of reactivity in an SN2 mechanism. A larger halogen atom allows for easier leaving group departure, which increases reactivity towards nucleophilic substitution. Hence, the reactivity order is RI > RBr > RCl > RF.

Step 2: Analyze Assertion (b) RF > RCl > RBr

When considering the boiling point order of alkyl halides, the general trend due to halogen mass is used: RI, with a heavier iodine, will have the highest boiling point followed by RBr, then RCl due to dispersion forces.

Step 3: Analyze Assertion (c) 3° > 2° > 1°

In terms of alkyl halide stability, a tertiary (3°) halide is more stable than a secondary (2°), which in turn is more stable than a primary (1°) alkyl halide due to hyperconjugation and inductive effects.

Step 4: Analyze Assertion (d) 1° > 2° > 3°

This is regarding the replacement of hydrogen atoms. Primary (1°) hydrogen atoms are more accessible for substitution than secondary (2°) and tertiary (3°) ones due to lesser steric hindrance.

Step 5: Establish Matches for (a) to (d) with (p) to (t)

Using the analysis: - (a) RI > RBr > RCl matches with (p) reactivity SN2, - (b) RF > RCl > RBr matches with (q) boiling point, - (c) 3° > 2° > 1° matches with (s) stability order, - (d) 1° > 2° > 3° matches with (t) H replacement.

Key Concepts

Alkyl Halide Reactivity

Alkyl halides are known for their reactivity, especially in nucleophilic substitution reactions like the SN2 mechanism. The reactivity of an alkyl halide largely depends on the nature of the leaving group—the larger and more polarizable the halogen, the easier it is for it to be replaced by a nucleophile. In the context of SN2 reactions, the order of reactivity for common alkyl halides is typically

• RI > RBr > RCl > RF

This order is due to iodine being the largest and most polarizable, making it the best leaving group among halogens. Bromine and chlorine follow suit but are less efficient. Fluorine, being very small and less polarizable, makes alkyl fluorides the least reactive in SN2 reactions.
This information helps us recognize which alkyl halides can be used in certain chemical reactions and how effectively they can be transformed or substituted.

Boiling Point Trends

The boiling point of alkyl halides is influenced by several factors, including the mass of the halogen and the strength of intermolecular forces. Generally, the larger the halogen atom, the higher the boiling point of the alkyl halide. Therefore, the boiling point order of alkyl halides is found as follows:

• RI > RBr > RCl > RF

This is because iodine is the heaviest and its molecules experience stronger van der Waals forces, leading to higher boiling points. This trend is essential for predicting physical properties and separation processes like distillation in organic chemistry.
Understanding these trends aids in determining how different alkyl halides might behave in various temperatures and environments.

Alkyl Halide Stability

Stability in alkyl halides is largely associated with the structure of the carbon chain attached to the halogen. Tertiary alkyl halides (3°) tend to be more stable than secondary (2°) or primary (1°) ones due to effects like hyperconjugation and inductive stabilization. Therefore, the stability order is:

• 3° > 2° > 1°

In hyperconjugation, electrons from adjacent C-H bonds in the alkyl groups stabilize positive charge distributions, making the molecule more stable. Inductive effect refers to the electron-withdrawing or -donating effects that stabilize the molecule.
This principle is crucial for determining how an alkyl halide will react in chemical processes, influencing the selectivity and outcomes of reactions.

Hydrogen Atom Substitution

The ease of hydrogen atom substitution in alkyl chains is an interesting aspect of organic chemistry, often governed by steric effects. The order of substitution is typically 1° > 2° > 3°, primarily due to sterics, where less crowded (primary) substrates allow easier access for nucleophiles or radicals to replace a hydrogen atom.

• This reflects that primary carbon atoms are more accessible compared to secondary and especially tertiary ones.

As steric hindrance increases, like in secondary and tertiary structures, it becomes more difficult for attacking species to approach and effectively substitute hydrogen atoms.
Therefore, this knowledge can guide chemists in selecting the appropriate conditions and reactants for pursuing successful substitution reactions.