Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

The order of reactivity of alkyl halides towards elimination reaction is (a) \(1^{\circ}>2^{\circ}>3^{\circ}\) (b) \(2^{\circ}>1^{\circ}>3^{\circ}\) (c) \(3^{\circ}>2^{\circ}>1^{\circ}\) (d) \(3^{\circ}>1^{\circ}>2^{\circ}\)

Step-by-Step Solution

Answer

The order is (c) \(3^{\circ}>2^{\circ}>1^{\circ}\).

Step 1: Understanding Alkyl Halides

Alkyl halides are organic compounds containing a halogen atom bonded to an sp3 hybridized carbon atom. They are classified based on the carbon atom to which the halogen is attached: primary (1°), secondary (2°), and tertiary (3°). Elimination reactions involve the removal of atoms or groups from a molecule, often forming a double bond.

Step 2: Factors Affecting Elimination Reactions

In elimination reactions, especially E2 and E1 mechanisms, the stability of the formed alkene plays a crucial role. Tertiary alkyl halides form more stable alkenes due to greater hyperconjugation and inductive effects compared to secondary and primary ones. Thus, tertiary alkyl halides react more readily in elimination reactions.

Step 3: Analyzing Reactivity Trend

Given the increased stability of alkenes formed from tertiary alkyl halides, they are the most reactive towards elimination reactions. This is followed by secondary, and lastly primary alkyl halides due to lower alkene stability as the degree decreases. Therefore, the order of reactivity is tertiary > secondary > primary.

Step 4: Concluding the Correct Answer

Based on the analysis of stability and the mechanism of elimination reactions, the correct order of reactivity towards elimination for these alkyl halides is tertiary > secondary > primary. This corresponds to option (c) where the order is \(3^{\circ}>2^{\circ}>1^{\circ}\).

Key Concepts

Elimination Reactions

Elimination reactions are fundamental transformations in organic chemistry, where a molecule loses atoms or groups, typically resulting in the formation of a double bond. This process can effectively alter the structure and reactivity of a compound. These reactions are pivotal in forming alkenes from alkyl halides.

• Elimination reactions commonly occur in two main forms: E1 (unimolecular elimination) and E2 (bimolecular elimination).
• In general, elimination processes compete with substitution reactions, and selecting the desired pathway often depends on the reaction conditions.
• Heat and specific solvent choices can tip the scale towards elimination rather than substitution, emphasizing the importance of reaction environment.

Understanding the nature and mechanism of elimination reactions is crucial for correctly anticipating product formation and controlling reaction outcomes.

Stability of Alkenes

The stability of alkenes is a key factor that influences the order of reactivity in elimination reactions. Generally, alkenes are more stable when they are more substituted, meaning they have more carbon groups attached to the double-bonded carbons.

• Alkenes gain extra stability from hyperconjugation and the inductive effect of alkyl groups.
• Hyperconjugation involves the overlap of sigma bonds (usually C-H or C-C) with the adjacent empty p-orbitals, enhancing stability.
• The inductive effect refers to the electron-donating ability of alkyl groups, helping stabilize the positive charge in intermediary steps.

Thus, tertiary alkyl halides, which can form highly substituted and hence more stable alkenes, show higher reactivity towards elimination reactions.

E2 Mechanism

The E2 mechanism, or bimolecular elimination, is a one-step process where the elimination occurs simultaneously as the base removes a hydrogen atom and the leaving group departs. It requires a strong base and is particularly common for alkyl halides.

• The E2 reaction is favored with primary and secondary alkyl halides, as these allow the base to effectively abstract the hydrogen proton.
• The geometry of the reacting molecule is important; the hydrogen to be removed must be anti to the leaving group for optimal orbital alignment.
• E2 reactions follow second-order kinetics, meaning the rate depends on the concentration of both the alkyl halide and the base.

This mechanism is highly stereo-specific, often leading to the trans product due to the geometric constraints of the reaction.

E1 Mechanism

The E1 mechanism, or unimolecular elimination, is a two-step process that begins with the formation of a carbocation as the leaving group departs. This intermediate is then deprotonated by a base, forming the alkene.

• E1 reactions are favored by tertiary alkyl halides because they stabilize the carbocation well.
• These reactions typically occur under conditions of heat and without a strong base, as the carbocation formation is the rate-determining step.
• E1 mechanisms follow first-order kinetics, where the rate depends solely on the concentration of the substrate, not the base.

While E1 can lead to mixtures of alkene products due to possible rearrangements, it is integral in understanding the reactivity trends of alkyl halides towards elimination.