Q.73 Chemistry · Section A ⭐ 360° Option Elimination Active

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NEET 2024 Chemistry Question 73 Solution

Topic: Haloalkanes and Haloarenes · Official NTA Booklet Code T1

Question

The compound that will undergo Sₙ1 reaction with the fastest rate is

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Verified Against NTA Final Answer Key

Option (2) : (1-bromoethyl)benzene (PhCH(Br)CH3)

📚 Haloalkanes and Haloarenes Easy

📋 Given Parameters & Problem Setup

Substrate 1 Bromobenzene (PhBr)

Substrate 2 (1-bromoethyl)benzene (PhCH(CH3)Br)

Substrate 3 (Bromomethyl)cyclohexane (Cyclohexyl-CH2Br)

Substrate 4 Bromocyclohexane (Cyclohexyl-Br)

⚡

The 30-Second Exam Hall Shortcut: ⚡ Carbocation Stability SN1 Speed Rule

⏱️ Save ~35 seconds

SN1 rate ∝ Carbocation Stability! Secondary benzylic carbocation [PhCH⁺CH3] has resonance stabilization from the benzene ring PLUS 3 α-hydrogens -> Fastest SN1!

1 Rate-determining step of an SN1 reaction is the ionization to form a carbocation intermediate.

2 Inspect carbocations formed by each substrate:

3 - Substrate 1: Phenyl cation (sp² hybridized, extremely unstable; zero SN1 rate).

4 - Substrate 2: Secondary benzylic carbocation Ph-C⁺H-CH3 (resonance delocalized across all 3 ortho/para positions of the aromatic ring + 3 hyperconjugative α-hydrogens). Exceptionally stable!

5 - Substrate 3: Primary aliphatic carbocation Cyclohexyl-CH2⁺ (very unstable).

6 - Substrate 4: Secondary aliphatic carbocation Cyclohexyl⁺ (moderately stable, but lacks resonance).

7 Benzylic carbocation stability dominates all aliphatic cations.

8 Option (2) undergoes SN1 with the fastest rate in 5 seconds!

🛡️

Why You Can 100% Trust This Shortcut: NCERT Chemistry · Haloalkanes and Haloarenes (Reactivity order of alkyl halides in SN1 reactions: allylic and benzylic halides show high reactivity due to resonance stabilization).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Kinetics and Rate Law of the SN1 Mechanism

The unimolecular nucleophilic substitution (SN1) proceeds through a two-step mechanism. The slow, rate-determining step is the heterolytic cleavage of the C-Br bond to form a planar carbocation:

R{-}\text{Br} \xrightarrow{\text{slow, rds}} R^\oplus + \text{Br}^\ominus, \quad \text{Rate} = k[R{-}\text{Br}]

Step 2

Evaluation of Carbocation Intermediates

Analyzing the stability of the intermediate generated from each given substrate: 1. Bromobenzene: Phenyl cation C6H5⁺ has the positive charge in an sp² orbital orthogonal to the π-system; it cannot be stabilized by resonance and is energetically prohibitive. 2. (1-Bromoethyl)benzene: The resulting carbocation Ph-C⁺H-CH3 is a secondary benzylic carbocation. The positive charge is extensively delocalized into the benzene ring across four contributing resonance structures, enhanced by hyperconjugation from the methyl group. 3. (Bromomethyl)cyclohexane: Generates an unstable 1° carbocation. 4. Bromocyclohexane: Generates a 2° aliphatic carbocation without any resonance stabilization.

\text{Stability: } \text{PhCH}^\oplus\text{CH}_3 \;(\text{2° benzylic}) \gg \text{Cyclohexyl}^\oplus \;(\text{2° aliphatic}) > \text{Cyclohexyl-CH}_2^\oplus \;(\text{1°}) \gg \text{C}_6\text{H}_5^\oplus

Step 3

Conclusion on Relative Reaction Rates

By the Hammond postulate, the transition state leading to the 2° benzylic carbocation has the lowest activation energy (ΔG‡). Consequently, (1-bromoethyl)benzene undergoes solvolysis/SN1 with the fastest rate.

k_{SN1} \propto \exp\left(-\frac{\Delta G^\ddagger}{RT}\right) \implies \text{Fastest for Substrate (2)}

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360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Does not undergo SN1 due to instability of phenyl cation and partial double bond character of C-Br.

⚠️

Student Fallacy (Why candidates pick this trap): Assuming presence of benzene ring always accelerates SN1.

2 (2)

✓ 100% Correct

Verdict & Scientific Reason:

Forms an exceptionally stable 2° benzylic carbocation with aromatic resonance delocalization.

💡

Why this is airtight: None.

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Primary aliphatic halide; reacts predominantly via SN2, extremely slow via SN1.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing 1° aliphatic with benzylic.

4 (4)

✗ Incorrect

Verdict & Scientific Reason:

Secondary aliphatic carbocation lacks resonance stabilization.

⚠️

Student Fallacy (Why candidates pick this trap): Believing secondary aliphatic is comparable to secondary benzylic.

⚠️

NTA Trap Alert: Confusing Phenyl Halides with Benzylic Halides

Halogen directly on the ring (Bromobenzene) is INERT to SN1. Halogen on the side-chain carbon adjacent to the ring (Benzylic) is HYPER-REACTIVE to SN1!

🛡️ How to avoid the -1 mark: Check the carbon bearing bromine: on the ring = inert; one carbon away from the ring = benzylic (super fast).

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Core NEET Syllabus Standard

Chemistry Core Organic Chemistry · Haloalkanes and Haloarenes Standard Benchmark

Core Scientific Principle Tested:

"Benzylic halides show high reactivity in SN1 reactions because the carbocation formed is stabilized through resonance with the aromatic ring."

Syllabus Focus: Nucleophilic Substitution Reactions: SN1 Reactivity Factors

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Benzylic carbon shares the charge: ring resonance makes its speed huge and large!

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NEET 2024 Chemistry Question 73 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Carbocation Stability SN1 Speed Rule

Step-by-Step Rigorous Workflow & Mathematical Proof

Kinetics and Rate Law of the SN1 Mechanism

Evaluation of Carbocation Intermediates

Conclusion on Relative Reaction Rates

360° Option Elimination Engine Key Feature

NTA Trap Alert: Confusing Phenyl Halides with Benzylic Halides

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Haloalkanes and Haloarenes

Monochlorination Products (Including Stereoisomers) of 2-Methylbutane

Leaving Group Ability and S_N2 Reaction Rates of 1-Iodobutane vs. 1-Chlorobutane

Distinction Between Allylic Halide and Benzylic Halide

Multistep Reaction Sequence: Synthesis of Ethylbenzene via Wurtz-Fittig Reaction

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