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Haloalkanes and Haloarenes - Mechanism of Nucleophilic Substitution (SN1 and SN2)

Grade 12CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Substitution Nucleophilic Bimolecular (SN2S_N2): A concerted, single-step mechanism where the nucleophile attacks the substrate from the side opposite to the leaving group. It follows second-order kinetics.

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Stereochemistry of SN2S_N2: Results in complete inversion of configuration, known as Walden Inversion. Reactivity order: CH3X>1∘>2∘>3∘CH_3X > 1^\circ > 2^\circ > 3^\circ due to increasing steric hindrance.

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Substitution Nucleophilic Unimolecular (SN1S_N1): A two-step mechanism. Step 1 (Slow) involves the ionization of the C−XC-X bond to form a carbocation intermediate. Step 2 (Fast) is the attack by the nucleophile.

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Stereochemistry of SN1S_N1: If the starting material is optically active, the product is a racemic mixture (50%50\% inversion, 50%50\% retention) because the nucleophile can attack the planar sp2sp^2 hybridized carbocation from either side.

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Reactivity Order for SN1S_N1: 3∘>2∘>1∘>CH3X3^\circ > 2^\circ > 1^\circ > CH_3X, governed by the stability of the carbocation intermediate (Tertiary>Secondary>PrimaryTertiary > Secondary > Primary).

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Allylic and Benzylic halides: These show high reactivity towards SN1S_N1 reactions because the resulting carbocations are stabilized by resonance.

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Nature of Leaving Group: For both mechanisms, the reactivity follows the order: R−I>R−Br>R−Cl>R−FR-I > R-Br > R-Cl > R-F because the C−IC-I bond is the weakest.

📐Formulae

RateSN2=k[R−X][Nu−]Rate_{S_N2} = k[R-X][Nu^-]

RateSN1=k[R−X]Rate_{S_N1} = k[R-X]

Stability:(CH3)3C+>(CH3)2CH+>CH3CH2+>CH3+Stability: (CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+

💡Examples

Problem 1:

Between CH3CH2CH2CH2BrCH_3CH_2CH_2CH_2Br (1-bromobutane) and CH3CH2CH(Br)CH3CH_3CH_2CH(Br)CH_3 (2-bromobutane), which one will react faster in an SN2S_N2 reaction and why?

Solution:

CH3CH2CH2CH2BrCH_3CH_2CH_2CH_2Br (1-bromobutane) will react faster.

Explanation:

In SN2S_N2 reactions, the nucleophile attacks from the back side. 1∘1^\circ alkyl halides like 1-bromobutane have less steric hindrance compared to 2∘2^\circ alkyl halides like 2-bromobutane, making it easier for the nucleophile to approach the CC atom.

Problem 2:

Predict the order of reactivity of the following toward SN1S_N1 reaction: C6H5CH2BrC_6H_5CH_2Br, C6H5CH(C6H5)BrC_6H_5CH(C_6H_5)Br, C6H5CH(CH3)BrC_6H_5CH(CH_3)Br.

Solution:

C6H5CH(C6H5)Br>C6H5CH(CH3)Br>C6H5CH2BrC_6H_5CH(C_6H_5)Br > C_6H_5CH(CH_3)Br > C_6H_5CH_2Br.

Explanation:

Reactivity in SN1S_N1 depends on the stability of the carbocation. The carbocation (C6H5)2CH+(C_6H_5)_2CH^+ is stabilized by two phenyl rings via resonance, C6H5CH(CH3)+C_6H_5CH(CH_3)^+ is stabilized by one phenyl ring (resonance) and one methyl group (+I effect), and C6H5CH2+C_6H_5CH_2^+ is stabilized by only one phenyl ring.