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Reactions of Haloalkanes - Nucleophilic Substitution Reactions

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Estimated time: 6 minutes
CBSE: Class 12

Key Points: Reactions of Haloalkanes — Nucleophilic Substitution Reaction

The C–X bond in alkyl halides is polarised (Cδ+–Xδ–), making alkyl halides reactive towards nucleophiles.

Two Types of SN Reactions

SN1 (Unimolecular Nucleophilic Substitution):

  • First-order kinetics: Rate = k[RX] (depends only on substrate concentration)
  • Two-step mechanism: Step 1 (slow) — ionisation to form carbocation; Step 2 (fast) — attack by nucleophile.
  • Intermediate: Trigonal planar carbocation.
  • More substituted alkyl halides react faster (more stable carbocation).
  • Reactivity order: R₃CX > R₂CHX > RCH₂X (3° > 2° > 1°)
  • Gives a racemic mixture (optically inactive product) because the nucleophile can attack from both faces.
  • For aryl/vinyl halides: Ar₂CX > Ar₂CHX > ArCH₂X = CH₂=CHX > CH₂=CHCH₂X

SN2 (Bimolecular Nucleophilic Substitution):

  • Second-order kinetics: Rate = k[RX][Nu] (depends on both substrate and nucleophile concentration)
  • One-step mechanism (concerted): Nucleophile attacks from the back side as leaving group departs simultaneously → Transition State is formed.
  • Results in Walden Inversion (inversion of configuration at the carbon — stereochemistry inverted).
  • Reactivity order: Methyl halide > Primary > Secondary > Tertiary (CH₃X > 1° > 2° > 3°)
  • The SN2 reaction rate depends on the concentration of both alkyl halide and nucleophile.

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mechanism of substitution reactions [00:02:49]
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mechanism of substitution reactions
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Carbocation Stability
00:09:37  undefined
Haloalkanes and Haloarenes part 17 (Nucleophilic Substitution reaction)
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Haloalkanes and Haloarenes part 18 (Substitution SN1 reaction)
00:14:19  undefined
Haloalkanes and Haloarenes part 19 (Substitution SN2 reaction)
00:12:13  undefined
Haloalkanes and Haloarenes part 20 (Numerical SN1, SN2 reaction)
00:08:08  undefined
Haloalkanes and Haloarenes part 21 (Stereochemistry, optical isomers)
00:26:56  undefined

Related QuestionsVIEW ALL [124]

CCl4 is insoluble in water because:-

The following questions are case-based questions. Read the passage carefully and answer the questions that follow:

Nucleophilic Substitution:
Nucleophilic substitution reaction of haloalkane can be conducted according to both SN1 and SN2 mechanisms. SN1 is a two-step reaction, while SN2 is a single-step reaction. For any haloalkane, which mechanism is followed depends on factors such as the structure of haloalkane, properties of leaving group, nucleophilic reagent and solvent.

Influences of solvent polarity:
In SN1 reaction, the polarity of the system increases from the reactant to the transition state, because a polar solvent has a greater effect on the transition state than the reactant, thereby reducing activation energy and accelerating the reaction. In SN2 reaction, the polarity of the system generally does not change from the reactant to the transition state, and only charge dispersion occurs. At this time, the polar solvent has a great stabilizing effect on Nu than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate (SN1) of tertiary chlorobutane at 25°C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24).

The reaction rate (SN2) of 2-bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has an influence on both SN1 and SN2 reactions but with different results. Generally speaking, a weak polar solvent is favourable for SN2 reaction, while a strong polar solvent is favourable for SN1. Generally speaking, the substitution reaction of tertiary haloalkane is based on SN1 mechanism in solvents with a strong polarity (for example ethanol containing water).

Answer the following questions:

(a) Why racemisation occurs in SN1? (1)

(b) Why is ethanol less polar than water? (1)

(c) Which one of, the following in each pair is more reactive towards SN2 reaction? (2)

(i) CH3 – CH2 – I or CH3CH2 – Cl

(ii)

OR

(c) Arrange the following in the increasing order of their reactivity towards SN1 reactions: (2)

(i) 2-Bromo-2-methylbutane, 1-Bromo-pentane, 2-Bromo-pentane

(ii) 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3- methylbutane

Answer the following question.
Write one stereochemical difference between SN1 and SN2 reactions.

Give reason for the following:

The product formed during SN1 reaction is a racemic mixture.

Which of the following undergoes nucleophilic substitution exclusively by SN1 mechanism?

The reaction of C6H5–CH=CH–CH3 with HBr produces:

Consider the reactions,

(i) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5OH] \underset{}{(CH3)2CH - CH2OC2H5 + HBr}}\\
\end{array}\]

(ii) \[\begin{array}{cc}
\phantom{}\ce{\underset{}{(CH3)2CH - CH2Br} ->[C2H5O-] \underset{}{(CH3)2CH - CH2OC2H5 + Br-}}\\
\end{array}\]

The mechanisms of reactions (i) and (ii) are respectively:

Write the structure of the major product in each of the following reaction :

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