Electrophilic and Nucleophilic Substitution Reactions – Detailed Notes & Mind Map

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1. Definition of Substitution Reactions

A Substitution Reaction is a type of chemical reaction in which one functional group in a molecule is replaced by another functional group or atom.

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2. Electrophilic Substitution Reaction (ESR)

 Definition:

An Electrophilic Substitution Reaction is a chemical reaction where an electrophile (E⁺) replaces a hydrogen atom in an aromatic compound (most commonly benzene and its derivatives).

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General Reaction:

Aromatic Compound (Ar–H) + E⁺ → Ar–E + H⁺

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Mechanism of Electrophilic Aromatic Substitution (EAS):

Example: Nitration of Benzene

• Step 1 – Generation of Electrophile (NO₂⁺):

  $$HNO₃ + H₂SO₄ → NO₂^+ + HSO₄^- + H₂O$$

• Step 2 – Formation of Arenium Ion (σ-complex):
  Electrophile (NO₂⁺) attacks benzene → forms carbocation intermediate.

• Step 3 – Rearomatization:
  Loss of a proton (H⁺) restores aromaticity and produces nitrobenzene.

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Example Reactions:

1. Nitration:
   Benzene + HNO₃ → Nitrobenzene + H₂O (catalyst: H₂SO₄)

2. Halogenation:
   Benzene + Br₂ → Bromobenzene + HBr (catalyst: FeBr₃)

3. Sulfonation:
   Benzene + H₂SO₄ → Benzene sulfonic acid + H₂O

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Characteristics of ESR:

• Occurs mainly in aromatic compounds.

• Requires a catalyst (often Lewis acids like AlCl₃, FeCl₃, FeBr₃).

• Proceeds via carbocation (arenium ion) intermediate.

• Electron-donating groups (–OH, –OCH₃) activate the ring (ortho/para directing).

• Electron-withdrawing groups (–NO₂, –CN) deactivate the ring (meta directing).

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3. Nucleophilic Substitution Reaction (NSR)

 Definition:

A Nucleophilic Substitution Reaction is a chemical reaction where a nucleophile (Nu⁻) replaces a leaving group (X) attached to a carbon atom.

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General Reaction:

R–X + Nu⁻ → R–Nu + X⁻

Where:

• R–X = Alkyl Halide

• Nu⁻ = Nucleophile (e.g., OH⁻, CN⁻, NH₃)

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Two Main Types of NSR:

SN1 Mechanism (Unimolecular):

• Occurs in tertiary alkyl halides mainly.

• Involves two steps:

  1. Formation of Carbocation (Slow, Rate-Determining Step):
     R–X → R⁺ + X⁻

  2. Nucleophile attacks Carbocation:
     R⁺ + Nu⁻ → R–Nu

• Rate Law:
  Rate = k[R–X]

• Characteristics:

  • First order reaction.

  • Carbocation intermediate.

  • Racemization occurs in chiral centers.

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SN2 Mechanism (Bimolecular):

• Occurs mostly in primary alkyl halides.

• Involves a single concerted step:
  R–X + Nu⁻ → [Transition State] → R–Nu + X⁻

• Rate Law:
  Rate = k[R–X][Nu⁻]

• Characteristics:

  • Second order reaction.

  • Inversion of configuration at the carbon center (Walden inversion).

  • No carbocation intermediate.

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Examples of Nucleophilic Substitution:

1. Hydrolysis of Alkyl Halide:
   CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻

2. Cyanide Substitution:
   CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl

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Factors Affecting NSR:

• Nature of Substrate:

  • Primary alkyl halides favor SN2.

  • Tertiary alkyl halides favor SN1.

• Strength of Nucleophile:
  Stronger nucleophiles favor SN2.

• Leaving Group:
  Good leaving groups (I⁻, Br⁻, Cl⁻) increase the rate.

• Solvent:

  • Polar protic solvents favor SN1.

  • Polar aprotic solvents favor SN2.

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4. Key Differences:

Feature	Electrophilic Substitution	Nucleophilic Substitution
Reaction Type	Electrophile attacks the compound	Nucleophile attacks the compound
Common in	Aromatic Compounds	Alkyl Halides
Catalyst Needed	Yes (AlCl₃, FeCl₃)	No catalyst required usually
Intermediate	Arenium ion (σ-complex)	Carbocation (SN1) or Transition State (SN2)
Rate Law	Depends on electrophile & substrate	SN1: First order; SN2: Second order
Examples	Nitration, Halogenation	Hydrolysis, Cyanation

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5. Important Tips to Remember

• In Electrophilic Substitution, the aromaticity is temporarily lost in the arenium ion intermediate and restored after substitution.

• In Nucleophilic Substitution, inversion of configuration happens in SN2, whereas racemization happens in SN1.

• ESR occurs mainly with benzene and its derivatives, while NSR applies to alkyl halides.