Study Notes on Electrophilic and Nucleophilic Substitution Reactions

CHM 211: Electrophilic and Nucleophilic Substitution Reactions

Electrophilic and nucleophilic substitution reactions are common in organic compounds.

Definition: When two atoms are covalently bonded together, each atom takes its electron from the bond, resulting in free radicals as reaction intermediates.
Illustrated as follows:
- Chemical Reaction:
  A + B
  ightarrow A^{ullet} + B^{ullet}
- Single-barbed arrows indicate the movement of single electrons.
Characteristics of Radicals:
- Highly reactive and short-lived species.
Common catalysts include light, heat, or hydrogen peroxide.

Definition: Occurs between atoms of different electronegativity, resulting in one atom taking both bonding electrons.
Resulting ions:
- Positively charged carbon atom intermediate: Carbocation or Carbonium Ion.
- Negatively charged carbon: Carbanion.

Primary Carbonium: Attached to one carbon atom. Example:
- H_3C^+
Secondary Carbonium: Attached to two carbon atoms. Example:
- H_3C-CH_3 (with positive charge on one carbon).
Tertiary Carbonium: Attached to three carbon atoms, most stable. Example:
- H_3C-CH(CH_3)-CH_3.
Stability Order (decreasing stability):
- ext{tert-Butyl cation (3°)} > ext{Isopropyl cation (2°)} > ext{Ethyl cation (1°)} > ext{Methyl cation (least stable)}.

A detailed step-by-step description of how reactants are converted to products involving:
- Movement of electrons and cleavage of bonds.
- Spatial arrangement of atoms (stereochemistry).
- Generally, reactions proceed via the most stable intermediate.

Electrophiles: Electron-deficient species seeking electrons from electron-rich intermediates.
- Also referred to as "electron lovers."
- Examples: H^+, ext{Br}^+, ext{NO}_2^+, ext{BF}_3, ext{AlCl}_3.
- Some neutral compounds (Lewis acids) can also act as electrophiles.
Nucleophiles: Electron-rich species seeking electron-deficient sites to donate electrons.
- Also called "nucleus-loving" species.
- Examples: OH^-, CN^-, Cl^-, Br^-, I^-, ROH, NH_3.
- Neutral compounds with lone pairs often act as nucleophiles.
- Note: Certain anion salts like FeCl_4^-, BF_4^-, AlBr_4^- are not considered nucleophiles.

A reaction where an electrophile replaces an atom or group in a reactant molecule, commonly occurring in aromatic compounds (e.g., benzene).
Benzene is electron-rich due to pi bond delocalization, repelling nucleophiles, allowing electrophiles to replace hydrogen atoms.
Types of electrophilic substitution reactions include:
- Nitration
- Halogenation
- Sulphonation
- Friedel-Crafts reactions

Nitration:
- Reaction:
  ext{Benzene} + ext{HNO}_3/ ext{H}_2 ext{SO}_4
  ightarrow ext{Nitrobenzene} + ext{H}_2 ext{O}
- Electrophile: Nitronium ion generated from H_2SO_4 and HNO_2.
Halogenation:
- Reaction:
  ext{Benzene} + ext{Br}_2 ext{ (in presence of catalyst)}
  ightarrow ext{Bromobenzene} + ext{HBr}
- Catalyst: FeBr_3 or FeCl_3
- Chlorination and Iodination conditions are specified similarly.
- Iodination requires an oxidizing agent (e.g., H_2O_2).
Sulphonation:
- Reaction:
  ext{Benzene} + ext{H}_2 ext{SO}_4
  ightarrow ext{Benzene Sulphonic Acid} + ext{H}_2 ext{O}
- Electrophile: SO₃ (neutral).
Friedel-Crafts Alkylation:
- Reaction:
  ext{Benzene} + ext{(CH}_3)_2 ext{CHCl} + ext{AlCl}_3
  ightarrow ext{(1-Methyl Ethyl) Benzene} + ext{HCl}
- Electrophile: R+ from alkyl halide interacting with Lewis acid.
- Limitations include deactivation by substituents, lack of reactivity for aryl and alkenyl halides, and activation toward multiple substitutions.
Friedel-Crafts Acylation:
- Reaction:
  ext{Benzene} + ext{Acetyl Chloride} + ext{AlCl}_3
  ightarrow ext{Acetophenone} + ext{HCl}
- Acyl group replaces hydrogen in Friedel-Crafts reaction.

Consists of two stages:
1. Generation of Electrophile: From a neutral molecule, often by a catalyst.
2. Nucleophile Attack: Pi-electrons attack the generated electrophile, restoring aromaticity, leading to substitution.

A nucleophile replaces an atom or group in a reactant molecule (substrate).
The atom or group replaced is called the leaving group.
Mechanism involves heterolysis, yielding a nucleophile and a leaving group:
ext{Nu} + ext{R-X}
ightarrow ext{Nu-R} + ext{X}^-

SN1 Mechanism (Unimolecular):
- Stepwise reaction where the leaving group detaches, forming a carbocation first, followed by nucleophile attack.
- First-order kinetics as the rate depends solely on the substrate.
- Example with tert-Butyl Chloride reacting with OH- shows independence from nucleophile concentration.
Racemization occurs due to the formation of achiral carbocation:
- An optically active compound becomes a racemic mixture during this process.
SN2 Mechanism (Bimolecular):
- Occurs in one concerted step where bond breaking and formation happen simultaneously.
- Second-order kinetics as the rate depends on both substrate and nucleophile concentrations.

Rate equation exemplifies dependence:
ext{Rate} = k[ ext{R-X}][ ext{Nu}^-]
Stereochemistry of SN2 Reactions:
- Backside attack leads to inversion of configuration, producing enantiomers.
- Example demonstrates this through a Walden inversion transformation with specific configurations noted.

Predict the products of the following reactions:
1. Conduct reactions involving the application of catalysts and reagents.
2. Analyze the mechanisms for nucleophilic and electrophilic substitution reactions based on their described pathways and stereochemical outcomes.