Slide 1: Nitrogen Containing Organic Compounds - Two Types of Discrete Mechanisms

  • In organic chemistry, there are two types of discrete mechanisms that involve nitrogen-containing organic compounds.
  • These mechanisms are the nucleophilic substitution and the elimination reactions.
  • Let’s take a closer look at each of them in this lecture.

Slide 2: Nucleophilic Substitution Reaction

  • Nucleophilic substitution reaction involves the substitution of a nucleophile (electron-rich species) for a leaving group in an organic compound.
  • This reaction occurs in two steps:
    1. Attack of the nucleophile on the carbon atom attached to the leaving group.
    2. Departure of the leaving group, resulting in the displacement of the leaving group by the nucleophile.

Slide 3: Nucleophilic Substitution Reaction - SN1 Mechanism

  • The SN1 (Substitution Nucleophilic Unimolecular) mechanism is a two-step reaction.
  • It proceeds via the formation of a carbocation intermediate.
  • In the first step, the leaving group departs, creating a carbocation.
  • In the second step, the nucleophile attacks the carbocation, resulting in the formation of the substitution product.

Slide 4: Example of SN1 Reaction

Example: Conversion of tert-butyl chloride to tert-butyl alcohol using sodium hydroxide.

  1. tert-butyl chloride reacts with water to form a tert-butyl carbocation.
  1. Sodium hydroxide acts as a nucleophile and attacks the carbocation, resulting in the substitution of the chloride ion with the hydroxyl group.
  1. The final product is tert-butyl alcohol. Equation: $$\text{tert-butyl chloride + sodium hydroxide → tert-butyl alcohol}$$

Slide 5: Nucleophilic Substitution Reaction - SN2 Mechanism

  • The SN2 (Substitution Nucleophilic Bimolecular) mechanism is a single-step reaction where the nucleophile directly replaces the leaving group.
  • The attack of the nucleophile and departure of the leaving group occur simultaneously.
  • This mechanism involves a backside attack by the nucleophile, resulting in the inversion of configuration.

Slide 6: Example of SN2 Reaction

Example: Conversion of methyl chloride to methyl alcohol using sodium hydroxide.

  1. Sodium hydroxide attacks the carbon atom attached to the chloride group from the backside, displacing the chloride ion.
  1. The configuration of the product is inverted due to backside attack.
  1. The final product is methyl alcohol. Equation: $$\text{methyl chloride + sodium hydroxide → methyl alcohol}$$

Slide 7: Elimination Reaction

  • Elimination reactions involve the removal of two substituents (usually a leaving group and a proton) from a molecule to form a double bond or pi bond.
  • These reactions occur in two major mechanisms: E1 and E2.

Slide 8: Elimination Reaction - E1 Mechanism

  • The E1 (Elimination Unimolecular) mechanism is a two-step reaction.
  • In the first step, the leaving group departs, forming a carbocation intermediate.
  • In the second step, a base abstracts a proton from an adjacent carbon, resulting in the formation of the double bond.

Slide 9: Example of E1 Reaction

Example: Conversion of tert-butyl chloride to 2-methylpropene using sodium ethoxide.

  1. tert-butyl chloride reacts with sodium ethoxide to form a tert-butyl carbocation.
  1. The base (sodium ethoxide) abstracts a proton from an adjacent carbon, resulting in the formation of a double bond.
  1. The final product is 2-methylpropene. Equation: $$\text{tert-butyl chloride + sodium ethoxide → 2-methylpropene}$$

Slide 10: Elimination Reaction - E2 Mechanism

  • The E2 (Elimination Bimolecular) mechanism is a single-step reaction where the base removes a proton and a leaving group from an adjacent carbon simultaneously.
  • The base attacks the hydrogen atom while the leaving group departs, forming a double bond. These are the two types of discrete mechanisms involving nitrogen-containing organic compounds.

Slide 11: Electrophilic Aromatic Substitution

  • Electrophilic aromatic substitution (EAS) is a reaction in which an electrophile replaces a hydrogen atom on an aromatic compound.
  • This reaction proceeds through the formation of a carbocation intermediate and involves resonance stabilization.
  • Two common mechanisms are the addition-elimination mechanism (Aromatic Substitution SN1) and the concerted mechanism (Aromatic Substitution SN2).

Slide 12: Addition-Elimination Mechanism (Aromatic Substitution SN1)

  • In the addition-elimination mechanism, the electrophile first adds to the aromatic ring to form a sigma complex.
  • In the second step, the sigma complex loses a proton to regenerate the aromaticity.

Slide 13: Example of Addition-Elimination Mechanism

Example: Nitration of benzene to form nitrobenzene using a mixture of concentrated sulfuric acid and concentrated nitric acid.

  1. The nitronium ion (NO2+) is generated by the reaction of concentrated sulfuric acid and concentrated nitric acid.
  1. The electrophilic nitronium ion adds to the benzene ring, forming a sigma complex.
  1. The sigma complex loses a proton to regenerate the aromaticity.
  1. The final product is nitrobenzene. Equation: $$\text{Benzene + Nitronium Ion → Nitrobenzene}$$

Slide 14: Concerted Mechanism (Aromatic Substitution SN2)

  • In the concerted mechanism, the nucleophilic pi electrons on the aromatic ring attack the electrophile while simultaneously pushing out the leaving group.
  • This process occurs in a single step without the formation of a carbocation intermediate.

Slide 15: Example of Concerted Mechanism

Example: Bromination of benzene to form bromobenzene using elemental bromine (Br2) as the electrophile.

  1. The electrophilic bromine molecule adds to the benzene ring, with the pi electrons attacking the bromine atom.
  1. The sigma complex is formed and stabilized by resonance.
  1. The leaving group (bromide ion) is pushed out, resulting in the formation of bromobenzene.
  1. The final product is bromobenzene. Equation: $$\text{Benzene + Bromine → Bromobenzene}$$

Slide 16: Reduction Reactions

  • Reduction reactions involve the gain of electrons or the decrease in oxidation state of an atom or molecule.
  • Reduction can be achieved by adding hydrogen (H2) gas, metal hydrides, or other reducing agents.
  • Common reducing agents include lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4).

Slide 17: Example of Reduction Reaction with H2

Example: Conversion of nitrobenzene to aniline using hydrogen gas (H2) and a catalyst such as palladium.

  1. Nitrobenzene is subjected to hydrogenation using hydrogen gas and a catalyst.
  1. The nitrogen in the nitro group accepts two electrons from hydrogen gas, reducing it to an amine group.
  1. The final product is aniline. Equation: $$\text{Nitrobenzene + H2 → Aniline}$$

Slide 18: Example of Reduction Reaction with NaBH4

Example: Reduction of a ketone to an alcohol using sodium borohydride (NaBH4) as the reducing agent.

  1. Sodium borohydride donates a hydride ion (H-) to the carbonyl carbon, resulting in the formation of an alkoxide intermediate.
  1. The alkoxide is then protonated by water, leading to the formation of the alcohol.
  1. The final product is an alcohol. Equation: $$\text{Ketone + NaBH4 → Alcohol}$$

Slide 19: Oxidation Reactions

  • Oxidation reactions involve the loss of electrons or an increase in oxidation state.
  • Common oxidizing agents include potassium permanganate (KMnO4), potassium dichromate (K2Cr2O7), and chromic acid (H2CrO4).

Slide 20: Example of Oxidation Reaction

Example: Oxidation of primary alcohol (ethanol) to aldehyde (acetaldehyde) using potassium dichromate (K2Cr2O7) as the oxidizing agent.

  1. The primary alcohol is oxidized by the potassium dichromate, which is reduced in the process.
  1. The alcohol group is converted to an aldehyde group, resulting in the formation of acetaldehyde.
  1. The final product is acetaldehyde. Equation: $$\text{Ethanol + K2Cr2O7 → Acetaldehyde}$$

Slide 21:

  • Nitrogen-containing organic compounds can undergo two types of discrete mechanisms in organic chemistry.
  • These mechanisms are nucleophilic substitution (SN1 and SN2) and elimination (E1 and E2) reactions.
  • Let’s explore each of these mechanisms in more detail.

Slide 22: Nucleophilic Substitution - SN1 Mechanism

  • SN1 stands for Substitution Nucleophilic Unimolecular mechanism.
  • It is a two-step reaction involving the formation of a carbocation intermediate.
  • In the first step, the leaving group departs, leaving behind a carbocation.
  • In the second step, the nucleophile attacks the carbocation, leading to the substitution of the leaving group with the nucleophile.

Slide 23: Nucleophilic Substitution - SN2 Mechanism

  • SN2 stands for Substitution Nucleophilic Bimolecular mechanism.
  • It is a one-step reaction where the nucleophile directly replaces the leaving group.
  • The nucleophile attacks the carbon atom attached to the leaving group from the backside, resulting in the displacement of the leaving group.

Slide 24: Examples of Nucleophilic Substitution Reactions

  1. SN1 Example: Conversion of tert-butyl bromide to tert-butanol using sodium hydroxide.
    • The leaving group (bromide) departs, forming a tert-butyl carbocation.
    • Sodium hydroxide acts as a nucleophile and attacks the carbocation, resulting in the substitution of the bromide with the hydroxyl group.
    • The final product is tert-butanol.
  1. SN2 Example: Conversion of methyl iodide to methanol using sodium hydroxide.
    • Sodium hydroxide attacks the carbon atom attached to the iodide group from the backside, displacing the iodide ion.
    • The configuration of the product is inverted due to backside attack.
    • The final product is methanol.

Slide 25: Elimination Reactions - E1 Mechanism

  • E1 stands for Elimination Unimolecular mechanism.
  • It is a two-step reaction involving the formation of a carbocation intermediate.
  • In the first step, the leaving group departs, forming a carbocation.
  • In the second step, a base abstracts a proton from an adjacent carbon atom, resulting in the formation of a double bond.

Slide 26: Elimination Reactions - E2 Mechanism

  • E2 stands for Elimination Bimolecular mechanism.
  • It is a one-step reaction where the base removes a proton and the leaving group simultaneously.
  • The base attacks the hydrogen atom while the leaving group departs, forming a double bond.

Slide 27: Examples of Elimination Reactions

  1. E1 Example: Conversion of tert-butyl chloride to 2-methylpropene using sodium ethoxide.
    • The leaving group (chloride) departs, forming a tert-butyl carbocation.
    • The base (sodium ethoxide) abstracts a proton from an adjacent carbon, resulting in the formation of a double bond.
    • The final product is 2-methylpropene.
  1. E2 Example: Conversion of 2-bromobutane to butene using potassium ethoxide.
    • Potassium ethoxide removes the proton from the beta carbon while the bromide ion departs.
    • The formation of a double bond occurs simultaneously with the removal of the leaving group.
    • The final product is butene.

Slide 28: Electrophilic Aromatic Substitution

  • Electrophilic aromatic substitution (EAS) is a reaction where an electrophile replaces a hydrogen atom on an aromatic compound.
  • It proceeds through the formation of a carbocation intermediate.
  • Two common mechanisms are the addition-elimination mechanism (SN1) and the concerted mechanism (SN2).

Slide 29: Electrophilic Aromatic Substitution - Addition-Elimination Mechanism

  • In the addition-elimination mechanism, the electrophile first adds to the aromatic ring, forming a sigma complex.
  • In the second step, the sigma complex loses a proton to regenerate the aromaticity.

Slide 30: Electrophilic Aromatic Substitution - Concerted Mechanism

  • In the concerted mechanism, the nucleophilic pi electrons on the aromatic ring attack the electrophile while simultaneously pushing out the leaving group.
  • This process occurs in a single step without the formation of a carbocation intermediate.
  • The configuration at the reaction center is retained.