02/27

Review of Previous Material

• Yesterday covered organic chemistry overview of thermodynamics and kinetics.

• Introduced substitution reactions, which involve:

  • Nucleophiles: Electron-rich species.

  • Electrophiles: Electron-poor species (referred to as substrate).

  • Characterized by:

    • Loss of leaving group

    • Nucleophilic attack

• Mechanism steps are not chosen by chemists but observed.

Substitution Reactions

Two Types of Mechanism

1. Concerted Mechanism (SN2)

   • Nucleophilic attack and leaving group loss occur at the same time.

   • Represented with two arrows indicating bond formation and bond breaking.

   • Example: Nucleophile attracts to electrophile where a leaving group (Lg) is present, builds partial positive charge.

   • Chemical equilibrium might be indicated using equilibrium arrows.

   • The outcome is the nucleophile replacing the leaving group.

2. Stepwise Mechanism (SN1)

   • Involves two steps:

     • Leaving group departs first, forming a carbocation.

     • Nucleophile attaches second.

   • Depicts an intermediate state during the process.

   • Rate-determining step is the formation of the carbocation (slowest step).

Reaction Rates

• The rate of SN2 mechanisms depends on the concentration of both nucleophile and substrate:

  • Rate equation: Rate = k [Nucleophile][Substrate].

  • Bi-molecular nature is indicated by the number 2 in SN2.

• SN1 reaction's rate-determining step is the formation of the carbocation.

Reactivity Trends in SN2 Reactions

1. Tertiary Alkyl Halides - Slowest, nearly unreactive in normal conditions.

2. Secondary Alkyl Halides - Moderate reactivity.

3. Primary Alkyl Halides - Faster reactivity, significantly increased rate.

4. Methyl Halides - Most reactive due to accessibility, very low steric hindrance.

General Observations

• Steric hindrance (

  • More crowded electrophiles lead to lower reaction rates.

  • Reaction rates from least to most reactive: Tertiary < Secondary < Primary < Methyl.

• It's emphasized that understanding the mechanism rather than just memorizing trends is crucial for success in examinations.

Importance of Stereochemistry

• SN2 reactions result in inversion of stereochemistry due to the mechanism of attack:

  • Nucleophiles approach from the backside of the leaving group.

  • For example, if an R configuration is present, it will convert to S in the product.

• An example was demonstrated showing how a chirality center is transformed by nucleophilic attack and leave group departure.

Transition States and Mechanistic Diagrams

• Transition states are high-energy states during a reaction and are visually represented.

• Diagrams illustrate how nucleophiles attack and leaving groups depart.

• The concept of energy of activation is introduced.