Chem Oct. 20th

Overview of Course Topics

  • Discussion about exams and coursework schedules.
    • Bio Statistics due Saturday.
    • Biochemistry next week.
    • Courses: Calculus, Psychology.
    • Prioritizing study around ongoing exams: Microbiology on Friday, Biology on Saturday.

Module Five: Substitution and Elimination Reactions

  • Important reactions involving SN2 and E2 mechanisms.
  • Focus on alkyl halides:
    • Alkyl halides are effective in demonstrating substitution and elimination because they have strong leaving groups.
    • Examples of good leaving groups: I⁻, Br⁻, Cl⁻.
  • Introduction to sulfonates as another type of leaving group:
    • Sulfonates are stable conjugate bases derived from strong acids, thus making them good leaving groups.
    • They can leave as O⁻ which resonates over multiple oxygen atoms, providing stability.
    • Example: CF₃ as an inductively withdrawing group enhances stability of the leaving group.

Importance of Leaving Groups in Reactions

  • Good leaving groups are essential for substitution or elimination reactions:
    • Hydroxide (OH⁻) is a poor leaving group (described as a "crappy leaving group").
    • Converting alcohols (OH) to sulfonates can enhance reactivity by improving leaving group quality:
    • Process: Convert alcohol (OH) to a sulfonate using tosyl chloride (tosyl chloride reacts with alcohol to form a tosylate).
    • Tosylate is a good leaving group, allowing for nucleophilic substitution or elimination reactions without changing the stereochemistry.
  • Alternatively, strong acids can be used to protonate alcohols, transforming them into good leaving groups (like alcohol → alkyl halides).

Mechanisms for Converting Alcohols to Leaving Groups

  • Method 1: Sulfonate () Conversion.
    • Alcohols can be converted to tosylates, which then act as excellent leaving groups in nucleophilic reactions.
    • Pyridine aides in reaction as a base to deprotonate the product.
  • Method 2: Protonation with Strong Acid.
    • Protonation using acids like HBr will produce good leaving groups as water, which is a weak base.
    • This allows for conversion of alcohols to alkyl halides, facilitating various reactions learned in Module Five.

Reactions Involving Carbohydrates

  • Carbohydrate reactions and mechanisms covered:
    • Discusses how protonating alcohols leads to possible rearrangements to create stable carbocations:
    • Rearrangement of carbocations is essential for achieving thermodynamic stability:
      • Secondary > Primary and Tertiary rearrangements stated as preferred.

Chemistry of Alkenes (Module Six)

  • Introduction to alkenes and their electron-rich nature:
    • A pi bond characterizes alkenes as nucleophilic (potential bases as well).
    • Electrophiles can react with alkene pi bonds, breaking pi bonds to form sigma bonds, leading to product formation.
    • Reactions can be run under low or high temperatures, affecting the enthalpy and resultant products.

Additions Across Pi Bonds

  • Key mechanisms driving alkene reactions: formation of new bonds through reactions with electrophiles.
  • Discussion of lewis structure and resonance stabilization in carbocations supports product formation predictions.
  • The proficiency in predicting products based on alkene reactivity with useful electrophiles leads to successful organic chemical synthesis.

Catalytic Acid Reactions (Hydration)

  • In hydration reactions, H₂O and H₂SO₄ are used as catalyst and protonating agents, respectively.
  • The mechanism involves:
    • Protonation of water creates hydronium (H₃O⁺), facilitating the reaction.
    • Water acts as a better nucleophile due to stronger acidity.
  • Understanding that using acids like H₂SO₄ avoids rapid nucleophilic substitution from good nucleophiles (like Br⁻)
  • Protonation forms carbocations that undergo subsequent nucleophilic attack, resulting in alcohol formation.