Introduction

• Inquiry about Russian speakers in the class, indicated that a piece was read beautifully but was not understood by others.

Course Transition into Alkenes

• Introduction of the new unit focusing on alkene reactions.

• Mention of technical issues with video links on Blackboard due to financial arrangements; alternative access through recordings on Canvas.

Overview of Alkene Reactions

• Summary of previous unit focused on how to create alkenes via reactions:

  • Utilization of E2 reactions leading to double bond formation with good leaving groups.

  • Methods of creating alkenes from secondary/tertiary alcohols through elimination processes.

• Transition to new unit focusing on manipulating alkenes through addition reactions.

Key Concepts of Addition Reactions

• General definition: Addition reactions involve adding something across double bonds in alkenes.

• Visual Representation of Addition Reaction:

  • Alkene (C=C) + X (first component) + Y (second component) → Product with X and Y added to each carbon of the double bond.

• Possible reactions identified:

  1. Hydrogenation (adding $H_2$)

  2. Hydrohalogenation (adding hydrogen halides)

  3. Hydration (adding water)

• Historical significance of addition reactions referenced through traditional names and their role in organic chemistry.

Importance of Flashcards for Reactions

• Encouragement to build a systematic collection of flashcards for each type of reaction similar to a toolbox:

  • Example: Hammer for nails, screwdriver for screws.

• Expectation of developing 50-60 reactions by the end of the course and the necessity for consistent study and usage of these flashcards.

Role of Pi Bonds in Reactions

• Pi bonds can act in two major roles:

  • Weak Bronsted-Lowry base accepting protons in presence of strong acids like HCl, HBr, or HI.

  • Nucleophile attracting positive charges due to the electron-dense area being located away from the sigma bond region.

• Electrophilic addition noted, highlighting the interaction of alkenes with electrophiles.

Addition vs. Elimination Reactions

• Addition reactions typically reduce the number of moles from reactants to products, resulting in less disorder (entropy).

• Elimination reactions increase the number of moles and hence result in more disorder.

• **Spontaneity and Thermodynamics:

  • Addition reactions driven by enthalpy (ΔH) and entropy (ΔS): **

  • ΔG = ΔH - TΔS; involves conditions such as pressure and temperature influencing the direction of favorability.

• Observations:

  • Addition reactions tend to be exothermic (negative ΔH) due to formation of stronger sigma bonds from the breaking of a pi bond.

  • At low temperatures, addition reactions are more spontaneous.

  • At higher temperatures, the reaction rates increase but may yield less product due to unfavorable entropy changes.

Hydrohalogenation Reaction

• Notable addition reaction discussed is hydrohalogenation (adding HBr, HCl, or HI).

• Following hydrohalogenation:

  • Formation of alkyl halides where chirality may be induced, leading to racemic mixtures in certain conditions due to symmetry of the alkene.

Markovnikov's Rule

• Definition: The halogen adds to the more substituted carbon, while hydrogen adds to the less substituted carbon ("them's as has gets").

  • Historical significance of Markovnikov's contribution with references to the original context.

• Mechanistic insights:

  • The rational behind Markovnikov's rule lies in carbocation stability; tertiary carbocations are favored over secondary and primary.

• If the alkene has a higher degree of substitution at one of its carbons, then the hydrogen will attach to the least substituted carbon due to generating a more stable transition state after protonation.

Anti-Markovnikov Addition

• Occurrence of anti-Markovnikov addition discussed; situation arises due to the presence of peroxides which generate free radicals.

• Explanation of how peroxides lead to a different reaction pathway, resulting in the opposite regiochemistry observed in standard Markovnikov addition reactions.

Reactions in Synthesis

• Understanding of functional groups and positional isomerism using Markovnikov vs. anti-Markovnikov addition crucial in synthesis problems.

• Ability to synthesize various products by manipulating reactions during lab work.

• Anticipated discussions of practical examples and mechanisms later in the unit's teachings.

Conclusion and Homework Direction

• Assignments: Practical predictions of reaction products to be discussed in the next session.

• Encouragement to engage with the material and understand underlying principles to excel in problem-solving scenarios.