02/26

Recap of Previous Lecture

• Objective: Align knowledge across diverse backgrounds in general chemistry (Gen Chem).

• Focus on a one-step reaction model before moving to multi-step reactions.

• Reaction Coordinate Diagram:

  • X-axis: Reaction coordinate, reflecting the progress of the reaction.

  • Y-axis: Energy levels, depicting the energy required to convert reactants to products.

  • Concepts introduced:

    • ΔG (Gibbs Free Energy): Difference in energy between reactants and products, indicates stability.

    • Ea (Energy of Activation): Energy required to initiate the reaction, related to the reaction's kinetics.

• Distinct separation of thermodynamics and kinetics: High-stability products do not guarantee rapid formation.

Kinetics and Thermodynamics

• Introduction of concepts to evaluate reaction dynamics:

  • Reaction rate: Speed at which reactants convert into products.

  • Rate Constant (k): Empirical measurement influencing reaction speeds.

  • Order of reaction determined by the exponents in the rate law equations relevant to reactants’ concentrations.

• Kinetics definition: Focus on making qualitative assessments of reactant concentration and its effect on reaction rates.

• Example of First-Order and Second-Order Reactions: Responses to concentration changes directly affect reaction rates.

Importance of Concentration in Reactions

• Analogy using stock market trading to elucidate how higher concentrations lead to increased reaction rates.

• Emphasis on practical laboratory applications where raising concentrations may speed reactions, but could also lead to unwanted pathways additional products.

Factors Affecting Reaction Rates

• Highlighting two primary aspects:

  • Energy of Activation (Ea): Key determinant of reaction speed; lower Ea leads to faster reactions.

  • Temperature: Higher temperatures increase molecular movement and collisions, consequently accelerating reactions. Example: Milk spoilage as a function of temperature control.

Revisiting Concepts of Substitution Reactions

• Transition to Topic 6: Alkyl Halides and Substitution Reactions

  • Definition of alkyl halides involves the presence of carbon (R group) and a halogen (X).

  • Classification of alkyl halides: Primary, Secondary, and Tertiary based on the number of carbon groups attached to the alpha carbon.

  • Incorporation of mechanisms using curved arrow notation in reaction equations.

  • Role of Nucleophiles and Electrophiles:

    • Nucleophiles: Electron-rich species that donate electron pairs (e.g., bases).

    • Electrophiles: Electron-poor species that accept electron pairs (e.g., carbocations, alkyl halides).

Criteria for Substitution Reactions

• Necessary components for substitution reactions include:

  • Substrate: The molecule that contains the leaving group.

  • Leaving Group: The halogen or similar compound that facilitates the reaction by departing from the substrate.

• Factors that influence the ability of leaving groups—electronegativity and size affecting stabilization after departure.

  • Leaving Group Criteria: Must effectively create a partial positive charge on the substrate and stabilize itself post-reaction.

Classifying Alkyl Halides

• Alkyl halides can be classified using criteria that consider the number of carbon substituents connected to the alpha carbon:

  1. Methyl Halides: No beta carbons.

  2. Primary Alkyl Halides: One R-group attached to alpha carbon and one halogen.

  3. Secondary Alkyl Halides: Two R-groups attached.

  4. Tertiary Alkyl Halides: Three R-groups attached.

• Each classification influences the rate and mechanism of substitution reactions.

Mechanisms of Substitution Reactions

• Overview of structure of substitution reactions including:

  • Loss of a leaving group.

  • Nucleophilic attack from the nucleophile.

• SN1 and SN2 reactions:

  • SN1: Unimolecular nucleophilic substitution that features a two-step reaction process.

  • SN2: Bimolecular nucleophilic substitution occurring in one concerted step.