lecture recording on 12 March 2025 at 13.50.40 PM

Deuterium as an Isotope of Hydrogen

• Deuterium (D): An isotope of hydrogen with a heavier mass due to the presence of one neutron.

  • Referred to as 2H or D2 instead of H2.

  • Can replace hydrogen in chemical reactions to trace mechanisms.

Importance in Chemical Reactions

• Swapping H for D can distinguish reaction pathways:

  • Helps visualize where additions occur in reactions.

  • Example: Replacing H2 in reactions with D2 to track deuterium.

Symmetry in Molecular Structures

• Symmetrical molecules experience uniformity in addition reactions:

  • Adding D (or H) does not matter which side due to symmetry.

  • Non-symmetrical structures will exhibit different addition patterns.

Chiral Centers and Reaction Products

• Adding deuterium creates a chiral center:

  • Chiral center: A carbon attached to four different groups can create stereoisomers.

  • Prior addition of H results in two identical groups; D changes this scenario.

• Different products arise from nucleophile (like CHCl3) addition:

  • This can offer various isomers and configurations for products (wedge vs. dash).

Possible Stereoisomers with Deuterium and Chlorine

• With one D and one Cl:

  • Chiral centers introduce the potential for 4 stereoisomers.

  • Variations include:

    • D on wedge, Cl on dash.

    • D on dash, Cl on wedge.

• Products are influenced by how D and Cl are oriented relative to each other.

Regiochemistry vs. Stereochemistry

• Regiochemistry: Concerns the position where substituents attach to a molecule:

  • Hydrogen addition to the less substituted carbon (Markovnikov's rule).

  • Major products informed by stabilizing carbocations (primary vs. tertiary stability).

Carbocations Stability and Inductive Effects

• Carbocations: Carbons with a positive charge; classified as primary, secondary, or tertiary based on substituents:

  • Tertiary carbocation is the most stable due to inductive effects (more carbon groups stabilize + charge).

  • Methyl carbocation is least stable; fewer electron-donating adjacent groups.

Hyperconjugation in Carbocation Stability

• Hyperconjugation: Increased stability from adjacent carbon-sigma bonds overlapping with empty p-orbitals:

  • Greater overlap leads to enhanced stability in more complex carbocations (secondary versus primary).

Thermodynamics in Reaction Mechanisms

• Reactions include two key steps:

  1. Activation Energy: Energy for starting materials to transform into intermediates.

  2. Endothermic vs. Exothermic: Determined by stability of reactants, intermediates, and products.

Reaction Coordinate Diagram

• Y-axis: Represents Gibbs free energy or relative stability.

• X-axis: Progress of the reaction;

  • Displays activation energies, transition states, and relative stabilities of reactants (high energy) vs. products (lower energy).

Transition States and Mechanistic Steps

• Transition states mark the highest energy points during the formation of products:

  • Often represented as dashed lines to indicate their unstable nature.

• Importance of recognizing which states are intermediates versus transition states during reaction profiling.

Summary of Key Points

• Switching H for D provides clarity in reaction mechanisms.

• Creation of chiral centers leads to increased stereoisomer variations.

• Stability of carbocations influences the regiochemistry of reactions.

• Reaction profiles provide insights into the kinetics and thermodynamics of chemical processes.