Organic Reactions

Organic Chemical Reactions

  • Organic reactions are vital for understanding biological systems, as most biological molecules are carbon-based.

  • Reaction mechanisms explain how molecules transform through bond making and breaking.

Reaction Mechanisms: Arrow Notation

  • Curved arrows show electron movement during bond making and breaking.

  • Half-headed arrows (fish hooks) represent one electron transfer in homolytic bond cleavage, forming radicals (e.g., X•X• and Y•Y•).

  • Full-headed arrows represent two electron transfer in heterolytic bond cleavage, forming charged intermediates.

Singly Bonded Functional Groups

  • C–XC–X bonds (where X = halogen, OH, OR, NH2, etc.) are polar because X is more electronegative than C (e.g., Cδ+C^{\delta+} – Xδ−X^{\delta-}.)

  • Cδ+C^{\delta+} atom is electrophilic.

  • Nucleophiles donate electron pairs to Cδ+C^{\delta+} forming bonds.

  • Good leaving groups (X) and nucleophiles enable:

    • Nucleophilic Substitution: Bond broken and bond formed.

    • Elimination: Double bond formation.

Nucleophilic Substitution

  • Example: Methyl-group transfer in biological systems (e.g., adrenaline production).

  • Possible Reaction Sequences:

    • OPTION 1: X leaves as Nu:−Nu:^- attaches; rate = k[C−X]k [C-X] (if first step is rate determining).

    • OPTION 2: Nu:−Nu:^- attaches as X departs; rate = k[C−X][Nu:−]k [C-X] [Nu:^-] (SN2 reaction).

SN2 Reaction

  • Rate = k[C−X][Nu:−]k [C-X] [Nu:^-] (one-step, second-order reaction).

  • Nu:−Nu:^- attaches as X departs via a transition state.

  • Example: CH3Br+NaOH⟶CH3OH+NaBrCH3Br + NaOH \longrightarrow CH3OH + NaBr