chapter 3: acids and bases

  • brønsted-lowry acids donate a proton
    • a conjugate base results when an acid gives up a proton
  • brønsted-lowry bases accept a proton
    • a conjugate acid results when a base accepts a proton
  • the making and breaking of bonds involves electron movement
  • a mechanism is a step by step pathway from reactants to products showing bonds breaking, forming, and the order in which they occur
  • quantitative strength analysis uses numerical data to compare acid strength
  • Ka is the acid-dissociation constant of an acid dissolved in water
    • the measurement of an acid’s strength when water is the base
  • if an acid is strong, the Ka will be > 1
  • ==pKa = -logKa==
    • lower pKa → a stronger acid
  • for a brønsted-lowry acid-base pair, pKa + pKb = 14
  • mineral acids: pKa = 0
  • acetic acid: pKa = 4.75
    • lower pKa → more acidic
  • water/small ROHs: pKa = 16
  • the stronger an acid, the weaker its conjugate base
  • equilibrium favors the weaker acid and weaker base
    • higher pKa → weaker acid
  • the stronger the acid, the more stable its conjugate base
    • better, thermodynamically favored
  • when an acid loses a proton, it forms the conjugate base, which has a lone pair of electrons that resulted from the loss of H+
    • to determine the stability of a conjugate base, look at the stability of the lone pair
  • the more effectively a conjugate base can stabilize its negative charge (lone pair) the stronger the acid from which it was derived
  • ARIO affects the stability of a negative charge
    • the type of atom that carries the charge
    • the larger the atom, the more stable a negative charge will be
      • size is the most important factor
      • as the size of the other atom increases, the H is more loosely held and the bond is easier to break
    • the more electronegative atom will better stabilize the negative charge
      • as the bond to H becomes more polarized, H becomes more positive and the bond to H is easier to break
    • resonance
    • most important!
    • resonance stabilizes a negative charge by spreading it out across multiple atoms
    • induction
    • induction can also stabilize a formal negative charge by spreading it out
    • more electron withdrawing groups → more stable conjugate base
    • the closer the electron withdrawing groups is to the negative charge → the more stable the conjugate base
    • the type of orbital where the charge resides
    • the closer the electrons are held to the nucleus, the more stable the entity, the more tightly the electrons are held
      • so sp is most acidic and most stable
    • the shorter the atomic orbital, the closer these electrons are held to the nucleus
    • not needed if we have quantitative values (pKa)
    • only to be used as a guideline
  • more resonance structures → more delocalization → more stable
  • difference in acidity is due to the relative stability of their conjugate bases
  • do not include counter ions (spectator ions) when writing a reaction
  • a lewis acid accepts a pair of electrons
    • electrophile
  • a lewis base donates a pair of electrons
    • nucleophile
  • water can be acid or base, depending