Acids and Bases

• Bronsted-Lowry Acid

  Proton donor
• Bronsted-Lowry Base

  Proton acceptor
• Conjugate acid


  Formed when a base accepts a proton from a suitable donor
• Conjugate base


  Formed when an acid loses a proton
• Equilibrium constant (Ka)


  Measure of the strength of an acid in dilute aqueous solution
• Strong acids


  • Characterized by large values of Ka, essentially every molecule transfers a proton to water
• Weak acids


  • Have small Ka values
• pKa


  Convenient way to express acid strength, equal to -log(Ka)
• Relative strength of an acid and its conjugate base

  The stronger the acid, the weaker the conjugate base. The weaker the acid, the stronger the conjugate base.

  
• Electrophile


  Loves electrons, acid
• Nucleophile


  Attacks molecules with positive charge, base
• Factors affecting acidity in binary compounds

  • Bond length (shorter = stronger, lower acidity)
  • Bond strength (longer = weaker, higher acidity)
  • Bond polarity (smaller electronegativity difference favors higher acidity)
• Push-pull electronic effects on the proton

  Affect the strength of oxyacids
• Ranking 1.0M solutions of HBrO, HIO and HClO in order of increasing pH

  • HIO < HBrO < HClO
• Acid strength


  Ability to donate a proton to water, expressed by Ka or pKa
• Positivity of H


  The more positive the H, the more acidic and the higher the acid strength
• Stability of conjugate base

  The more stable the conjugate base, the more willing the acid is to donate its proton, the higher the acid strength

  
• Electronic effects

  • Electron release or electron donating effect (+) increases electron density
  • Electron withdrawal or electron accepting effect (-) decreases electron density
• Resonance effect


  • +R groups render the benzene ring more electron rich, especially at ortho and para positions
  • -R groups render the ring less electron rich, especially at meta positions
• Inductive effect


  • Inhibited by -I EN atoms/groups, groups with orbital EN, groups with excess + charge
  • +I effect exhibited by groups with excess - charge, alkyl groups
• Steric effects


  • Generally associated with bulky groups, can exhibit -I, +R but +R>-I, net effect is electron release or withdrawal
• Acidic form


  Predominates when pH < pKa
• Buffer solution

  Maintains a nearly constant pH within certain parameters
• Lewis acid

  Electron pair acceptor

  
• Lewis base

  Electron pair donor

  
• Basicity


  Tendency to donate an electron pair, exhibited by amines with a lone pair on N
• Factors affecting basicity


  • Presence of electron releasing groups makes the electron pair more available, hence more basic
  • Presence of electron withdrawing groups makes the electron pair less available, hence less basic
• Bonsted-Lowry Acids and Bases is also known as Protonic Theory