Chem 334 - Fall 1998 - Organic Chemistry I
Portland State University - Dr. Carl C. Wamser

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Chapter 3 Notes: Acid-Base Reactions

Bronsted-Lowry Acid/Base

• acid - donates H+
• base - accepts H+
  NH3 + H2O <==> NH4+ + OH-
  base + acid <==> acid + base
• note conjugate acid-base pairs
  (differ by H+)

Acidity Constant (Ka)

• HA + H2O <==> H3O+ + A-

usually simplified to
HA <==> H+ + A-

• Ka = [H+] [A-] / [HA]

Acid Strength (pKa)

• stronger acids have higher Ka
  for HCl, Ka = 10E7
  for CH3COOH, Ka = 10E-5
  (acetic acid, found in vinegar)
  
• pKa = - log Ka
• stronger acids have a lower pKa
  for HCl, pKa = -7
  for CH3COOH, pKa = 5

pH and pKa

• Ka = [H+][A-]/[HA]
• pKa = pH - log([A-]/[HA])
• for pH = pKa, [A- ] = [HA]
• for pH < pKa, HA predominates
• for pH > pKa, A- predominates
  e.g., for acetic acid at pH = 7
  [CH3COO-] > [CH3COOH]

Structural Effects on Acid Strength

• electronegativity
  HF > H2O > NH3 > CH4
• weaker bond to H
  HI > HBr > HCl > HF
• inductive effects - electron withdrawal
  H2SO4 > H2SO3
  Cl-CH2-COOH > CH3-COOH
• hybridization with greater s-character
  sp C-H > sp2 C-H > sp3 C-H
• delocalization
  RCOOH > ROH
  

Acid-Base Reactions

CH3COOH + OH- <==> CH3COO- + H2O
acid (pKa = 5) + base <==> base + acid (pKa = 15.7)
reaction favored for stronger acid

reaction favored to the right

Acid-Base Reactions

CH3COOH + H2O <==> CH3COO- + H3O+
acid (pKa = 5) + base <==> base + acid (pKa = 1.7)
reaction favored to the left

Acid-Base Reactions

HC=CH + Na+NH2- <==> HC=C-Na+ + NH3
acid (pKa = 25) + base <==> base + acid (pKa = 35)
reaction favored to the right

Acid-Base Reactions

CH3OH + CN- <==> CH3O- + HCN
acid (pKa = 16) + base <==> base + acid (pKa = 9)
reaction favored to the left

Lewis Acids

• acid - accepts an electron pair
• base - donates an electron pair
• (making a new covalent bond)

H+ + H2O <==> H3O+
L acid + L base <==> new O-H bond

BF3 + NH3 <==> F3B-NH3
L acid + L base <==> new B-N bond