Chapter 3 Notes: Acid/Base Reactions

Chem 334 - Fall 1999 Organic Chemistry I Dr. Carl C. Wamser

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