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FACTORS THAT INCREASE ACIDITY

FACTORS THAT INCREASE ACIDITY. STABILIZATION OF A CONJUGATE BASE. We will study the factors that lead to lower energy (stabilization) in the conjugate base. STABILIZATION FACTORS. 1 Resonance. 2 Electronegativity. stabilization. 3 Size of Atoms. 4 Hybridization. A -.

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FACTORS THAT INCREASE ACIDITY

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  1. FACTORS THAT INCREASE ACIDITY STABILIZATION OF A CONJUGATE BASE

  2. We will study the factors that lead to lower energy (stabilization) in the conjugate base. STABILIZATION FACTORS 1 Resonance 2 Electronegativity stabilization 3 Size of Atoms 4 Hybridization A- 5 Inductive Effects 6 Charge 7 Solvation Stabilization of the conjugate base makes the acid stronger. HA 8 Steric Effects* * usually destabilize

  3. => Electronegativity As the bond to H becomes more polarized, H becomes more positive and the bond is easier to break.

  4. ELECTRONEGATIVITY When comparing two acids in the same period ... placing the negative charge on a more electronegative element in the conjugate base leads to a stronger acid

  5. EFFECT OF ELECTRONEGATIVITY increasing electronegativity pKa Values >45 45 20 34 35 15 16 18 5 3.5 conjugate bases :

  6. Predict whether trifluoroacetic acid will be a stronger or weaker acid than acetic acid. Ka F O F C C O H F 10-4.74 acetic acid 10-0.23 Fluorine is more electro-negative than hydrogen. Anion is more stable. trifluoroacetic acid more acidic acid

  7. Size • As size increases, the H is more loosely held and the bond is easier to break. • A larger size also stabilizes the anion. When comparing two acids in the same group ... placing the negative charge on a larger atom in the conjugate base leads to a stronger acid.

  8. EFFECT OF ATOMIC SIZE - - - - I F Cl Br increasing atom size pKa Values 3.5 16 -7 7 -9 4 -10 3 1.36 A 1.81 A 1.95 A 2.16 A

  9. => Resonance • Delocalization of the negative charge on the conjugate base will stabilize the anion, so the substance is a stronger acid. • More resonance structures usually mean greater stabilization.

  10. RESONANCE EFFECTS increasing quality of resonance pKa Values 45 18 28 10 25 30 5 25 15 20 9

  11. Which molecule is the stronger acid, ethanol or acetic acid? Ka 10-16 10-4.74 more stable anion because of resonance and inductive effects ethanol acetic acid the stronger acid

  12. RESONANCE IN THE ACETATE ION equivalent structures charge on oxygens O C H C 3 O O + -H _ C H C O H 3 base _ acetic acid O C H C 3 O acetate ion

  13. O O O _ _ _ O O _ PHENOLATE ION RESONANCE _ - More structures, but not better than acetate. Non-equivalent structures charge on carbon and oxygen

  14. DEPENDENCE OF ACIDITY ON SIZE AND ELECTRONEGATIVITY Acidity PERIOD Electronegativity GROUP Size Acidity

  15. EVALUATION OF ACID STRENGTH

  16. EVALUATION OF ACID STRENGTH HA + H2O H3O+ + A- In water, all acids form hydronium ion, the important factor of difference is the conjugate base. The difference between a strong acid and a weak acid is in the stability of the conjugate base. A- WEAK ACID has strong conj. base (=higher energy) E N E R G Y A- STRONG ACID has weak conj. base (=lower energy) ionization easier HA

  17. A model for the ionization of an acid. weak acid ΔGº Energy important Reaction = -RTln Ka

  18. It is a general principle that the more stable the anion the more acidic is the acid. Energy Reaction weak acid ΔGº = -RTln Ka

  19. Consider another acid where the anion is more stable. Will this acid be stronger or weaker? Energy Reaction weak acid stronger acid ΔGº = -RTln Ka

  20. Consider another acid where the anion is less stable. Will this acid be stronger or weaker? Energy Reaction weak acid stronger acid ΔGº = -RTln Ka

  21. Consider another acid where the anion is less stable. Will this acid be stronger or weaker? ΔGº = -RTln Ka Energy Reaction weak acid weaker acid

  22. Which molecule is the stronger acid, ethanol or acetic acid? Ka 10-16 10-4.74 more stable anion because of resonance and inductive effects ethanol acetic acid the stronger acid

  23. RESONANCE IN THE ACETATE ION equivalent structures charge on oxygens O C H C 3 O O + -H _ C H C O H 3 base _ acetic acid O C H C 3 O acetate ion

  24. C H C H O H + C H C H O H 3 2 3 2 Energy Reaction Identify the stronger base?

  25. Circle the stronger acid with an explanation for your choice. same base

  26. Circle the stronger acid with an explanation for your choice. ΔGº ΔGº Energy Reaction

  27. Buffers and pH changes

  28. In chemistry, particularly biology, a large number of compounds are acids and bases. dopamine Biological fluids are often buffered (constant pH) an it is useful to know the predominant species present at a given pH.

  29. when p H = -log [H] a useful concept: Biological fluids are often buffered (constant pH) an it is useful to know the predominant species present at a given pH.

  30. Consider acetic acid with a Ka = 10-4.74 at pH = 4.74 at lower pH, more acidic than 4.74, acetic acid is the major species present

  31. Consider acetic acid with a Ka = 10-4.74 at pH = 4.74 at lower pH, more acidic than 4.74, acetic acid is the major species present

  32. Consider acetic acid with a Ka = 10-4.74 at pH = 4.74 at lower pH, more acidic than 4.74, acetic acid is the major species present at higher pH, less acidic than 4.74, acetate ion is the major species present

  33. If acetic acid is introduced into the blood what will be the predominant species present? Will it be acetate ion or acetic acid? at pH = 4.74 The pH of blood is maintained at pH 7.4 If the pH of blood was 4.74 then the acetate ion would be equal to the acetic acid ion concentration. If the pH is raised to 7.4 will the concentration of acetate ion increase or decrease?

  34. If acetic acid is introduced into the blood what will be the predominant species present? Will it be acetate ion or acetic acid? pH 4.74  7.4 If the pH is raised to 7.4 (more basic) will the concentration of acetate ion increase or decrease?

  35. If acetic acid is introduced into the blood what will be the predominant species present? Will it be acetate ion or acetic acid? Acetate ion is the major species present if acetic acid is introduced into blood. at pH = 4.74 If the pH is raised to 7.4 will the concentration of acetate ion increase or decrease?

  36. a useful concept: then when

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