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1.15 How Structure Affects Acid Strength

The Main Ways Structure Affects Acid Strength. The strength of the bond to the atom from which the proton is lostThe electronegativity of the atom from which the proton is lostElectron delocalization in the conjugate base. Bond Strength. Bond strength is controlling factor when comparing acidity o

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1.15 How Structure Affects Acid Strength

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    1. 1.15 How Structure Affects Acid Strength

    2. The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost The electronegativity of the atom from which the proton is lost Electron delocalization in the conjugate base

    3. Bond Strength Bond strength is controlling factor when comparing acidity of hydrogen halides.

    4. Bond Strength Recall that bond strength decreases in a group in going down the periodic table. Generalization: Bond strength is most important factor when considering acidity of protons bonded to atoms in same group of periodic table (as in HF, HCl, HBr, and HI). Another example: H2S (pKa = 7.0) is a stronger acid than H2O (pKa = 15.7).

    5. The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost The electronegativity of the atom from which the proton is lost Electron delocalization in the conjugate base

    6. Electronegativity Electronegativity is controlling factor when comparing acidity of protons bonded to atoms in the same row of the periodic table.

    7. Electronegativity

    8. Electronegativity The equilibrium becomes more favorable as A becomes better able to bear a negative charge. Another way of looking at it is that H becomes more positive as the atom to which it is attached becomes more electronegative.

    9. Bond Strength Versus Electronegativity Bond strength is more important when comparing acids in which the proton that is lost is bonded to atoms in the same group of the periodic table. Electronegativity is more important when comparing acids in which the proton that is lost is bonded to atoms in the same row of the periodic table.

    10. Acidity of Alcohols

    11. Acidity of Alcohols Electronegative substituents can increase the acidity of alcohols by drawing electrons away from the —OH group.

    12. Inductive Effect The greater acidity of CF3CH2OH compared to CH3CH2OH is an example of an inductive effect. Inductive effects arise by polarization of the electron distribution in the bonds between atoms.

    13. Electrostatic Potential Maps The greater positive character of the proton of the OH group of CF3CH2OH compared to CH3CH2OH is apparent in the more blue color in its electrostatic potential map.

    14. Another Example of the Inductive Effect

    15. The Main Ways Structure Affects Acid Strength The strength of the bond to the atom from which the proton is lost The electronegativity of the atom from which the proton is lost Electron delocalization in the conjugate base

    16. Electron Delocalization Ionization becomes more favorable if electron delocalization increases in going from right to left in the equation. Resonance is a convenient way to show electron delocalization.

    17. Nitric Acid

    18. Nitric Acid Nitrate ion is stabilized by electron delocalization.

    19. Nitric Acid Negative charge is shared equally by all three oxygens.

    20. Acetic Acid

    21. Nitric Acid Acetate ion is stabilized by electron delocalization.

    22. Nitric Acid Negative charge is shared equally by both oxygens.

    23. 1.16 Acid-Base Equilibria

    24. Generalization The equilibrium in an acid-base reaction is favorable if the stronger acid is on the left and the weaker acid is on the right.

    25. Example of a Strong Acid

    26. Example of a Weak acid

    27. Important Points A strong acid is one that is stronger than H3O+. A weak acid is one that is weaker than H3O+. A strong base is one that is stronger than HO–. A weak base is one that is weaker than HO–. The strongest acid present in significant quantities when a strong acid is dissolved in water is H3O+. The strongest acid present in significant quantities when a weak acid is dissolved in water is the weak acid itself.

    28. Predicting the Direction of Acid-Base Reactions

    29. Predicting the Direction of Acid-Base Reactions

    30. 1.17 Lewis Acids and Lewis Bases

    31. Definitions Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis An acid is an electron pair acceptor. A base is an electron pair donor.

    32. Lewis Acid-Lewis Base Reactions The Lewis acid and the Lewis base can be either a neutral molecule or an ion.

    33. Example: Two Neutral Molecules

    34. Example: Ion + Neutral Molecule

    35. Example: Ion + Neutral Molecule

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