1 Lewis acid = a substance that accepts an electron pair Lewis Acids & Bases Lewis base = a substance that donates an electron pair

2 Reaction of a Lewis Acid and Lewis Base New bond formed using electron pair from the Lewis base.New bond formed using electron pair from the Lewis base. Coordinate covalent bondCoordinate covalent bond Notice geometry change on reaction.Notice geometry change on reaction.

3 Formation of hydronium ion is also an excellent example. Lewis Acids & Bases Electron pair of the new O-H bond originates on the Lewis base.Electron pair of the new O-H bond originates on the Lewis base.

4 Lewis Acid/Base Reaction

5 Other good examples involve metal ions. Lewis Acids & Bases

6 The combination of metal ions (Lewis acids) with Lewis bases such as H 2 O and NH > COMPLEX IONS > COMPLEX IONS Lewis Acids & Bases

7 Reaction of NH 3 with Cu 2+ (aq)

8 [Ni(H 2 O) 6 ] NH 3 ---> [Ni(NH 3 ) 6 ] 2+ [Ni(H 2 O) 6 ] NH 3 ---> [Ni(NH 3 ) 6 ] 2+ + DMG Lewis Acids & Bases See page 924

9 Lewis Acid-Base Interactions in Biology The heme group in hemoglobin can interact with O 2 and CO. The Fe ion in hemoglobin is a Lewis acid O 2 and CO can act as Lewis bases Heme group

10 Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. [Cu(H 2 O) 4 ] 2+ + H 2 O ---> [Cu(H 2 O) 3 (OH)] + + H 3 O + Lewis Acids & Bases

11 This explains why water solutions of Fe 3+, Al 3+, Cu 2+, Pb 2+, etc. are acidic. Lewis Acids & Bases This interaction weakens this bond Another H 2 O pulls this H away as H +

12 Amphoterism of Al(OH) 3

13 This explains AMPHOTERIC nature of some metal hydroxides. Al(OH) 3 (s) + 3 H + --> Al H 2 O Here Al(OH) 3 is a Brønsted base. Al(OH) 3 (s) + OH - --> Al(OH) 4 - Here Al(OH) 3 is a Lewis acid. Lewis Acids & Bases

14 Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. AgCl(s) e Ag + + Cl - K sp = 1.8 x Ag NH 3 --> Ag(NH 3 ) 2 + K form = 1.6 x AgCl(s) + 2 NH 3 e Ag(NH 3 ) Cl - K net = __________________ K net = __________________ Lewis Acids & Bases

15 Why? Why are some compounds acids?Why are some compounds acids? Why are some compounds bases?Why are some compounds bases? Why do acids and bases vary in strength?Why do acids and bases vary in strength? Can we predict variations in acidity or basicity?Can we predict variations in acidity or basicity? Why are some compounds acids?Why are some compounds acids? Why are some compounds bases?Why are some compounds bases? Why do acids and bases vary in strength?Why do acids and bases vary in strength? Can we predict variations in acidity or basicity?Can we predict variations in acidity or basicity?

16 Why is CH 3 CO 2 H an Acid? 1. The electro- negativity of the O atoms causes the H attached to O to be highly positive. 2. The O—H bond is highly polar. 3. The H atom of O—H is readily attracted to polar H 2 O. Figure 17.9

17 Trichloroacetic acid is much stronger acid owing to the high electronegativity of Cl, which withdraws electrons from the rest of the molecule. This makes the O—H bond highly polar. The H of O—H is very positive. Acetic acid Trichloroacetic acid K a = 1.8 x K a = 0.3

18 These ions are BASES. They become more and more basic as the negative charge increases. As the charge goes up, they interact more strongly with polar water molecules. NO 3 - CO 3 2- Basicity of Oxoanions PO 4 3-