Lewis AcidBase Theory

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Lewis Acid-Base Theory
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• Acid
• substance capable of accepting an e- pair

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Lewis acid

• must have an empty valence level orbital
• i.e. H has an empty 1s orbital which can accept
  an e- pair

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Thus, H is an acid under all three theories

• Arrhenius
• Bronsted-Lowry
• Lewis

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Lewis Acid-Base TheoryAcid substance capable of
accepting an e- pair

• Base
• substance capable of donating an e- pair

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Examples of Lewis bases

• OH- , NH3 , F-
• all have unbonded pairs of e- available for
  donation

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Elements of Group 13 (3A) form compounds that
make excellent Lewis acids
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B 1s2 2s2 2p1
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B 1s2 2s2 2p1
forms sp2 hybrid for bonding purposes
sp2 2p
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unhybridized p-orbital
sp2 hybrid orbital
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another typical Lewis acid-base reaction
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..
BF3 NH3
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Reaction of a Lewis Acid and Lewis Base

• New bond formed using electron pair from the
  Lewis base.
• Coordinate covalent bond
• Notice geometry change on reaction.

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Lewis Acids Bases

• Formation of hydronium ion is also an excellent
  example.

• Electron pair of the new O-H bond originates on
  the Lewis base.

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Lewis Acid/Base Reaction
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NO!
?
H3BO3 H2O H2BO3- H3O
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H3BO3 2H2O B(OH)4- H3O
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is Al(OH)3 an acid or base?
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Amphoterism of Al(OH)3
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• This explains AMPHOTERIC nature of some metal
  hydroxides.
• Al(OH)3(s) 3 H ? Al3 3 H2O
• Here Al(OH)3 is a Brønsted base.
• Al(OH)3(s) OH- ? Al(OH)4-
• Here Al(OH)3 is a Lewis acid.

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Transition metal ions also very good Lewis
Acids
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• Other good examples involve metal ions.

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..
Zn2 HOH ?
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Zn2 gt Ar 4s0 3d10 4p0
Zn(H2O)42
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Reaction of NH3 with Cu2(aq)
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Formation of complex ions is a
Lewis acid-base reaction
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Lewis Acid-Base Interactions in Biology

• The heme group in hemoglobin can interact with O2
  and CO.
• The Fe ion in hemoglobin is a Lewis acid
• O2 and CO can act as Lewis bases

Heme group
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Lewis Bronstead Arrhenius