Transcript Lewis Acids and Bases

Title: Lesson 5: Lewis Acids and Bases
Learning Objectives:
– Understand that a Lewis acid is a lone pair acceptor and a Lewis base is a
lone pair donor.
– Define what a nucleophile and an electrophile is.
Lewis theory focuses on electron pairs
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Gilbert Lewis – Remember chapter 4? Lewis structures!
Interpreted the Bronsted-Lowry theory in a different way – electron pairs
instead of protons.
Curly arrow is convention used to show donation of electron pairs.
H+ accepts the electron pairs.
Nitrogen atom in Ammonia donates electron pairs.
Lewis definition:
A Lewis acid is a lone pair acceptor
A Lewis base is a lone pair donor
Bronsted-Lowry definition:
A Bronsted-Lowry acid is a proton donor
A Bronsted-Lowry base is a proton acceptor
Compare and learn these!
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Lewis Acids and Bases
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Acid: electron pair acceptor
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Species with an incomplete octet/outer-shell
Base: electron pair donor
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Species with a lone pair
Gilbert Lewis
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For example:
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So how are these definitions really different?
By definition Lewis bases and Bronsted-Lowry bases are the same.
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They are species which must have a lone pair of electrons.
By definition Lewis acids are broader than Bronsted-Lowry acids.
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No longer restricted to H+
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A Lewis acid is any species that can accept a lone pair
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So all Bronsted-Lowry acids are Lewis acids... But now you need to include molecules with an incomplete
valence shell...
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Lewis acid-base reactions result in the formation of a covalent bond, which will always be a dative bond (a.k.a.
coordinate covalent bond) because both the electrons come from the base
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The arrow on the
covalent bond
shows where the
coordinate bond is
coming from.
Example:
Lewis
acid
Lewis
base
Boron has an incomplete octet, so it is able to accept an
electron pair
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Example: Cu2+(aq) + 6H2O(l) →[Cu(H2O)6]2+(aq)
Lewis
acid
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Lewis
base
Metals in the middle of the periodic table often form ions with vacant orbitals in their d subshell.
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are able to act as Lewis acids and accept lone pairs of electrons when they bond with
ligands to form complex ions.
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Ligands, as donors of lone pairs, are therefore acting as Lewis bases
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Ligands
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Typical ligands found in complex ions include H2O, CN- and NH3.
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They all have lone pairs of electrons, the defining feature of their Lewis base
properties.
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Some questions
Which of the following species would exhibit Lewis acid behaviour?
1.
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CH4, AlCl3, H2O, BH3, H+, Cu2+, NH3, NH4+
Which of the following species would exhibit Lewis base behaviour?
2.
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H2O, OH-, NH3, CO2, NH4+, C2H5OH, Cl-
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Acid-Base Theory Comparison
Theory
Definition of acid
Definition of base
Bronsted-Lowry
Proton donor
Proton acceptor
Lewis
Electron pair acceptor
Electron pair donor
Lewis acid
Bronsted-Lowry acid
Nucleophiles and Electrophiles
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Nucleophile (‘likes nucleus’) – electron rich species that donates lone pairs to form a
covalent bond... E.g. Lewis base
Electrophile (‘likes electrons’) – electron deficient species that accepts lone pairs to
form a covalent bond E.g. Lewis Acid
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Solutions
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