Title: Lesson 5: Lewis Acids and Bases Learning Objectives: Understand that a Lewis acid is a lone...
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Transcript of Title: Lesson 5: Lewis Acids and Bases Learning Objectives: Understand that a Lewis acid is a lone...
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.
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Lewis theory focuses on electron pairs
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 acceptorA Lewis base is a lone pair donor
Bronsted-Lowry definition:A Bronsted-Lowry acid is a proton donorA Bronsted-Lowry base is a proton acceptorCompare and learn
these!
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Lewis Acids and Bases
Acid: electron pair acceptor Species with an incomplete octet/outer-shell
Base: electron pair donor Species with a lone pair
For example:
Gilbert Lewis
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So how are these definitions really different?
By definition Lewis bases and Bronsted-Lowry bases are the same. They are species which must have a lone pair of electrons.
By definition Lewis acids are broader than Bronsted-Lowry acids. No longer restricted to H+
A Lewis acid is any species that can accept a lone pair So all Bronsted-Lowry acids are Lewis acids... But now you need to include molecules with
an incomplete valence shell... 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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Example:
Boron has an incomplete octet, so it is able to accept an electron pair
Lewis acid
Lewis base
The arrow on the covalent bond shows where the coordinate bond is coming from.
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Example: Cu2+(aq) + 6H2O(l) →[Cu(H2O)6]2+
(aq)
Metals in the middle of the periodic table often form ions with vacant orbitals in their d subshell.
They are able to act as Lewis acids and accept lone pairs of electrons when they bond with ligands to form complex ions.
Ligands, as donors of lone pairs, are therefore acting as Lewis bases
Lewis acid Lewis
base
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Ligands Typical ligands found in complex ions include H2O, CN- and
NH3.
They all have lone pairs of electrons, the defining feature of their Lewis base properties.
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Some questions1. Which of the following species would exhibit Lewis acid
behaviour?
CH4, AlCl3, H2O, BH3, H+, Cu2+, NH3, NH4+
2. Which of the following species would exhibit Lewis base behaviour?
H2O, OH-, NH3, CO2, NH4+, C2H5OH, Cl-
Acid-Base Theory ComparisonTheory Definition of acid Definition of base
Bronsted-Lowry Proton donor Proton acceptor
Lewis Electron pair acceptor Electron pair donor
Lewis acid
Bronsted-Lowry acid
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Nucleophiles and Electrophiles
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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