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.

Main Menu 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 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!

Main Menu 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

Main Menu 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

Main Menu 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.

Main Menu Example: Cu 2+ (aq) + 6H 2 O(l) → [Cu(H 2 O) 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

Main Menu Ligands  Typical ligands found in complex ions include H 2 O, CN - and NH 3.  They all have lone pairs of electrons, the defining feature of their Lewis base properties.

Main Menu Some questions 1. Which of the following species would exhibit Lewis acid behaviour?  CH 4, AlCl 3, H 2 O, BH 3, H +, Cu 2+, NH 3, NH Which of the following species would exhibit Lewis base behaviour?  H 2 O, OH -, NH 3, CO 2, NH 4 +, C 2 H 5 OH, Cl -

Acid-Base Theory Comparison TheoryDefinition of acidDefinition of base Bronsted-LowryProton donorProton acceptor LewisElectron pair acceptorElectron pair donor Lewis acid Bronsted-Lowry acid

Main Menu 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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