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 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!

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: 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

Main Menu

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.

Main Menu

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

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

Main Menu

Main Menu

Solutions

Main Menu