2.11 Acids and Bases: The Lewis Definition
The Lewis definition is broader than the Brønsted-Lowry definition
Lewis acid – an electrophile
•
A substance with a vacant low energy orbital that can accept an
electron pair from a base
• All electrophiles are Lewis acids
Lewis base – a nucleophile
•
•
A substance that donates an electron lone pair to an acid
All nucleophiles are Lewis bases
Acids and Bases: The Lewis Definition
Lewis Acids and the Curved Arrow Formalism
• To accept an electron pair a Lewis acid must have either:
• A vacant, low-energy orbital
• A polar bond to hydrogen so that it can donate H+
• Various metal cations, such as Mg2+, are Lewis acids
because they accept a pair of electrons when they form a
bond to a base
Acids and Bases: The Lewis Definition
• Compounds of group 3A elements, such as BF 3 and AlCl3
are Lewis acids
• Have unfilled valence orbitals and can accept electron
pairs from Lewis bases
• Many transition metals, such as TiCl4, FeCl3, ZnCl2, and
SnCl4 are Lewis acids
Acids and Bases: The Lewis Definition
Curved arrow formalism
•
•
•
Indicates the direction of
electron flow from the base to
the acid
Always means that a pair of
electrons moves from the atom
at the tail of the arrow to the
atom at the head of the arrow
For the reaction of boron
trifluoride, a Lewis acid, with
dimethyl ether, a Lewis base.
All movement of electrons from
the Lewis base to the Lewis
acid is indicated by a curved
arrow
Acids and Bases: The Lewis Definition
Further examples of Lewis acids
Acids and Bases: The Lewis Definition
Lewis bases
• A compound with a pair of nonbonding electrons that it can
use in bonding to a Lewis acid
• Definition of Lewis base similar to Brønsted-Lowry definition
• H2O acts as a Lewis base
•
Has two nonbonding electrons on oxygen
Acids and Bases: The Lewis Definition
• Most oxygen- and nitrogen- containing organic compounds
are Lewis bases
•
They have lone pair electrons
Acids and Bases: The Lewis Definition
• Some compounds can act as both acids and bases
• Some compounds have more than one atom with a lone pair
of electrons
•
•
•
Reaction normally occurs only once in such instances
The more stable of the two possible protonation products is
formed
Occurs with carboxylic acids, esters, and amides
Worked Example 2.6
Using Curves Arrows to Show Electron Flow
Using curved arrows, show how acetaldehyde,
CH3CHO, can act as a Lewis base.
2.12 Noncovalent Interactions between
Molecules
Noncovalent Interactions
• Also called intermolecular forces or van der Waals forces
• Types of noncovalent interactions include:
•
Dipole-Dipole forces
•
Occur between polar molecules as a result of electrostatic
interactions among dipoles
Forces are either attractive or repulsive
•
Attractive geometry is lower in energy and therefore predominates
•
Attractive
Repulsive
Noncovalent Interactions between Molecules
•
Dispersion forces
•
•
•
•
Attractive dispersion forces in nonpolar molecules are caused by
temporary dipoles
One side of the molecule may have a slight excess of electrons
relative to the opposite side, giving the molecule a temporary dipole
Temporary dipole in one molecule causes a nearby molecule to
adopt a temporarily opposite dipole resulting in a small attraction
between the two molecules
Arise because the electron distribution within molecules is constantly
changing
Noncovalent Interactions between Molecules
•
Hydrogen Bonds
•
•
•
A weak attraction between a hydrogen atom bonded to an
electronegative O or N and an electron lone pair on another O
or N atom
Strong dipole-dipole interaction involving polarized O-H and
N-H bonds
Important noncovalent interaction in biological molecules
Noncovalent Interactions between Molecules
•
Effects of Hydrogen Bonding
•
•
•
Causes water to be a liquid rather than a gas at room
temperature
Holds enzymes in the shapes necessary for catalyzing biological
reactions
Causes strands of deoxyribonucleic acid (DNA) to pair up and
coil into a double helix
Noncovalent Interactions between Molecules
Hydrophilic (water-loving)
• Dissolves in water
• Table sugar
•
Has ionic charges, polar –OH groups, in its structure
Hydrophobic (water-fearing)
• Does not dissolve in water
• Vegetable oil
•
Does not have groups that form hydrogen bonds
3.1 Functional Groups
Functional group
• An atom or a group of atoms that is part of a larger molecule
and that has a characteristic chemical reactivity
• Structural features that allow for classification of compounds
into families
Functional Groups
Functional Groups
Functional Groups
Functional Groups