2.7 Acids and Bases: The Brønsted-Lowry
Definition
Two frequently used definitions of acidity
•
The Brønsted-Lowry definition
Lewis definition
Brønsted-Lowry acid
• A substance that donates a hydrogen ion (proton; H+) to a base
Brønsted-Lowry base
• A substance that accepts a hydrogen ion (proton; H+) from an acid
•
Acids and Bases: The Brønsted-Lowry
Definition
Conjugate acid
• The product that results from protonation of a Brønsted-Lowry
base
Conjugate base
• The anion that results from deprotonation of a Brønsted-Lowry
acid
In a general sense
Acids and Bases:
The Brønsted-Lowry Definition
Water can act either as an acid or as a base
2.8 Acid and Base Strength
Acids differ in their ability to donate H+
• The exact strength of a given acid, HA, in water
solution is described using the equilibrium constant
Keq for the acid-dissociation equilibrium
A- + H3O+
HA + H2O
Keq =
H3O+ AHA H2O
Acid and Base Strength
The concentration of water, [H 2O], remains nearly constant at
55.5 M at 25 C
• Can rewrite equilibrium expression using new quantity called
the acidity constant Ka
Acidity constant Ka
• A measure of acid strength in water
• For any weak acid HA, the acidity constant is given by the
expression Ka
HA + H2O
Ka = Keq H2O =
A- + H3O+
H3O+ AHA
Acid and Base Strength
HA + H2O
A- + H3O+
• Equilibria for stronger acids favor the products (to the right)
and thus have larger acidity constants
• Equilibria for weaker acids favor the reactants (to the left)
and thus have smaller acidity constants
Acid strengths are normally expressed using pKa values
pKa
• The negative common logarithm of the Ka
pKa = -log Ka
• Stronger acids (larger Ka) have smaller pKa
• Weaker acids (smaller Ka) have larger pKa
Acid and Base Strength
Acid and Base Strength
Self ionization of water
H2O + H2O ⇄ OH- + H3O+
(acid) (solvent)
Ka
[H3 O ][A ] [H3 O ][OH ]
[HA]
[H2 O]
[H3O+][OH-] = [1.0 x 10-7][1.0 x 10-7] = Kw = ion product constant for water
• Kw = 1.00 x 10-14
• [H2O] = 55.4 M at 25.0 C
[ Kw ] [1.0 10-14 ]
1.81 10-14
[H2 O]
55.4
pKa 15.74
Ka
Acid and Base Strength
• Strong acid (BrØnsted-Lowry)
• One that loses H+ easily
• Conjugate base holds on to the H+ weakly (weak
base)
• Strong acid has weak conjugate base
• Weak acid (BrØnsted-Lowry)
• One that loses H+ with difficulty
• Conjugate base holds on to the H+ strongly (strong
base)
• Weak acid has strong conjugate base
2.9
Predicting Acid-Base Reactions from pKa
Values
• An acid will donate a proton to the conjugate base of a
weaker acid
• The conjugate base of a weaker acid will remove the
proton from a stronger acid
Predicting Acid-Base Reactions from pKa
Values
• Product of conjugate acid must be weaker and less reactive
than the starting acid
• Product of conjugate base must be weaker and less
reactive than the starting base
Worked Example 2.4
Predicting Acid Strengths from pKa Values
Water has pKa = 15.74, and acetylene has pKa = 25. Which
is the stronger acid? Does hydroxide ion react with
acetylene?
Worked Example 2.5
Calculating Ka from pKa
According to the data in Table 2.3, acetic acid has
pKa = 4.76. What is its Ka?
2.10 Organic Acids and Organic Bases
Most biological reactions involve organic acids and organic bases
Organic acid
•
•
Positively polarized hydrogen atom
Two main kinds of organic acids
1.
Contains a hydrogen atom bonded to an oxygen atom (O-H)
2.
Contains a hydrogen atom bonded to a carbon atom next to a C=O
double bond (O=C-C-H)
Organic Acids and Organic Bases
Conjugate base
• Anion stabilized by having its negative charge on a highly
electronegative atom
• Anion stabilized by resonance
Methanol
Acetic Acid
Acetone
Organic Acids and Organic Bases
• Conjugate bases from methanol, acetic acid, and acetone
• The electronegative oxygen atoms stabilize the negative
charge in all three
Organic Acids and Organic Bases
Carboxylic acids
• Contain the –CO2H grouping
• Occur abundantly in all living organisms
• Involved in almost all metabolic pathways
• At cellular pH of 7.3 carboxylic acids are usually dissociated
and exist as their carboxylate anions, –CO 2
Organic Acids and Organic Bases
Organic bases
• Characterized by the presence of an atom with a lone pair of
electrons that can bond to H+
• Nitrogen-containing compounds are common organic bases
and are involved in almost all metabolic pathways
• Oxygen-containing compounds can act both as acids and as
bases
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.