1. Acids
and
Bases
Unit 2

2. Arrhenius Acids and Bases
In 1884, Svante Arrhenius proposed these definitions
acid: a substance that produces H3O+ ions aqueous solution
base: a substance that produces OH- ions in aqueous solution
this definition of an acid is a slight modification of the original Arrhenius definition, which was that an acid produces H+ in aqueous solution
today we know that H+ reacts immediately with a water molecule to give a hydronium ion (H3O+)

3. Brønsted-Lowry Definitions
Acid: a proton (H+) donor
Base: a proton acceptor

4. Conjugate Acids & Bases
conjugate base: the species formed from an acid when it donates a proton to a base
conjugate acid: the species formed from a base when it accepts a proton from an acid
acid-base reaction: a proton-transfer reaction
conjugate acid-base pair: any pair of molecules or ions that can be interconverted by transfer of a proton

5. Conjugate Acids & Bases
Brønsted-Lowry definitions do not require water as a reactant
consider the following reaction between acetic acid and ammonia

6. Conjugate Acids & Bases
we can use curved arrows to show the flow of electrons in an acid-base reaction

7. Conjugate Acids & Bases
Many organic molecules have two or more sites that can act as proton acceptors
in these molecules, the favored site of protonation is the one in which the charge is more delocalized
question: which oxygen of a carboxylic acid is protonated?

8. Weak Acids and Bases
We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as:
water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L
we can combine these constants to give a new constant, Ka, called an acid dissociation constant

9. Weak Acids and Bases

10. Molecular Structure and Acidity
The overriding principle in determining the relative acidities of uncharged organic acids is the stability of the anion, A-, resulting from the loss of a proton
the more stable the anion, the greater the acidity of HA
Ways to stabilize anions include having the negative charge
on a more electronegative atom
on a larger atom
delocalized through resonance
delocalized by the inductive effect
in an orbital with more s character

11. Effect of Electronegativity on pKa
As the bond to H becomes more polarized, H becomes more positive and the bond is easier to break.

12. Effect of Size on pKa
As size increases, the H is more loosely held and the bond is easier to break.
A larger size also stabilizes the anion.

13. Effect of Resonance on pKa
If the negative charge on an atom can be delocalized over two or more atoms, the acidity of that compound will be greater than when the negative charge cannot be delocalized.
The ethoxide anion is less acidic than the acetate ion simply because the acetate ion can delocalize the negative charge.
Methanesulfonic acid can delocalize the charge in three different resonance forms, making it more acidic than the acetate ion.

14. Lewis Acids and Bases Lewis definition
Acids accept electrons. (electrophile)
Bases donate electrons. (nucleophile)
Hint for recognizing the base – look for electrons!!!!
Either a lone pair or pi bonded electrons seek out electrophiles.

15. Lewis Acids and Bases
Acid-base reactions can take place with or without a proton.
Lewis bases are species that are able to donate a pair of electrons - called nucleophiles (lover of nuclei).
Lewis acids are species that can accept this same pair of electrons – called electrophiles (lover of electrons).
The reactions are drawn using curved arrow formalism (movement of electrons represented with arrows).

16. Lewis Acids and Bases
Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of electrons
Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of electrons - + A + : B A B
new covalent bond
formed in this Lewis
acid-base reaction
Lewis
acid
Lewis
base

17. Lewis Acids and Bases : H H : B r : + : C H - C - H + : B r : C H - C3 - C - H 3 + : B r - : C H 3 - C - H 3 : H H H H
sec-
Butyl cation
(a carbocation)
Bromide
ion
2-Bromobutane C H 3 2 C H 3 2 F F + - : O : + B F : O B - F C H 3 2 F C H 3 2 F
Diethyl ether
(a Lewis base)
Boron trifluoride
(a Lewis acid)
A BF 3
-ether complex