Acid-Base Chemistry

Acid-Base Chemistry

HA + :B A- + HB+

Ka is the equilibrium constant for the dissociation of an acid into H+ and its conjugate base. Ka values are typically measure in water.

acid base conjugate base

conjugate acid

We define pKa = -logKa just as pH = -log[H3O+]

Weak acids have small Ka and large pKa valuesStrong acids have large Ka and small pKa values

Remember – when comparing acids, the weaker acid will have…the stronger conjugate base.

HA H+ + A-HA H+ + A-

HA

AHa a

aaK

HA and A- are a conjugate acid-base pair.HB+ and B: are also a conjugate acid-base pair.

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N C H S

H

N C H S

H

N C H O

H

HN C H

H

O

H

H O NCH NCH

OH

A Brønsted acid is simply an electrophile that loses H+ to a base. The acidic site is the hydrogen atom. A Brønsted base is simply a nucleophile that makes a bond to the hydrogen atom of an acid. The basic site is the atom/bond donating the electrons used to make this bond.

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Acid Ka pKa

HClO4 1010 -10

HF 7.2 x 10-4 3.1

NH4+ 6.3 x 10-10 9.2

H2O 2.0 x 10-16 15.7

H2 10-35 35

some typical pKa values

Acid-Base Chemistry

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So, for an equilibrium between two conjugate acid/base pairs, the equilibrium will always favour the side which has…the weakest acid.

What does this mean for ‘real life chemistry’?

OH

+ CH3CO2

O

+ CH3CO2H

KapKa

1.0 x 10-10

101.8 x 10-5

4.72

So, if you wanted this equilibrium to be product favoured, you would use a base…whose conjugate acid was weaker than phenol. 4

pKa Compound types

-10 mineral acids: H2SO4, HI, HBr, HCl, sulfonic acids RSO3H

0 H3O+, H3PO4

5 Carboxylic acids, HF, thiophenols ArSH, HN3

10 Weak inorganic acids (H2S, HCN, NH4+, amine salts RNH3

+, phenols ArOH,thiols (RSH), aromatic amides ArCONH2

15 H2O, alcohols ROH, amids RCONH2)

20 Ketones (the alpha proton H-CH2COR)

25 Esters (the alpha proton H-CH2CO2R), alkynes RCCH, nitriles (H-CH2CN)

30 Anilines ArNH2

40 Ammonia NH3, amines RNH2, benzylic protons ArCH3

45 Arenes ArH and alkenes RCH=CH2

50 Alkanes

Table 5.1 (page 144)

In water, there can be no acid stronger than…H3O+

…and no base stronger than…-OH 5

Down a group...

HO H

HS H

15.7

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Trends in Acidity

Across the period...

Acid Conjugate Base pKa

H3C H

H2N H

HO H

F H

15.7

36

48

3.1

6

H3C H

C

H

H

C H

48

45

25

O

H

H

O

H

-2

-4 to -10

Pi Bonds…

7

Resonance...

O

H3CH2C O

O

O H

H

H

17

5

10

pKa

8

Resonance eff ects greatly enhance the acidity of "activated" C—H bonds...

N C CH2

H 25

O O

CH3

CH4

36

48

919

H

H9

Inductive and Field effects

H2CX C

O

O H

pKa4.874.763.172.902.862.592.461.68

XCH3HIBrClFCNNO2

+ H2OH2CX C

O

O

+ H3O+

10

ClH2C C

O

OH

Cl2HC C

O

OH

Cl3C C

O

OH

2.85 1.48 0.70

These effects are…additive.

COH

O

COH

O

COH

O

COH

O

Cl

Cl

Cl

2.86 4.05 4.52

4.82

These effects…drop off rapidly with increasing number of bonds.

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pKa + pKb for a conjugate acid/base pair = 14

Bases

Although potentially confusing, when evaluating base strength we often look at the pKa of its conjugate acid. The higher the pKa of the conjugate acid, the stronger the base.

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N:

R3N:

H2N

N

H O

R O

Conjugate acid pKa of conjugate acid

N H

R3NH

H2O

ROH

H2NH

NH

5

11

15

16-18

38

40

Base

13

The same things that affect acidity, also affect basicity...

Across the periodic table...

H3N > H2O > HF

Down a column...

H2N: > H2P: HO > HS

H3N > H3P H2O > H2S

Hybridization...

NH N:

Conj. pKa: 12.20 5.3

O

H

O

-2 -4 to -10

14

NHH

NHH

NOOpKa of conjugate is 4.62

pKa of conjugate is 1.00

Resonance…

NH NH2

Et2NH Et2NH2

base CA pKa

11.0

12.2

Sterics…

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Inductive effects...

NH3 CH3NH2 (CH3)2NH (CH3)3N

pKa of conj. 9.25 10.65 10.73 9.78

Amphiprotic species

CH3OH + H2SO4

CH3OH + NH2

CH3OH2+ + HSO4

-

CH3O + NH3

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Acids, bases, electrophiles, and nucleophiles.

For the following molecule, identify which nitrogen atom is more basic. Why?

O

CCH2N

NH2

H H

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Acids, bases, electrophiles, and nucleophiles.

For the following molecule, identify the acidic and basic sites and rank them in terms of reactivity (by category; don’t compare acid to base). Rationalize your rankings.

O

CO

O

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Acids, bases, electrophiles, and nucleophiles.

For the following molecule, identify the acidic, basic, electrophilic, and nucleophilic sites and rank them in terms of reactivity (by category). Rationalize your rankings.

O

CCH3C

C

H H

CH

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Acids, bases, electrophiles, and nucleophiles.

When solving a mechanism, we can use the different kinds of sites (A, B, E, Nu) to help us.e.g. How can we rationalize the following reaction?

O

C CH

H

C

H

Cl

D D

H3C OO

C CC

H

D

DOH3C

HH

+ + Cl

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Acids, bases, electrophiles, and nucleophiles.

•It is incorrect to mix-and-match the terms acid and electrophile, or base and nucleophile. They are not interchangeable.

e.g. I – is a good nucleophile but not a good base

•It is, however, entirely possible for a molecule to be both an acid and an electrophile – or both a base and a nucleophile. It is even possible for the same molecule to be an acid, a base, a nucleophile and an electrophile all at the same time. In that case, how it reacts will depend on the other species in the reaction flask (since usually the most nucleophilic site reacts with the most electrophilic site – assuming it can reach).

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