copyright © 2010 pearson education, inc. brønsted–lowry acids and bases an acid is a proton...

Copyright © 2010 Pearson Education, Inc.

Brønsted–Lowry Acids and BasesAn acid is a proton donorA base is a proton acceptor

acid base

H2O H3OHBr Br+ +

NH3 H2O+ NH4 + OHacidbase

Note that water can act as an acid or a base


Brønsted–Lowry Acids and Bases

acid base

H2O H3OHBr Br+ +

The remaining species after the proton has been donated is the conjugate base.

conjugate acid

conjugate base

Every acid–base reaction involving proton transfer

has two conjugate acid–base pairs.

HBr Br+ +OH H

O

HHH

The resulting species after the proton has been accepted is the conjugate acid.


pH

The concentration of hydrogen ions is used as a measure of acidity

This concentration is expressed as pH

pH = – log[H3O+]

The higher the concentration, the more acidic the solution and the lower the pH


pH

Neutral water: [H3O+] = 1.0 × 10–7 M

pH = – log[H3O+] = 7

pH < 7.00 Acidic solution

pH = 7.00 Neutral solution

pH > 7.00 Basic solution

H2O + H2O OH H3O+


The Acidity Constant, Ka

The strength of an acid is represented by its ionization constant (acidity constant), Ka

Ka=product of concentrations of ionized species

concentration of intact acid

HA + H2O A H3O+

Ka =A H3O

HA



The Ka implies the concentrations of the acid and the ions

Ka > 1 Ionized products greater than intact acid.

Ka < 1 Ionized products less than intact acid.

Ka >> 1 Ionization goes to completion (strong acid).(e.g., > 103)

Ka << 1 Ionization does not occur to an appreciable amount. (e.g., < 10–3)


pKa = – log (Ka)


Since the Ka values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use


Compare pKa and Ka Values

pKa 14121086420

strong acids weak acids

Ka10-1410-1010-610-2

The smaller the value of the pKa

the stronger the acid.

-2

102


Acid Strength

HA + H2O H3O+ + A-

HA

A-

A-

Has a strong conj. base(higher energy)

WEAK ACID

STRONG ACID

ENERGY

ionizationeasier

The difference between a strong acid and a weak acid can be described by the stability of the conjugate base.

Has a weak conj. base(lower energy)


Acid StrengthA more stable conjugate base means a

stronger acid.

HA

stabilization

ENERGY

A-

A-


Acid StrengthFactors that influence stability of the

conjugate base include:• Resonance• Electronegativity• Atomic Size• Hybridization• Inductive Effects


Resonance Effects

More or better resonance structures of the conjugate base lead to a stronger acid.


Resonance Effects

18

10

5

45

30

25

20

9

28

25

15

pKa Valuesincreasing qualityof resonance

R OH

OH

R C

O

OH

R CH3

CH3

CH3O C

O

CH3

R C

O

CH3

R C

O

CH2 C R

O

R NH2

NH2

R C

O

NH2


Resonance EffectsThe Acetate Ion

acetate ion

acetic acid

CH3 C OH

O-H+

Base

CH3 C O

O

CH3 C O

O

Resonance Stabilized Equivalent structures (charges on oxygens)


Resonance Effects

O

-

More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen)

O O O O O

The Phenolate Ion


Electronegativity

Placing the negative charge on a more electronegative element (from the same period) in the conjugate base leads to a stronger acid.


ElectronegativitypKa Values

increasing electronegativity

C

O

CH3R 20

15

5

C

O

NH2R

C

O

OHR

CH4

NH3

H2O

HF

>45

34

16

3.5

RCH3

RNH2

ROH

45

35

18

Consider the conjugate bases



Consider the conjugate bases


C

O

CH3R 20

15

5

C

O

NH2R

C

O

OHR

C

H

H

H

NH

H

OH

F

CH4

NH3

H2O

HF

> 45

34

16

3.5

RCH3

RNH2

ROH

45

35

18


Atomic Size

Placing the negative charge on a larger atom (from the same group) in the conjugate base leads to a stronger acid.


Atomic SizepKa Values

increasing size

H2O

H2S

H2Se

H2Te

16

7

4

3

Consider the ionic radii

C

O

OHR

C

O

SHR

C

S

SHR

HF

HCl

HBr

HI

3.5

– 7

– 9

– 10


F–

Cl–

I–

Br–


Consider the ionic radii

increasingsize

HF

HCl

HBr

HI

3.5

–7

–9

–10

H2O

H2S

H2Se

H2Te

16

7

4

3

C

O

OHR

C

O

SHR

C

S

SHR

1.36 Å

1.81 Å

1.95 Å

2.16 Å


Hybridization

• More s character in the orbital bearing the negative charge in the conjugate base leads to a stronger acid.


Hybridization

sp3

sp2

sp

> 45

35

25

As electrons in hybrid orbitals become closer to the nucleus, they are lower in energy

-1.74

-7

:

:

:pKa

pKa

C

H

H

H

H

C CH

H

H

H

C CH H

C

C

C

O

H

HH

CR

RO H


Inductive EffectsElectron-withdrawing effects due to

differences in electronegativity pull electron density away from the negatively charged end of the conjugate base, lowering the energy and stabilizing the conjugate base, making the acid stronger.

R CO

OC

O

OO SR

O

O

O


Inductive EffectsElectron-donating effects due to

differences in electronegativity push electron density toward the negatively charged end of the conjugate base, increasing the energy and destabilizing the conjugate base, making the acid weaker.

R CO

OC

O

OO SR

O

O

O


Inductive EffectsElectron-withdrawing Groups

F, Cl, Br, O, N R, CH3, B, Si

electronegative elements pull electron density away from carbon

alkyl groups and elements less electronegative than carbon push electron density toward carbon

Remember, the electron-withdrawing and -donating groups work throughthe bond system, while resonance groups work through the system.

Electron-donating Groups

Cl C

CH3 C


Inductive EffectsChlorine helps to stabilize – CO2

– by withdrawing electrons

Cl CO

O

This effect diminishes with distance—it extends for about 3 bonds

C CO

CCl O


Inductive Effects

3.13

2.87

2.81

2.66

4.75

2.81

1.29

0.65

pKa Values


increasing substitution

I CH2COOH

Br CH2COOH

Cl CH2COOH

F CH2COOH

CH3 COOH

CH2 COOHCl

CH COOHCl

Cl

C COOHCl

Cl

Cl


Inductive Effects

2.814.75pKa:

Increasing substitution

C

O

OHCH3 C

O

OHCH2Cl C

O

OHCCl

Cl

Cl 0.65

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