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2-1Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Introduction to Introduction to Organic Organic

ChemistryChemistry2 ed2 ed

William H. BrownWilliam H. Brown

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2-2Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Acids Acids and and

BasesBases

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2-3Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Brønsted-Lowry DefinitionsBrønsted-Lowry Definitions• Acid: a proton donor• Base: a proton acceptor

+

Proton donor

Protonacceptor

-O H

H

OH

H

O HH + +O H H

+

Protonacceptor

Protondonor

+O H

H

OH

H

N H

H

H

H

H

H + +N H H

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2-4Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Conjugate Acids & BasesConjugate Acids & Bases• When an acid donates a proton, it is converted to

its conjugate base• When a base accepts a proton, it is converted to

its conjugate acid

+ +

Acid Base Conjugate base

Conjugate acid

H3O+

NH3 H2O NH4+

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2-5Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Strong Acids and BasesStrong Acids and Bases• Strong acid - completely ionized in aqueous

solution. Examples are:• HCl, HBr, HI, HNO3, HClO4, and H2SO4

• Strong base - completely ionized in aqueous solution. Examples are:• LiOH, NaOH, KOH, Ca(OH)2, and Ba(OH)2

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2-6Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Weak Acids and BasesWeak Acids and Bases• Acetic acid is a weak acid

• it is incompletely ionized in aqueous solution

Base(weaker base)

Acid(weaker acid)

Conjugate baseof CH3CO2H

(stronger base)

Conjugate acidof H2O

(stronger acid)

CH3COH + H2 O

+ H3 O+

CH3CO-

O

O

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2-7Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Weak Acids and BasesWeak Acids and Bases• The equation for the ionization of a weak acid,

HA, in water and the acid ionization constant, Ka, for this equilibrium are

=

HA H3O++A

-

[H3O+][A

-]

[HA]Ka

+ H2O

= Keq[H2O]

pKa = - log Ka

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2-8Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Weak Acids and BasesWeak Acids and Bases

Acid Formula pKaConjugate Base

ethanol

water

bicarbonate ion

ammonium ion

carbonic acid

acetic acid

sulfuric acidhydrogen chloride

10.33

15.7

15.9

4.76

6.36

9.24

-5.2

-7

CH3CH2OH CH3CH2O-

H2O HO-

HCO3-

CO32-

NH4+ NH3

H2CO3 HCO3-

CH3CO2H CH3CO2-

H2SO4 HSO4-

HCl Cl -

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2-9Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Structure & AcidityStructure & Acidity• Electronegativity

• the relative acidity of HA within a period is determined by the stability of A-

• the greater the electronegativity, the greater the stability of A-

pKa 51 38 15.7 3.5

Electronegativityof A inA-H

2.5 3.0 3.5 4.0Increasing acid strength

H3C-H H2N-H HO-H F-H

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2-10Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Structure & AcidityStructure & Acidity• Resonance - delocalization of charge in A-

• carboxylic acids and alcohols both contain an -OH group

• a carboxylic acid is a stronger acid than an alcohol

+CH3 COH CH3 CO-

H3O+ pKa = 4.76+ H2O

+CH3CH2 OH CH3CH2 O- H3O+

An alcohol An alkoxideion

+H2O pKa = 15.9

O O

A carboxylicacid

A carboxylateion

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2-11Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Structure & AcidityStructure & Acidity• Resonance - delocalization of charge in A-

• delocalization of charge stabilizes the carboxylate ion relative to the alkoxide ion

••

••

•• ••

••••

••

•• •• ••

••••••

••

These contributing structures are equivalent; the carboxylate anion is stabilized

by delocalization of the negative charge

+CH3 C

O

O

C

O

O

CH3

O H

C

O

CH3

H3O+

+ H2O

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2-12Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Structure & AcidityStructure & Acidity• Inductive effect

• the electron-withdrawing inductive effect of the adjacent carbonyl group weakens the O-H bond of the carboxylic acid

• Carboxylic acids are more acidic than alcohols• delocalization of charge in the carboxylate ion• electron-withdrawing inductive effect of adjacent C=O

CH3 CO

O

H

δ-

δ+

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2-13Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Position of equilibriumPosition of equilibrium• Favors reaction of the stronger acid and stronger

base to give the weaker acid and weaker base

++Stronger acid

Stronger base

Weaker base

Weaker acid

HA B-A- HB

(weaker acid)(stronger acid)pKa 9.24pKa 4.76

+ +

Acetic acid Ammonium ion

CH3CO2H NH3 CH3CO2-

NH4+

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2-14Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Position of equilibriumPosition of equilibrium• Stronger acid and stronger base react to give

weaker acid and weaker base

Carbonic acidpKa 6.36

PhenolpKa 9.95

++C6 H5OH HCO3- C6 H5O

-H2 CO3

Bicarbonateion

Phenoxideion

+ +

Acetic acidBicarbonateion

Acetateion

Carbonic acidpKa 4.76 pKa 6.36

CH3 CO2H HCO3- CH3 CO2

- H2 CO3

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2-15Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Position of equilibriumPosition of equilibrium• Stronger acid and stronger base react to give

weaker acid and weaker base

Carbonic acidpKa 6.36

PhenolpKa 9.95

++C6 H5OH HCO3- C6 H5O

-H2 CO3

Bicarbonateion

Phenoxideion

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2-16Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Lewis Acids and BasesLewis Acids and Bases• Lewis acidLewis acid: accepts a pair of electrons to form a

new covalent bond• Lewis baseLewis base: donates a pair of electrons to form a

new covalent bond

Lewis base

Lewis acid

+ B AA Bnew covalent bondformed in this Lewisacid-base reaction

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2-17Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Lewis Acids and BasesLewis Acids and Bases

Lewis base(donates an

electron pair)

Lewis acid(accepts an

electron pair)

-+ O-H H-O

H

+ H-O-H

H

H O H

CH3-C+

H

CH3

+ Cl - CH3-C-Cl

H

CH3A carbocation(a Lewis acid)

Chloride ion(a Lewis base)

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2-18Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Acids Acids and and

BasesBasesEnd Chapter 2End Chapter 2