4-1 acidsandbases chapter 4. 4-2 4.1 arrhenius acids and bases in 1884, svante arrhenius proposed...

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Acids Acids and and BasesBases

Chapter 4Chapter 4

4-4-22

4.1 4.1 Arrhenius Acids and BasesArrhenius Acids and Bases

In 1884, Svante Arrhenius proposed these definitions • acid:acid: a substance that produces H3O+ ions

aqueous solution

• base:base: a substance that produces OH- ions in aqueous solution

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Arrhenius Acids and BasesArrhenius Acids and Bases

• 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

H+(aq) + H2O(l) H3O+(aq)Hydronium ion

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4.2 4.2 Brønsted-Lowry DefinitionsBrønsted-Lowry Definitions

Acid:Acid: a proton donor Base: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

: ::

: ::

::

: :

::

4-4-55

A.A. Conjugate Acids & Bases Conjugate Acids & Bases

• acid-base reaction:acid-base reaction: a proton-transfer reaction

• conjugate baseconjugate base: the species formed from an acid when it donates a proton to a base

• conjugate acid:conjugate acid: the species formed from a base when it accepts a proton from an acid

+

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

: ::

: ::

::

: :

::

conj.base

conj.acid

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Conjugate Acids & BasesConjugate Acids & Bases

• conjugate acid-base pair:conjugate acid-base pair: any pair of molecules or ions that can be interconverted by transfer of a proton

HCl(aq) H2O(l) Cl-(aq) H3O+(aq)+ +WaterHydrogen

chlorideHydronium

ionChloride

ion(base)(acid) (conjugate

acid of H2O)(conjugate

base of HCl)

conjugate acid-base pair


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• Brønsted-Lowry definitions do not require water as a reactant

• consider the following reaction between acetic acid and ammonia

NH4+CH3COOH CH3COO-

NH3+ +

Acetic acid Ammonia

(acid)


Acetate ion

Ammoniumion

(base) (conjugate baseacetic acid)

(conjugate acidof ammonia)


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• we can use curved arrows to show the flow of electrons in an acid-base reaction

CH3-C-OO

H N HH

H CH3-C-O -

OH-N-H

H

H+ +

Acetic acid(proton donor)

Acetate ion

:

:: ::

:: :: :

Ammonia(proton acceptor)

Ammoniumion

4-4-99

B.B. Conjugate Acids & Bases Conjugate Acids & Bases

Many organic molecules have two or more sites that can act as proton acceptors in this chapter, we limit our discussion to carboxylic

acids, esters, and amides 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?

CH3-C-O-H

O+ H2SO4 CH3-C-O-H

HO+

CH3-C-O-HH

O+

+ HSO4-or

A(protonation

on the carbonyl oxygen)

B(protonation

on thehydroxyl oxygen)

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Conjugate Acids & BasesConjugate Acids & Bases for protonation on the carbonyl oxygen, we can

write three contributing structures two place the positive charge on oxygen, one

places it on carbon

A-1 and A-3 make the greater contribution because all atoms have complete octets

the positive charge is delocalized over three atoms and is greater on the two equiv. oxygens

++

CH3-C-O-H

OH

CH3-C-O-H

OH

CH3-C

OH

++CH3-C-O-H

O

CH3-C=O-H

OH

CH3-C=O-H

OH+

A-1(C and O have

complete octets)

A-2(C has incomplete

octet)

A-3(C and O have

complete octets)

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Conjugate Acids & BasesConjugate Acids & Bases for protonation on the hydroxyl oxygen, we can

write two contributing structures

B-2 makes only a minor contribution because of charge separation and adjacent positive charges

therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen

CH3-C-O-H

O

H

+

B-1

CH3-C-O-H

O

H+

+

B-2(charge separation and

adjacent positive charges)

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Problem 4.3Problem 4.3 Does proton transfer to an amide group occur preferentially on the amide oxygen or the amide nitrogen?

CH3-C-N-H

H

O+ HCl

+

CH3-C-N-H

OH

H

H

CH3-C-N-H

H

O+

+ Cl -or

A(protonation

on theamide oxygen)

B(protonation

on theamide nitrogen)

Acetamide(an amide)

4-4-1313

C.C. Pi Electrons As Basic Sites Pi Electrons As Basic Sites Proton-transfer reactions occur with compounds

having pi electrons, for example the pi electrons of carbon-carbon double and triple bonds the pi electrons of 2-butene, for example, react

with HBr by proton transfer to form a new C-H bond

the result is formation of a carbocationcarbocation, a species in which one of its carbons has only six electrons in its valence shell and carries a charge of +1

CH3-CH=CH-CH3 + H-Br CH3-C-C-CH3 + Br -

H

H

+

2-Butene sec-Butyl cation(a 2° carbocation)

H

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Pi Electrons As Basic SitesPi Electrons As Basic Sites

Problem 4.4Problem 4.4 Draw Lewis structures for the two possible carbocations formed by proton transfer from HBr to 2-methyl-2-butene

CH3-C=CH-CH3 + H-Br

2-Methyl-2-butene

CH3

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4.3 4.3 Acids & Base StrengthsAcids & Base Strengths

The strength of an acid is expressed by an equilibrium constant the acid dissociation of acetic acid is given by the

following equation

H3O+

+CH3CO-

OH2O+CH3COH

O

Acetic acid Water Acetateion

Hydroniumion

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Weak Acids and BasesWeak 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, KKaa, called an acid dissociation constantacid dissociation constant

HA + H2O

[H3O+][A

-]

[HA][H2O]Keq

A- + H3O

+

=

Keq[H2O]Ka[H3O+][A-]

[HA]= =

4-4-1717HI I-

HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6H5COOH C6H5COO-

CH3COOH CH3COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6H5OH C6H5O-

HCO3-

CO32-

CH3NH2CH3NH3+

H2O HO-

CH3CH2OH CH3CH2O-

HC CH HC C-H2 H-NH3 NH2

-CH2=CH2 CH2=CH

-CH3CH3 CH3CH2

-Acid Formula pKa Conjugate BaseEthane

Ammonia

EthanolWater

Bicarbonate ionPhenol

Ammonium ion

Carbonic acidAcetic acid

3525

Benzoic acidPhosphoric acid

Sulfuric acidHydrogen chlorideHydrogen bromideHydrogen iodide

51

38

10.33

15.715.9

4.766.36

9.24

9.95

-5.2-7

-9-8

4.19

2.1-1.74Hydronium ion

Strongerconjugate

base

Weakerconjugate

base

Weaker acid

Stronger acid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

AcetyleneHydrogen

Ethylene 44

pKa values, Table 4-1pKa values, Table 4-1

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4.4 4.4 Acid-Base EquilibriaAcid-Base Equilibria

Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base

CH3COOH NH3 CH3COO-

NH4++ +

Ammonia(stronger base)

Acetate ion(weaker base)

Acetic acidpKa 4.76

(stronger acid)

Ammonium ionpKa 9.24

(weaker acid)

pKeq = 4.76 - 9.24 = -4.48

Keq = 3.0 x 104

4-4-1919

Acid-Base EquilibriaAcid-Base Equilibria Consider the reaction between acetic acid and

sodium bicarbonate we can write the equilibrium as an ionic equation we omit Na+ because it does not undergo any

chemical change in the reaction

equilibrium lies to the right carbonic acid forms, which then decomposes to

carbon dioxide and water

CH3COH

OHCO3

- CH3CO-

O

H2CO3

Bicarbonate ion Acetate ionAcetic acidpKa 4.76

(stronger acid)

Carbonic acidpKa 6.36

(weaker acid)

+ +

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4.5 4.5 Molecular Structure and AcidityMolecular 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 a more stable the anion increase 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

4-4-2121

Molecular Structure and AcidityMolecular Structure and Acidity

A. Electronegativity of the atom bearing the negative charge • within a period, the greater the electronegativity of the atom

bearing the negative charge, the more strongly its electrons are held, the more stable the anion is, and the stronger the acid

CH

HH

NH

H

O H

EthanepKa 51

MethylaminepKa 38

MethanolpKa 16

i

Conjugate base

CH3 C –H

H

CH3 N –

H

CH3 O –

Methylamide ion

Methoxide ion

Ethyl anionCH3

CH3

CH3

Acid

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B. Size of the atom bearing the negative charge• within a column of the Periodic Table, acidity is related

to the size of the the atom bearing the negative charge• atomic size increases from top to bottom of a column• the larger the atom bearing the charge, the greater its

stability

S HCH3 CH3 O – CH3 S –O HCH3

MethanethiolpKa 7.0

(stronger acid)

Methoxide ion

(stronger base)

Methanethiolateion

(weaker base)

MethanolpKa 16

(weaker acid)

+ +

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C. Resonance delocalization of charge in A-

• the more stable the anion, the farther the position of equilibrium is shifted to the right

• compare the acidity alcohols and carboxylic acids

• ionization of the O-H bond of an alcohol gives an anion for which there is no resonance stabilization

CH3CH2O-H H2O CH3CH2O - H3O++

An alcohol An alkoxide ion

+ pKa = 15.9

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• ionization of a carboxylic acid gives a resonance-stabilized anion

• the pKa of acetic acid is 4.76

• carboxylic acids are stronger acids than alcohols as a result of the resonance stabilization of the carboxylate anion

equivalent contributing structures; the carboxylate anion is stabilized bydelocalization of the negative charge.

+CH3 C

O

O

C

O

O

CH3O H

C

O

CH3 H3O+

+ H2O

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D. Electron-withdrawing inductive effect• the polarization of electron density of a covalent

bond due to the electronegativity of an adjacent covalent bond

C-CH2O-H

H

H

H

C-CH2O-H

F

F

F

EthanolpKa 15.9

2,2,2-TrifluoroethanolpKa 12.4

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Electron-withdrawing inductive effect, cont.

• stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative charge

CF3-CH2-OH CF3-CH2-CH2-OH CF3-CH2-CH2-CH2-OH

2,2,2-Trifluoro-ethanol

(pKa 12.4)

3,3,3-Trifluoro-1-propanol(pKa 14.6)

4,4,4-Trifluoro-1-butanol

(pKa 15.4)

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• we also see the operation of the inductive effect in the acidity of halogen substituted carboxylic acids

Butanoic acid

pKa 4.82

4-Chlorobutanoicacid

pKa 4.52


pKa 3.98


pKa 2.83

OH

O

OH

O

OH

O

OH

O

Cl

ClCl

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Table 4-2 Acidity: Table 4-2 Acidity: akanes, alkenesakanes, alkenes & akynes& akynes

E. Hybridization • for anions differing only in the hybridization of the charged atom,

the greater the % s character to the hybrid orbital of the charged atom, the more stable the anion

• consider the acidity of alkanes, alkenes, and alkynes (given for comparison are the acidities of water and ammonia)

CH3CH2-H CH3CH2–

CH2=CH-H CH2=CH–

H2N-H H2N–

HC C H HC C–

HO-H HO–

Weak Acid

Alkyne

Alkene

Alkane

Water

25

44

51

15.7

ConjugateBase pKa

I n c r e a s i n g a c i d i t y

Ammonia 38

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4.6 4.6 Lewis Acids and BasesLewis Acids and Bases

Lewis acid:Lewis acid: any molecule or ion that can form a new covalent bond by accepting a pair of electrons

Lewis base:Lewis base: any molecule or ion that can form a new covalent bond by donating a pair of electrons

Lewis base

Lewis acid

+ BA new covalent bondformed in this Lewisacid-base reaction

:+-

A B

4-4-3030

Lewis Acids and BasesLewis Acids and Bases

• examples :

: :

Br -CH3-C C-CH3

H H

H

CH3-C C-CH3

H H

H Br

Bromideion

sec-Butyl cation(a carbocation)

2-Bromobutane

++

: :

::

+ -

Diethyl ether(a Lewis base)

Boron trifluoride (a Lewis acid)

O

CH3CH2

CH3CH2

F

F

O

CH3CH2

CH3CH2

B-F

F

F

+ B

A BF3-ether complex

F:: :

4-4-3131

Acids and Acids and BasesBases

End Chapter 4End Chapter 4

4-1 acidsandbases chapter 4. 4-2 4.1 arrhenius acids and bases in 1884, svante arrhenius proposed...

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