4 4-1 Organic Chemistry William H. Brown & Christopher S. Foote.

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4-1

Organic Organic Chemistry Chemistry

William H. BrownWilliam H. Brown& Christopher S. Foote& Christopher S. Foote

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4-2

Acids Acids and and BasesBases

Chapter 4Chapter 4

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4-3

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

: ::

: ::

::

: :

::

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4-4

Conjugate Acids & basesConjugate Acids & bases 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

+ +

Acid Base Conjugate base

Conjugate acid

H3 O+

NH3 H2 O NH4+

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4-5

Conjugate Acids & BasesConjugate Acids & Bases Which is the favored site of protonation?

CH3-C-O-H

OH+

or CH3-C-O-H

O

H

+

A(protonation on thecarbonyl oxygen)

B(protonation on thehydroxyl oxygen)

CH3-C-O-H

OH+

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4-6

Conjugate Acids & BasesConjugate Acids & Bases Which is the favored site of protonation?

+ HCl CH3-C-N-H

OH+

H+

or

A(protonation on the

amide oxygen)

H

+

Acetamide(an amide)

CH3-C-N-H

O

H

CH3-C-N-H

O

H

H

B(protonation on

the amide nitrogen)

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4-7

Strong Acids and BasesStrong Acids and Bases Strong acid:Strong acid: completely ionized in aqueous

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

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

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4-8

Acids & Base StrengthsAcids & Base Strengths Acetic acid is a weak acid

• it is incompletely ionized in aqueous solution

Base (weaker base)

Acid (weaker acid)

Conjugate base (stronger base)

Conjugate acid (stronger acid)

CH3COH + H2O

CH3CO- + H3O

+

O

O

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4-9

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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4-10

Weak Acids and BasesWeak Acids and BasesAcid Formula pKa

Conjugate 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

CH3CH2 OH CH3CH2 O-

H2 O HO-

HCO3-

CO32-

NH4+ NH3

H2 CO3 HCO3-

CH3COOH CH3COO-

H2 SO4 HSO4-

HCl Cl -

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4-11

Weak Acids and BasesWeak Acids and Bases Equilibrium favors reaction of the stronger

acid and stronger base to give the weaker acid and weaker base

CH3COH NH3 NH4++ +

Acetic acidpKa 4.76

(stronger acid)

Ammonia(stronger base)

Acetate ion(weaker base)

Ammonium ionpKa 9.24

(weaker acid)

CH3CO-

O O

Keq = 3.0 x 104

pKeq = 4.76 - 9.24 = -4.48

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4-12

Structure and AcidityStructure and AcidityA. 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 A-, and the greater the acidity of the acid HA

pKa 51 38 16

Electronegativity of A in A-H

2.5 3.0 3.5

CH3CH2-H CH3NH-H CH3O-H

Anion CH3CH2- CH3NH- CH3O-

increasing anion stability

Acid

increasing acidity

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4-13

Structure and AcidityStructure and AcidityB. 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 negative charge, the greater its

stability, and the greater the acidity of HA

CH3O-CH3S-

CH3SH CH3OHMethanethiol

pKa 7.0Methanol

pKa 16

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4-14

Structure and AcidityStructure and AcidityC. Resonance Delocalization of Charge in A-

• for compounds with the same functional group, the more stable the anion A-, the stronger the acid HA

• compare an alcohol and a carboxylic acid• ionization of the O-H bond of an alcohol gives an

anion for which there is no resonance stabilization

:: + H3O

+

Ethanol Ethoxide ion

+ H2OpKa = 15.9

CH3CH2-O-H CH3CH2-O:-

::

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4-15

Structure and AcidityStructure and Acidity• ionization of a carboxylic acid gives a resonance-

stabilized anion

• a carboxylic acid is a stronger acid than an alcohol

Delocalization of the negative charge

+CH3 C

O

O

C

O

O

CH3

O H

C

O

CH3

H3O+

+ H2OpKa = 4.76

::

::

: :

:

:

:

:

:

:

::

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4-16

Structure and AcidityStructure and AcidityD. Electron-withdrawing inductive effect

• the polarization of electron density of a covalent bond due to the electronegativity of an adjacent covalent bond

CH3-C-OH

Acetic acidpKa 4.76

Cl-CH2-C-OH

Chloroacetic acidpKa 2.86

O

O

+C C

Cl O0.5-

O0.5-HH

-

Charge is distributedequally betweenthe two oxygens

The positive chargehelps neutralize the negative charge on the oxygens

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4-17

Structure and AcidityStructure and Acidity• stabilization of an anion by the inductive effect falls off

rapidly with increasing distance of the electronegative atom from the site of the negative charge

CH3CH2CH2COH

O

CH2CH2CH2COH CH3CHCH2COH

O

Cl

CH3CH2CHCOHO

Cl ClButanoic

acidpKa 4.82

4-Chlorobutanoicacid

pKa 4.52

3-Chlorobutanoicacid

pKa 3.98

2-Chlorobutanoicacid

pKa 2.83

O

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4-18

Structure and AcidityStructure and AcidityE. Hybridization

• for anions differing only in the hybridization of the charged atom, the greater the % of s character to the hybrid orbital of the charged atom, the more stable the anion, and the greater the acidity of HA

25

44

51

pKa

HC CH

C C

H

H C

HH

C

H

H

Carbon% s

sp 50

sp2 33

sp3 25

Acid Anion

HC C:-

C C

H

H

H

H C C: -

H

H

H H H

H H H

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4-19

Lewis Acids and BasesLewis Acids and Bases Lewis acid:Lewis acid: any molecule of ion that can form a

new covalent bond by accepting a pair of elections

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

Lewis base

Lewis acid

+ BA new covalent bondformed in this Lewisacid-base reaction

:+-

A B

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4-20

Lewis Acids and BasesLewis Acids and Bases

:

: :

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

CH3 CH2

CH3 CH2

F

F

O

CH3 CH2

CH3 CH2

B-F

F

F

+ B

A BF3-ether complex

F:: :

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4-21

Prob 4.8Prob 4.8 For each conjugate acid-base pair, identify the first

species as an acid or base and the second as its conjugate base or acid.(a) (b)

(c) (d)

(e) (f)

(g)

HCOOH, HCOO- NH4+, NH3

CH3CH2O-, CH3CH2OH

CH3CH3, CH3CH2-H2PO4

-, HPO42-

HCO3-, CO3

2-

CH3S-, CH3SH

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4-22

Prob 4.9Prob 4.9 Complete a net ionic equation for each proton-transfer

reaction. Label the original acid and its conjugate base.

(b)

(a)

CH3CH2O-

+NH3 HCl

+ HCl

(d)

(c) +HCO3- OH-

+CH3COO- NH4+

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4-23

Prob 4.10Prob 4.10 Complete a net ionic equation for each proton-transfer

reaction. Label the original acid and its conjugate base.

NH4+

OH-

CH3COO-

CH3NH3++

+(a)

(b)

+ OH-CH3NH3

+

+(c)

(d)

NH4+

CH3CH2O-

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4-24

Prob 4.11Prob 4.11 Write a structural formula for the conjugate acid

formed by treating each molecule or ion with HCl.

(a) (b)

(c) (d)

OCH3CH2OH HCH

(CH3)2NH HCO3-

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4-25

Prob 4.13Prob 4.13 Account for the greater stability of the anion derived

from acetone compared with the anion derived from ethane.

CH3CCH2-H

O

CH3CH2-H

AcetonepKa 22

EthanepKa 51

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4-26

Prob 4.16Prob 4.16 Arrange the compounds in each set in order of

increasing acid strength.

CH3CH2OH HOCO-

Ethanol Bicarbonate ion Benzoic acid

(a)

(b)

C6H5COH

HOCOHCarbonic acid

CH3COHAcetic acid

HClHydrogen chloride

O O

OO

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4-27

Prob 4.17Prob 4.17 Arrange the compounds in each set in order of

increasing base strength.

NH3 HOCO- CH3CH2O-(a)

(b)

OH-

HOCO- CH3CO-

(c)H2O NH3 CH3CO-

(c) CH3CO-NH2-

OH-

O

OO

O

O

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4-28

Prob 4.20Prob 4.20 Write an equation for the reaction of acetic acid, pKa

4.76, with each base. Which equilibria lie toward the left? Which lie toward the right?(a)NaHCO3 (b)

(c) (d)

NH3

H2O NaOH

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4-29

Prob 4.21Prob 4.21 Write an equation for the reaction of ethanol,

CH3CH2OH, pKa 15.9 with each base. Which equilibria lie toward the left? Which lie toward the right?

(a)HCO3- (b)

(c) (d)

OH-

NH2- NH3

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4-30

Prob 4.22Prob 4.22 Benzoic acid, C6H5COOH, pKa 4.19, is only slightly

soluble in water but its salt, C6H5COO-Na+, is quite soluble in water. In which solutions will benzoic acid dissolve more readily than in water?

(a)

(b)

(c)

Aqueous NaOH

Aqueous NaHCO3

Aqueous Na2CO3

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4-31

Prob 4.23Prob 4.23 4-Methylphenol, CH3C6H4OH, pKa 10.26, is only slightly

soluble in water, but its sodium salt, CH3C6H4O-Na +, is quite soluble in water. In which solutions will 4-methylphenol dissolve?

(a)

(b)

(c)

Aqueous NaOH

Aqueous NaHCO3

Aqueous Na2CO3

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4-32

Prob 4.24Prob 4.24 For an acid-base reaction, one way to determine the

predominant species at equilibrium is to say that the reaction arrows points to the acid with the higher value of pKa. Explain why this rule works.

pKa -1.74pKa 9.24

++NH4+ H2O NH3 H3O

+

+ +

pKa 9.24 pKa 15.7NH4

+ OH- NH3 H2O

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4-33

Prob 4.25Prob 4.25 Will acetylene react with sodium hydride according to

the following equation to form a salt and hydrogen?

HC CH + Na+H-

Acetylene Sodiumhydride

HC C-Na+ + H2

Sodiumacetylide

Hydrogen

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4-34

Prob 4.26Prob 4.26 Using the values of pKa given in Table 4.1, calculate

the equilibrium constant, Keq, for this acid-base reaction.

+ +H3 PO4 CH3CH2 OH H2 PO4- CH3CH2 OH2

+

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4-35

Prob 4.28Prob 4.28 Complete the equation for each Lewis acid-base

reaction.

(b)

(a) +

+

Cl

ClCH3-C-Cl

CH3-C+

Al-Cl

H-O-H

CH3

CH3

CH3

CH3

(c)+

+ Br -

+(d) CH3-O-H

CH3-CH-CH3

+CH3-CH-CH3

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4-36

Prob 4.29Prob 4.29 For each reaction, label the Lewis acid and the Lewis

base, and use curved arrows to show the flow of electrons in each reaction.

(a) +CH3-CH=CH2 H-Cl CH3-CH-CH2+

+ Cl -

(b) +CH3-C=CH2 Br-Br CH3-C-CH2-Br + Br -+

H

CH3 CH3

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4-37

Prob 4.30Prob 4.30 2,4-Pentanedione is a considerably stronger acid than

acetone. Write a structural formula for each conjugate base, and account for the greater stability of the conjugate base from 2,4-pentanedione.

CH3CCH2-H

O

CH3CCHCCH3

O

HAcetonepKa 22

2,4-PentanedionepKa 9

O

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4-38

Prob 4.32Prob 4.32 In which equation does the sec-butyl cation react as a

Lewis acid? In which equation does it react as a Brønsted-Lowry acid? Write Lewis structures for the reactants and products, and show by the use of curved arrows how each reaction occurs.

CH3-CH-CH2-CH3 H2O

CH3-CH-CH2-CH3 H2O

CH3-CH-CH2-CH3

OH H

CH3-CH=CH-CH3 H3O+

+

sec-Butyl cation

+

+

sec-Butyl cation

+

++

(1)

(2)

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4-39

Prob 4.34Prob 4.34 Write equations for the reaction of each compound

with H2SO4, a strong protic acid.

(a) (b)

(c) (d)

(e) (f)

CH3OCH3 CH3CH2SCH2CH3

CH3CCH3

CH3NCH3CH3CH2NHCH2CH3

CH3COCH3

O

CH3

O

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4-40

Prob 4.36Prob 4.36 Write a structural formula for the conjugate base

formed when each compound is treated with one mole of a base stronger than the compound’s conjugate base.

(a) (b)

(c) (d)

(e) (f)

HSCH2CH2NH2

CH3CHCOH

HOCCH2CH2SHHOCH2CH2C

H3NCH2CH2COH

(g) (h)HSCH2CH2OH

HOCH2CH2NH2

CHO

HO O O+

H3NCH2CH2CO-O+

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4-41

Prob 4.39Prob 4.39 For each pair of molecules or ions, select the stronger

base and write its Lewis structure.

(a) (b)

(c) (d)

(e) (f)

CH3NH- or CH3O-

NH3 or OH-

CH3CH2O- or H-

NH3 or H2O

(g) (h)HSO4- or OH-CH3COO- or HCO3

-

CH3S- or CH3O-

(i)

CH3COO- or OH-

OH- or Br-

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4-42

Prob 4.43Prob 4.43 When imidazole is dissolved in water, proton transfer

to it gives a cation. Is the cation better represented by A or B?

N

N

H

+ H2ON

N

HN

N

H

+ OH-

H

H

or

A BImidazole

+

+

:

:

:

:

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4-43

Acids Acids and and

BasesBasesEnd Chapter 4End Chapter 4