CH4 Acids and base.ppt

7/17/2019 CH4 Acids and base.ppt

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OrganicOrganic

ChemistryChemistry

William H. BrownWilliam H. Brown

Christopher S. FooteChristopher S. Foote

Brent L. IversonBrent L. Iverson


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AcidsAcids

andand

BasesBasesChapter 4Chapter 4


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Arrhenius Acids and BasesArrhenius Acids and Bases

In 1884, Svante Arrhenius proposed these

definitionsacid:acid:a substance that produces H3O+ions aqueous

solution

base:base:a substance that produces OH-ions in aqueoussolution

this definition of an acid is a slight modification of theoriginal Arrhenius definition, which was that an acid

produces H+in aqueous solution

today we know that H+reacts immediately with a watermolecule to give a hydronium ion

H+

(aq) +H2O(l) H3O+

(aq)

Hydronium ion


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Brnsted-Lowry DefinitionsBrnsted-Lowry Definitions

Acid:Acid:a proton donor

Base:Base:a proton acceptor+

Protondonor

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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Conjugate Acids & BasesConjugate Acids & Bases

conjugate baseconjugate base:the species formed from an acid when

it donates a proton to a baseconjugate acid:conjugate acid:the species formed from a base whenit accepts a proton from an acid

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

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

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

+(aq)+ +

WaterHydrogenchloride

Hydroniumion

Chlorideion

(base)(acid) (conjugate

acid of H2O)

(conjugate

base of HCl)

conjugate acid-base pair



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Brnsted-Lowry definitions do not require water as a

reactantconsider the following reaction between acetic acidand ammonia

NH4+

CH3COOH CH3COO-

NH3+ +

Acetic acid Ammonia

(acid)


Acetateion

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-O

O

H N H

H

H

CH3-C-O-

O

H-N-H

H

H

+ +

Acetic acid(proton donor)

Acetate ion

:

:::

:

:::

::

Ammonia(proton acceptor)

Ammoniumion


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Many organic molecules have two or more sites

that can act as proton acceptorsin this chapter, we limit our discussion to carboxylicacids, 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 isprotonated?

CH3-C-O-H

O

+ H2SO4 CH3-C-O-H

HO

+

CH3-C-O-HH

O+

+ HSO4-

or

A(protonationon the

carbonyl oxygen)

B(protonationon the

hydroxyl oxygen)


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for protonation on the carbonyl oxygen, we can write

three contributing structurestwo place the positive charge on oxygen, one places iton carbon

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

the positive charge is delocalized over three atomswith the greater share on the two equivalent oxygens

++

CH3-C-O-H

OH

CH3-C-O-H

OH

CH3-C

O H

++CH3-C-O-H

O

CH3-C=O-H

O H

CH3-C=O-H

O H

+

A-1(C and O havecomplete octets)

A-2(C has incomplete

octet)

A-3(C and O havecomplete octets)


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for protonation on the hydroxyl oxygen, we can write

two contributing structures

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

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

CH3-C-O-H

O

H

+

B-1

CH3-C-O-H

O

H+

+

B-2(charge separation andadjacent positive charges)


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Problem 4.3Problem 4.3Does proton transfer to an amide

group occur preferentially on the amide oxygenor 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(protonationon the

amide oxygen)

B(protonationon the

amide nitrogen)

Acetamide(an amide)


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Pi Electrons As Basic SitesPi Electrons As Basic Sites

Proton-transfer reactions occur with compounds

having pi electrons, as for example the pielectrons of carbon-carbon double and triplebonds

the pi electrons of 2-butene, for example, react withHBr by proton transfer to form a new C-H bond

the result is formation of acarbocationcarbocation, a species inwhich 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 4.4Draw Lewis structures for the two

possible carbocations formed by proton transferfrom HBr to 2-methyl-2-butene

CH3-C=CH-CH3 + H-Br

2-Methyl-2-butene

CH3


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Acids & Base StrengthsAcids & Base Strengths

The strength of an acid is expressed by an

equilibrium constantthe acid dissociation of acetic acid is given by thefollowing equation

H3O+

+CH3CO-

O

H2O+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 anacid dissociation constantacid dissociation constant

HA + H2O

[H3O+

][A-]

[HA][H2O]

Keq

A-

+H3O+

=

Keq[H2O]Ka[H3O

+][A

-]

[HA]= =


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HBr Br-

HCl Cl-

H2SO4 HSO4-

H2OH3O+

H3PO4 H2PO4-

C6HCOOH C6HCOO-

CH3

COOH CH3

COO-

H2CO3 HCO3-

H2S HS-

NH3NH4+

C6HOH C6HO-

HCO3-

CO32-

CH3

NH2

CH3

NH3

+H2O HO

-CH3CH2OH CH3CH2O

-HC CH HC C

-H2 H

-

NH3 NH2-CH

2=CH2 CH2=CH-

CH3CH3 CH3CH2-

Acid Formula pKa Conjugate Base

Ethane

Ammonia

Ethanol

Water

Bicarbonate ion

Phenol

Ammonium ion

Carbonic acid

Acetic acid

35

25

Benzoic acid

Phosphoric acid

Sulfuric acid

Hydrogen chloride

Hydrogen bromide

Hydrogen iodide

51

38

10.33

15.7

15.9

4.76

6.36

9.24

9.95

-5.2

-7

-9

-8

4.19

2.1

-1.74Hydronium ion

Stronger

conjugatebase

Weakerconjugate

base

Weaker

acid

Strongeracid

Methylammonium ion 10.64

Hydrogen sulfide 7.04

Acetylene

Hydrogen

Ethylene 44


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Acid-Base EquilibriaAcid-Base Equilibria

Equilibrium favors reaction of the stronger acid

and stronger base to give the weaker acid andweaker base

CH3COOH NH3 CH3COO-

NH4+

+ +

Ammonia(stronger base) Acetate ion(weaker base)Acetic acidpKa4.76(stronger acid)

Ammonium ionpKa9.24(weaker acid)

pKeq= 4.76 - 9.24 = -4.48

Keq= 3.0 x 10

4


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Acid-Base EquilibriaAcid-Base Equilibria

Consider the reaction between acetic acid and

sodium bicarbonatewe can write the equilibrium as a net ionic equation

we omit Na+because it does not undergo anychemical change in the reaction

equilibrium lies to the right

carbonic acid forms, which then decomposes tocarbon dioxide and water

CH3COH

O

HCO3-

CH3CO-

O

H2CO3

Bicarbonate ion Acetate ionAcetic acidpKa4.76

(stronger acid)

Carbonic acidpKa6.36

(weaker acid)

+ +


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Molecular Structure and AcidityMolecular Structure and Acidity

The overriding principle in determining the

relative acidities of uncharged organic acids isthe stability of the anion, A-, resulting from theloss of a proton

the more stable the anion, the greater the acidity of HA

Ways to stabilize anions include having thenegative charge

on a more electronegative atom

on a larger atomdelocalized through resonance

delocalized by the inductive effect

in an orbital with morescharacter


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A. Electronegativity of the atom bearing the

negative chargewithin a period, the greater the electronegativity of the atom

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

C

H

H

H

N

H

H

O H

EthanepKa51

Methylamine

pKa38

MethanolpKa16

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 sizeof 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

MethanethiolpKa7.0

(stronger acid)

Methoxideion

(stronger base)

Methanethiolateion

(weaker base)

MethanolpKa16

(weaker acid)

+ +


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C. Resonance delocalization of charge in A-

the more stable the anion, the farther the position ofequilibrium 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 anionthe pKaof acetic acid is 4.76

carboxylic acids are stronger acids than alcohols as aresult of the resonance stabilization of the carboxylate

anion

equivalent contributing structures;the carboxylate anion is stabilized by

delocalization 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 bonddue to the electronegativity of an adjacent covalentbond

stabilization by the inductive effect falls off rapidly

with increasing distance of the electronegative atomfrom the site of negative charge

C-CH2O-H

H

H

H

C-CH2O-H

"

"

"

EthanolpKa15.9

2,2,2-TrifluoroethanolpKa12.4

C"3-CH2-OH C"3-CH2-CH2-OH C"3-CH2-CH2-CH2-OH

2,2,2-Trifluoro-ethanol

(pK

a12.4)

3,3,3-Trifluoro-1-propanol

(pK

a14.6)

4,4,4-Trifluoro-1-butanol

(pK

a15.4)


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we also see the operation of the inductive effect in the

acidity of halogen substituted carboxylic acids

ButanoicacidpKa4.82

4-ChlorobutanoicacidpKa4.52



OH

O

OH

O

OH

O

OH

O

Cl

Cl

Cl


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E. Hybridization

for anions differing only in the hybridization of the charged atom,the greater the %scharacter to the hybrid orbital of the chargedatom, 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

Conjugate

Base pKa

Inc

Ammonia 38


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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 ofelectrons

Lewis base:Lewis base:any molecule of ion that can form a

new covalent bond by donating a pair ofelectrons

#e$is%ase

#e$isacid

+ BA ne$ c&'alen %&nd&r*ed in is #e$is

acid-%ase reaci&n

,+-

A B


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Lewis Acids and BasesLewis Acids and Bases

examples:

: :

Br -CH3-C C-CH3

H H

H

CH3-C C-CH3

H H

H Br

Br&*ide

i&n

sec-B.l cai&n

(a car%&cai&n)

2-Br&*&%ane

++

: :

:

:

+ -

/ie.l eer(a #e$is %ase)

B&r&n ri0&ride (a #e$is acid)

O

CH3CH2

CH3CH2

"

"

O

CH3CH2

CH3CH2

B-"

"

"

+ B

A B"3-eer c&*1le

",, ,


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Acids andAcids and

BasesBases

End Chapter 4End Chapter 4

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