Dr. Wolf's CHM 201 & 202 1- 1

1.141.14

What Happened to pWhat Happened to pKKbb??

Dr. Wolf's CHM 201 & 202 1- 2

About pAbout pKKaa and p and pKKbb

A separate “basicity constant” A separate “basicity constant” KKbb is not is not

necessary. necessary.

Because of the conjugate relationships in the Because of the conjugate relationships in the Brønsted-Lowry approach, we can examine Brønsted-Lowry approach, we can examine acid-base reactions by relying exclusively on acid-base reactions by relying exclusively on ppKKaa values. values.

Dr. Wolf's CHM 201 & 202 1- 3

ExampleExample

Which is the stronger base, ammonia (left) or Which is the stronger base, ammonia (left) or pyridine (right)?pyridine (right)?

Recall that the stronger the acid, the weaker the Recall that the stronger the acid, the weaker the conjugate base.conjugate base.

Therefore, the stronger base is the conjugate of Therefore, the stronger base is the conjugate of the weaker acid.the weaker acid.

Look up the pKLook up the pKaa values of the conjugate acids of values of the conjugate acids of

ammonia and pyridine in Table 1.7.ammonia and pyridine in Table 1.7.

NN••••

HH

HH

HH

••••NN

Dr. Wolf's CHM 201 & 202 1- 4

ExampleExample

NN

HHHH

HH

HH++

ppKKaa = 9.3 = 9.3 NN

HH

++ ppKKaa = 5.2 = 5.2

weaker acidweaker acid

stronger acidstronger acid

Therefore, ammonia is a Therefore, ammonia is a stronger base than pyridinestronger base than pyridine

Dr. Wolf's CHM 201 & 202 1- 5

1.151.15

How Structure Affects Acid StrengthHow Structure Affects Acid Strength

Dr. Wolf's CHM 201 & 202 1- 6

The Main Ways Structure Affects Acid StrengthThe Main Ways Structure Affects Acid Strength

The The strength of the bondstrength of the bond to the atom from to the atom from which the proton is lost. which the proton is lost.

The The electronegativityelectronegativity of the atom from which of the atom from which the proton is lost.the proton is lost.

Changes in Changes in electron delocalizationelectron delocalization on on ionization.ionization.

Dr. Wolf's CHM 201 & 202 1- 7

Bond StrengthBond Strength

Bond strength is controlling factor when Bond strength is controlling factor when comparing acidity of comparing acidity of hydrogen halideshydrogen halides..

HFHF HClHCl HBrHBr HIHI

ppKKaa3.13.1 -3.9-3.9 -5.8-5.8 -10.4-10.4

weakest acidweakest acid strongest acidstrongest acidstrongest H—X bondstrongest H—X bond weakest H—X bondweakest H—X bond

Dr. Wolf's CHM 201 & 202 1- 8

Bond StrengthBond Strength

RecallRecall that bond strength decreases in a group that bond strength decreases in a group in going down the periodic table.in going down the periodic table.

Generalization:Generalization: Bond strength is most Bond strength is most important factor when considering acidity of important factor when considering acidity of protons bonded to atoms in protons bonded to atoms in same groupsame group of of periodic table (as in HF, HCl, HBr, and HI). periodic table (as in HF, HCl, HBr, and HI).

Another example:Another example: H H22S (pS (pKKaa = 7.0) is a = 7.0) is a stronger acid than Hstronger acid than H22O (pO (pKKaa = 15.7). = 15.7).

Dr. Wolf's CHM 201 & 202 1- 9

The Main Ways Structure Affects Acid StrengthThe Main Ways Structure Affects Acid Strength

The strength of the bond to the atom from The strength of the bond to the atom from which the proton is lost.which the proton is lost.

The The electronegativityelectronegativity of the atom from which of the atom from which the proton is lost.the proton is lost.

Changes in Changes in electron delocalizationelectron delocalization on on ionization.ionization.

Dr. Wolf's CHM 201 & 202 1- 10

ElectronegativityElectronegativity

Electronegativity is controlling factor when Electronegativity is controlling factor when comparing acidity of protons bonded to atoms comparing acidity of protons bonded to atoms in the in the same rowsame row of the periodic table. of the periodic table.

Dr. Wolf's CHM 201 & 202 1- 11

ElectronegativityElectronegativity

ppKKaa6060 3636 15.715.7 3.13.1

weakest acidweakest acid strongest acidstrongest acid

least electronegativeleast electronegative most electronegativemost electronegative

CHCH44 NHNH33 HH22OO HFHF

Dr. Wolf's CHM 201 & 202 1- 12

ElectronegativityElectronegativity

The equilibrium becomes more favorable as The equilibrium becomes more favorable as AA becomes better able to bear a negative charge.becomes better able to bear a negative charge.

Another way of looking at it is that H becomes Another way of looking at it is that H becomes more positive as the atom to which it is more positive as the atom to which it is attached becomes more electronegative. attached becomes more electronegative.

HH AAOO

RR

HH

.... ....

RR

HH

.... OO HH AA––....

++

++++

Dr. Wolf's CHM 201 & 202 1- 13

Bond strength versus ElectronegativityBond strength versus Electronegativity

Bond strengthBond strength is more important when is more important when comparing acids in which the proton that is lost comparing acids in which the proton that is lost is bonded to atoms in the is bonded to atoms in the same group of the same group of the periodic table.periodic table.

ElectronegativityElectronegativity is more important when is more important when comparing acids in which the proton that is lost comparing acids in which the proton that is lost is bonded to atoms in the is bonded to atoms in the same row of the same row of the periodic table.periodic table.

Dr. Wolf's CHM 201 & 202 1- 14

Acidity of AlcoholsAcidity of Alcohols

ppKKaa

15.715.7

15.215.2

1616

1717

1818

Alcohols (RO—H) Alcohols (RO—H) resemble water resemble water (HO—H) in their (HO—H) in their acidity.acidity.

HO—HO—HH

CHCH33O—O—HH

CHCH33CHCH22O—O—HH

(CH(CH33))22CHO—CHO—HH

(CH(CH33))33CO—CO—HH

In many acids In many acids the the acidic protonacidic proton is bonded to is bonded to oxygen.oxygen.

Dr. Wolf's CHM 201 & 202 1- 15

Acidity of AlcoholsAcidity of Alcohols

Electronegative substituents can increase the Electronegative substituents can increase the acidity of alcohols by drawing electrons away acidity of alcohols by drawing electrons away from the —OH group.from the —OH group.

CHCH33CHCH22OHOH CFCF33CHCH22OHOH

1616 11.311.3weakerweaker strongerstronger

ppKKaa

Dr. Wolf's CHM 201 & 202 1- 16

Inductive EffectInductive Effect

The greater acidity of CFThe greater acidity of CF33CHCH22OH compared to OH compared to CHCH33CHCH22OH is an example of an OH is an example of an inductive effectinductive effect..

Inductive effects arise by polarization of the Inductive effects arise by polarization of the electron distribution in the bonds between electron distribution in the bonds between atoms.atoms.

CC CC OO HH

HH

HH

FF

FF

FF

++

Dr. Wolf's CHM 201 & 202 1- 17

Electrostatic Potential MapsElectrostatic Potential Maps

CHCH33CHCH22OHOH CFCF33CHCH22OHOH

The greater positive character of the proton of The greater positive character of the proton of the OH group of CFthe OH group of CF33CHCH22OH compared to OH compared to CHCH33CHCH22OH is apparent in the more blue color OH is apparent in the more blue color

in its electrostatic potential map. in its electrostatic potential map.

Dr. Wolf's CHM 201 & 202 1- 18

Another example of the inductive effectAnother example of the inductive effect

OO

CHCH33CC OO HH

OO

CCFF33CC OO HH

4.74.7 0.500.50weakerweaker strongerstronger

ppKKaa

Dr. Wolf's CHM 201 & 202 1- 19

The Main Ways Structure Affects Acid StrengthThe Main Ways Structure Affects Acid Strength

The strength of the bond to the atom from The strength of the bond to the atom from which the proton is lost. which the proton is lost.

The electronegativity of the atom from which The electronegativity of the atom from which the proton is lost.the proton is lost.

Changes in Changes in electron delocalizationelectron delocalization on on ionization.ionization.

Dr. Wolf's CHM 201 & 202 1- 20

Electron DelocalizationElectron Delocalization

Ionization becomes more favorable if electron Ionization becomes more favorable if electron delocalization increases in going from right to delocalization increases in going from right to left in the equation.left in the equation.

Resonance is a convenient way to show Resonance is a convenient way to show electron delocalization.electron delocalization.

HH AAOO

RR

HH

.... ....

RR

HH

.... OO HH AA––....

++

++++

Dr. Wolf's CHM 201 & 202 1- 21

Nitric AcidNitric Acid OO

++

––••••

OO NNHH••••

••••

•••• ••••

••••

••••OO

OO ••••

HH

HH

••••

HHOO

HH

HH

••••++

OO

++

––••••

OO NN••••

••••

•••• ••••

••••

••••OO

••••––

++

ppKKaa = -1.4 = -1.4

++

Dr. Wolf's CHM 201 & 202 1- 22

Nitric AcidNitric Acid

OO

++

––••••

OO NN••••

••••

•••• ••••

••••

••••OO

••••––Nitrate ion is stabilized by Nitrate ion is stabilized by

electron delocalization.electron delocalization.

Dr. Wolf's CHM 201 & 202 1- 23

Nitric AcidNitric Acid

––••••OO

++

••••

OO NN••••

••••

––

••••OO

••••

••••

••••

OO

++

––••••

OO NN••••

••••

•••• ••••

••••

••••OO

••••––

••••OO

++

––••••

OO NN••••

••••

••••

••••OO

••••

––•••• Negative charge is Negative charge is shared equally by all shared equally by all three oxygens.three oxygens.

Dr. Wolf's CHM 201 & 202 1- 24

Acetic AcidAcetic Acid OO

OO CCHH••••

••••

•••• ••••

OO ••••

HH

HH

••••

ppKKaa = 4.7 = 4.7

++

CHCH33

HHOO

HH

HH

••••++

++

OO

OO CC••••

••••

•••• ••••

••••––

CHCH33

Dr. Wolf's CHM 201 & 202 1- 25

Acetic AcidAcetic Acid

Acetate ion is stabilized by Acetate ion is stabilized by electron delocalization.electron delocalization.

OO

OO CC••••

••••

•••• ••••

••••––

CHCH33

Dr. Wolf's CHM 201 & 202 1- 26

Acetic AcidAcetic Acid

OO

OO CC••••

••••

•••• ••••

••••––

Negative charge is Negative charge is shared equally by shared equally by both oxygens.both oxygens.

••••OO

OO CC••••

••••

••••

––••••

CHCH33 CHCH33

Dr. Wolf's CHM 201 & 202 1- 27

1.161.16

Acid-Base EquilibriaAcid-Base Equilibria

Dr. Wolf's CHM 201 & 202 1- 28

GeneralizationGeneralization

The equilibrium in an acid-base reaction is The equilibrium in an acid-base reaction is favorable if the stronger acid is on the left and favorable if the stronger acid is on the left and the weaker acid is on the right.the weaker acid is on the right.

Stronger acid + Stronger baseStronger acid + Stronger base Weaker acid + Weaker baseWeaker acid + Weaker base

Dr. Wolf's CHM 201 & 202 1- 29

Example of a strong acidExample of a strong acid

HH

HH

.... OO HH BrBr––

.... ............

++HH BrBrOO

HH

HH

.... .... ....

........

ppKKaa = -5.8 = -5.8

stronger acidstronger acidppKKaa = -1.7 = -1.7

weaker acidweaker acid

++ ++

The equilibrium lies to the side of The equilibrium lies to the side of the weaker acid. (To the the weaker acid. (To the rightright))

Dr. Wolf's CHM 201 & 202 1- 30

Example of a weak acidExample of a weak acid ppKKaa = 4.7 = 4.7

weaker acidweaker acidppKKaa = -1.7 = -1.7

stronger acidstronger acid

HH—OCCH—OCCH33

OO•••• ••••

••••

••••++OO

HH

HH

•••• •••• O—O—HH

HH

HH

••••++

++ OCCHOCCH33

OO•••• ••••

••••

••••••••

––

The equilibrium lies to the side of The equilibrium lies to the side of the weaker acid. (To the left)the weaker acid. (To the left)

Dr. Wolf's CHM 201 & 202 1- 31

Important PointsImportant Points

A strong acid is one that is stronger than HA strong acid is one that is stronger than H33OO++..A weak acid is one that is weaker than HA weak acid is one that is weaker than H33OO++..

A strong base is one that is stronger than HOA strong base is one that is stronger than HO––..A weak base is one that is weaker than HOA weak base is one that is weaker than HO––..

The strongest acid present in significant The strongest acid present in significant quantities when a strong acid is dissolved in quantities when a strong acid is dissolved in water is Hwater is H33OO++..

The strongest acid present in significant The strongest acid present in significant quantities when a weak acid is dissolved in quantities when a weak acid is dissolved in water is the weak acid itself.water is the weak acid itself.

Dr. Wolf's CHM 201 & 202 1- 32

Predicting the Direction of Acid-Base ReactionsPredicting the Direction of Acid-Base Reactions PhenolPhenol

ppKKaa = 10 = 10

stronger acidstronger acid

WaterWaterppKKaa = 15.7 = 15.7

weaker acidweaker acid

HH—OC—OC66HH55

••••

••••++

––

The equilibrium lies to the side of the weaker The equilibrium lies to the side of the weaker acid. (To the acid. (To the rightright) Phenol is converted to ) Phenol is converted to phenoxide ion by reaction with NaOH.phenoxide ion by reaction with NaOH.

••••H—OH—O••••

••••––

OCOC66HH55

••••

••••++ ••••H—O—H—O—HH

••••

••••

Dr. Wolf's CHM 201 & 202 1- 33

Predicting the Direction of Acid-Base ReactionsPredicting the Direction of Acid-Base Reactions

PhenolPhenolppKKaa = 10 = 10

weaker acidweaker acid

Carbonic acidCarbonic acidppKKaa = 6.4 = 6.4

stronger acidstronger acid

The equilibrium lies to the side of the weaker The equilibrium lies to the side of the weaker acid. (To the left) Phenol is not converted to acid. (To the left) Phenol is not converted to phenoxide ion by reaction with NaHCOphenoxide ion by reaction with NaHCO33..

HH—OC—OC66HH55

••••

••••++

––OCOC66HH55

••••

••••++ ••••••••

••••

••••––

HOCOHOCO

OO

••••

••••HOCO—HOCO—HH

OO

Dr. Wolf's CHM 201 & 202 1- 34

1.171.17

Lewis Acids and Lewis BasesLewis Acids and Lewis Bases

Dr. Wolf's CHM 201 & 202 1- 35

DefinitionsDefinitions

ArrheniusArrhenius

An acid ionizes in water to give protons. A An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions.base ionizes in water to give hydroxide ions.

Brønsted-LowryBrønsted-Lowry

An acid is a proton donor. A base is a proton An acid is a proton donor. A base is a proton acceptor.acceptor.

LewisLewis

An acid is an electron pair acceptor. A base An acid is an electron pair acceptor. A base is an electron pair donor.is an electron pair donor.

Dr. Wolf's CHM 201 & 202 1- 36

Lewis Acid-Lewis Base ReactionsLewis Acid-Lewis Base Reactions

The Lewis acid and the Lewis base can be The Lewis acid and the Lewis base can be either a neutral molecule or an ion.either a neutral molecule or an ion.

Lewis acidLewis acid Lewis baseLewis base++

AA ++ A—BA—BBB••••

++ ––

AA ++ A—BA—BBB••••

++ ++

AA ++ A—BA—BBB••••

–– ––

––AA ++ A—BA—BBB••••

++

Dr. Wolf's CHM 201 & 202 1- 37

Example: Two Neutral MoleculesExample: Two Neutral Molecules

FF33BB ++ OO

CHCH22CHCH33

CHCH22CHCH33

•••• ••••–– ++

FF33BB OO

CHCH22CHCH33

CHCH22CHCH33

••••

Lewis acidLewis acid Lewis baseLewis base

Product is a stable substance. It is a liquid withProduct is a stable substance. It is a liquid witha boiling point of 126°C. Of the two reactants,a boiling point of 126°C. Of the two reactants,BFBF33 is a gas and CH is a gas and CH33CHCH22OCHOCH22CHCH33 with a with a

boiling point of 34°C.boiling point of 34°C.

Dr. Wolf's CHM 201 & 202 1- 38

Example: Ion + Neutral moleculeExample: Ion + Neutral molecule

Reaction is classified as a substitution. But noticeReaction is classified as a substitution. But noticehow much it resembles a Brønsted acid-base reaction.how much it resembles a Brønsted acid-base reaction.

++

Lewis baseLewis base Lewis acidLewis acid

H—OH—O••••••••

••••

––HH33CC—Br—Br ••••

••••

••••++H—OH—OCCHH33

••••

••••

••••

••••

••••

––•••• BrBr

++H—OH—O••••••••

••••

––HH—Br—Br ••••

••••

••••++H—O—H—O—HH

••••

••••

••••

••••

••••

––•••• BrBr

Dr. Wolf's CHM 201 & 202 1- 39

Example: Ion + Neutral moleculeExample: Ion + Neutral molecule

Brønsted acid-base reactions are a subcategory ofBrønsted acid-base reactions are a subcategory ofLewis acid-Lewis base reactions.Lewis acid-Lewis base reactions.

++

Lewis baseLewis base Lewis acidLewis acid

H—OH—O••••••••

••••

––HH33CC—Br—Br ••••

••••

••••++H—OH—OCCHH33

••••

••••

••••

••••

••••

––•••• BrBr

++H—OH—O••••••••

••••

––HH—Br—Br ••••

••••

••••++H—O—H—O—HH

••••

••••

••••

••••

••••

––•••• BrBr

Dr. Wolf's CHM 201 & 202 1- 40

End of Chapter 1End of Chapter 1