Acid-Base StrengthEdward A. Mottel

Department of Chemistry

Rose-Hulman Institute of Technology

04/18/23

Acid-Base Strength

The strength of an acid-base interactiondepends on the nature of the reacting species

and solvent effects.

Solvent effects can be eliminated by consideringgas phase reactions.

The strength of an acid is a measure of its abilityto dissociate and protonate another compound.

Non-aqueous solvents can be used todistinguish between strong acids.

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Gas Phase BasicityProton Affinity

An energy cycle can be used to determinethe strength of a base

in the absence of solvent effects.

B(g) + H(g)-HA,BH

BH(g)

+e- -HIE,H -e- HIE,BH

B(g) + H+(g)H = PA

BH+(g)

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Gas Phase BasicitySubstituted Amines

N3-

NH2-

NH2-

N(CH3)3 974basi

city

3084 kJ·mol-1

2565

1689

NH2CH3 919

NH3 872

NF3 604

NH(CH3)2 954

protonaffinityvalues

Solution Phase BasicitySubstituted Amines

NR3 The basicity of amines depends oninductive effects of substituentssteric effects

pKb

solvent effects

NF3

no basiccharacter

604

N(CH3)3

4.28

974

NH2CH3

3.36

919

NH3

4.74

872

NH(CH3)2

3.29

954PA

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Gas Phase AcidityProton Affinity

The conjugate base of stronger acidshave lower proton affinities.

HF(g)H = PA

H+(g) + F-(g)

CH3COO- 1459 kJ·mol-1

CF3COO- 1351 kJ·mol-1

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Gas Phase AcidityProton Affinity

I-

Br-

Cl-

F-

CH3COO-

OH-

1554

1459

1635

Con

jugate

Aci

d A

cid

ity

1315 kJ·mol-1

1354

1395

HI

HBr

HCl

HF

CH3COOH

H2O

Solution Phase AcidityAcid Strength in Solution

In water, HCl, HBr and HI are all consideredstrong acids.

Why are they all considered equally strongin water?

How can the strongest acid be identified?

Solution Phase AcidityAcid Strength in Solution

In which solvent would HCl ionize less?

CH3COOH

H2O

NH3

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Acid-Base ReactionsIonic and Covalent Contributions

Comparison of many acid-base reactionsindicates that both ionic and covalent factors

contribute to the enthalpy of reaction.

Each acid and base can be assignedan electrostatic (E) factor and

a covalent (C) factor.

-ΔHrx = EAEB + CACB

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Acid-Base ReactionsIonic and Covalent Contributions

OH

H

SH

H

K+

I - I

Which acid-base pair reactions are more exothermic?

EB = 2.28CB = 0.10

EB = 0.04CB = 1.56

EA = 0.50CA = 2.00

EA = 3.78CA = 0.10

-8.6

-3.1

-1.3 -0.3 kcal/mol

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Acid-Base ReactionsIonic and Covalent Contributions

-ΔHrx = EAEB + CACB

These factors provide ∙ an estimate of the enthalpy of reaction ∙ insight into the bonding nature of each species

I - IEA = 0.50CA = 2.00

K+ EA = 3.78CA = 0.10

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Acid-Base ReactionsIonic and Covalent Contributions

-ΔHrx = EAEB + CACB + RATB

More precise estimates can include acorrection factor for the transference of charge

from the base to the receptor acid.

K+ EA = 3.78CA = 0.10

RA = 20.79

OH

HEB = 2.28CB = 0.10TB = 0.43

(more important for ions)

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Acid-Base ReactionsIonic and Covalent Contributions

OH

H

K+

EB = 2.28CB = 0.10TB = 0.43

EA = 3.78CA = 0.10

RA = 20.79-17.6 kcal/mol

-ΔHrx = EAEB + CACB + RATB

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Acid-Base ReactionsIonic and Covalent Contributions

E, C, R & T values represent gas phase reactions.

F- has stronger ionic and covalent terms than Cl-, Br- or I-

F- 9.73 4.28 37.40

Cl- 7.50 3.76 12.30

Br- 6.74 3.21 5.86

I- 5.48 2.97 6.26

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Acid-Base ReactionsBond Energies

Three contributing factors to bond energy

covalent energy

Madelung (Coulombic) energy

electronegativity energy

-ΔHrx = EAEB + CACB + RATB

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Hard and Soft Acids and Bases

Hardnon-polarizable

high charge density

Softpolarizable

large, low charge

cations

anions

H+, Groups I & II,Ti4+, Cr3+, Fe3+, Co3+

Cu+, Ag+, Hg22+,

Hg2+, Pd2+, Pt2+

(class a) (class b)

N>>P>As>SbO>>S>Se>Te

F>Cl>Br>I

N<<P>As>SbO<<S<SeTe

F<Cl<Br<I

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Hard and Soft Acids and Bases

Dithiooxalate ioncan chelate on two sides.C

C

O

- O

S -

S

C

C- O

O

S

S -

C

CO

O

S

S

- -

prefers hardercations

prefers softercations

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Hard and Soft Acids and Bases

Dithiooxalate ioncan chelate on two sides.

C

CO

O

S

S

-

-

prefers hardercations

prefers softercations

C

CO

O

S

S

-

-

Fe3+ Cu+

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Capable of protonating compoundsthat don’t want to be protonated.

Causing isomerization in the most unlikely candidates.

More acidic than concentrated sulfuric acid.

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F F

H H

+

(CH3)3COH (CH3)3C+ + H3O+

I2 I2+ and/or I3

+

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Acids which are stronger than pure sulfuric acid.

Super acids are measuredwith the Hammett acidityfunction (Ho) Ho = pKBH+ - log

[BH+]

[B]

B is an indicator base:e.g., nitroaniline, benzene, picramide

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Superacid Acidity

100% sulfuric acid H2SO4 -11.9

hydrofluoric acid HF -11.0

perchloric acid HClO4 -13.0

fluorosulfuric acid HSO3F -15.6

triflic acid HSO3CF3 -14.6 trifluoromethanesulfonic acid

magic acidTM HSO3F-SbF5 -21 to -25

fluoroantimonic acidHF-SbF5 -21 to -28

Ho

non-aqueous systems

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(C6H5)3COH + superacid → (C6H5)3C+ + H3O+

trivalent carbocation(carbenium ion)

alkyl-Cl + superacid → alkyl+ + HCl

hydrocarbon + superacid → hydrocarbon-H+

pentacoordinate carbocation(carbonium ion)

George Olah 1994 Nobel Prize

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Formed by mixing

fluorine containingBronsted acidor metal oxide

fluorinated Lewis acid

HF, HSO3F,HSO3CF3

BF3, SbF5, TaF5

TiO2, SiO2

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2 HF + 2 SbF5 H2F+ +

F

F

FF

FF Sb

F

F

FF

F

Sb

-

→

HSO3F + SbF5

fluorosulfonicacid

antimony pentafluoridecomplexing agent

F

F

FF

FH-O-S—F

O

O

Sb→

Why are superacids so acidic?

magicacidTM

fluoroantimonic acid

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Hammett acidityis effectively the extension of pHbelow zero.

Ho = pKBH+ - log[BH+]

[B]

B is an indicator base:e.g., nitroaniline, benzene, picramide

BH+ B + H+ pKBH+

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Hammett acidityis determined

spectrophotometricallyby determining [BH+] and [B]

Ho = pKBH+ - log[BH+]

[B]

B is an indicator base:e.g., nitroaniline, benzene, picramide

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Applications

Isomerization/polymeriztion

R+ + C C C C+R

C CR C C+

C C

Heterogeneous catalystssolid superacidsZrO2-SO4, TiO2-SO4

for cracking & isomerizing alkanes

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Applications

Hydrocracking

heavy oilbitumen

H2

lower boiling distillate

also, coal liquifaction

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Redox Reactions

F-S-O-O-S-F

O

O

O

O

peroxydisulfuryldifluoride

+ I2 I

FO2SO

OSO2F

OSO2F

I2+

I3+

+ SO3F -

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HF + SO3 HSO3F

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Non-aqueous Solvents

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Acid-Base ReactionsIonic and Covalent Contributions

OH

H

SH

H

K+

I - I

Which acid-base pair reactions are more exothermic?

EB = 2.28CB = 0.10

EB = 0.04CB = 1.56

EA = 0.50CA = 2.00

EA = 3.78CA = 0.10

-8.6

-3.1

-1.3 -0.3 kcal/mol

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Superacids

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