An Introduction to Organic Reactions and Their Mechanism

Acids and Bases

3.4 Acid StrengthStrong acids:

completely ionized or completely dissociated

Forward reaction is predominated Most of HA is

dissociatedConjugated base is weak

and has low attraction for proton

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Acids StrengthWeak acids: partially ionized or dissolved Reverse reactions is predominated

Most of HA is undissociateConjugated base is strong and has high

attraction for protonHC2H3O4(aq) + H2O(l) H3O+(aq) +C2H3O4

-(aq)

3.5 The Strength of Acids and Bases: Ka and pKaThe acidity constant, Ka

a quantitative measure of the strength of an acid in solution.

It is known as the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions

Larger Ka, stronger the acidHA + H2O H3O+ + A Ka =

H3O+ A

HA

Acidity and pKaLogarithm scale of Ka with the base of 10pKa = - log KaThe larger pKa, the smaller extent of

dissociationWeaker acids

ExamplesAn acid (HA) has Ka = 10-7, what is its pKa?Another acid (HB) has Ka = 5, what is its

pKa?Which is the stronger acid?

Predicting the Strength of BasesThe strength of acids will decide the how

weak or strong its conjugated basesStronger the acids, the weaker will be its

conjugated baseAfter losing a proton, acid will become a

conjugated baseWhen a base accepts a proton, the resulting

chemical is called the conjugate acid of that original base

NH3 + H O H H N

H

H

H O H

Base AcidConjugatedacidpKa = 9.2

conjugated base

ExamplesThe pKa of anilinium ion (C6H5N+H3) is equal

to 4.6. On the basis of this fact, decide whether aniline (C6H5NH2) is a stronger or weak base than methylamine

3.6 Predicting the Outcome of Acid-Base ReactionsAcid-base reactions always favor the

formation of the weaker acid and weaker baseEquilibrium favor the formation of the most

stable (lowest potential energy) speciesStronger acid + stronger base weaker base

+ weaker acid O

C

R O H

+ Na O H

O

C

R O Na + H O H

stronger acidpKa = 3-5

stronger base Weaker base Weaker acid

ExampleConsider the mixing of an aqueous solution of

acetic acid, CH3CO2H (see Table 3.1), and NaOH. What acid base reaction, if any would take place

H2SO4(aq) + NH3(aq) NH4+(aq) + HSO4-(aq)

3.6 Water Solubility as the Result of Salt FormationRecall: Longer chain of carbon (4 or more)

will be less solubleHigher molecular weight of carboxylic

compounds do not dissolve in waterInstead, they dissolved in a strong base to form

a water-soluble saltO

O H Na O H

O Na

+ H O H

soluble in water

insoluble in water

Water SolubilityWe can predict that an amine will react with

aqueous hydrochloric acid

R NH2 + H O H

H

Cl H N

H

H

H O H

H

+

Stronger base stronger acidpKa = -1.74

weaker acidpKa = 9-10

weaker base

Water SolubilityMethylamine and most amines of low

molecular weight are very soluble in waterFormed a water-soluble salt from hydrochloric

acid

NH2 H O H

H

ClN

H

H

H

O H

H

+

Water insolubleWater soluble

Cl

3.7 Relationships between structure and AcidityThe strength of a Bronsted-Lowry acid

depends on the extend to which a proton can be separated from it and transferred to a base.Removing H+ breaking a bond

Making the conjugated base more electronegativeAccepting H+ forming a bond

Making the conjugated acid less electronegative

3.7 Relationships between structure and Acidity

Bond strength to proton decreases as we move down to columnDecreasing effectiveness of orbital overlap

between the hydrogen 1s orbital and the orbitals of successively larger element in the column

Less effective the orbital overlap, the weaker bond, stronger acid

Acidity increases from left to right

C CH H C C

H

H

H

H

C C

H

H

HH

H H> >

EthynepKa = 25 Ethene

pKa = 44EthanepKa = 50

More acidic H+ Less acidic H+

3.7 The Effect of HybridizationElectrons of 2s orbtials have lower energy

than those of 2p orbitals because these electrons are much closer to the nucleus

Having more s orbitals means that electrons of the anion will be lower in energy and more stable

3.7 The Effect of Hybridizationsp C – sH

hybridizationContain 50% s

character1s + 1pMore

electronegative spC

C CH H

EthynepKa = 25

3.7 The Effect of Hybridizationsp2C – sH

hybridizationContains 33.3% s

character1s + 2pLess electronegative

than spC

C C

H

H

H

H

EthenepKa = 44

3.7 The Effect of Hybridizationsp3C-sH

hybridizationContains 25% s

character1s + 3pLeast

electronegative C

C C

H

H

HH

H H

EthanepKa = 50

3.7 The Effect of HybridizationRelative basicity of the carbanions

Ethynide ion is the weakest baseThe more electronegative carbon, the more

stable the anion

H3C CH2 > CH2 CH > HC C