Chapter 3
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Transcript of Chapter 3
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