Chapter 3- Acids and Bases
An Introduction to organic reactions
and their mechanisms
Four Categories of Organic Reactions
1) Substitution- one group replaces another
2) Additions- All parts of the adding agent appear in
the product. Two compounds become one.
3) Eliminations- One molecule loses the elements of
another small molecule
4) Rearrangements-A molecule undergoes a
reorganization of its constituent parts
Homolysis vs Heterolysis
• Heterolysis- one of the atoms in a bond get
both shared electrons when the bond breaks.
The products are ions.
• Homolysis- bonds break evenly, with each
atom getting half the shared electrons. The
products are radicals.
Heterolysis
• Heterolysis usually occurs on polarized bonds
• Often needs other molecules “help” to
separate charges
Bronsted-Lowry Acids and Bases
• Acid- a substance that can donate, or lose, a
proton.
• Base- a substance that can accept, or remove,
a proton
Conjugates
• Conjugate base- molecule or ion that forms
when an acid loses its proton
• Conjugate acid- molecule or ion that forms
when a base accepts a proton
• Ex
Leveling Effect
• Hydronium ions and Hydroxide ions are the
strongest acid/base you can have in aqueous
solutions
• This does not mean there aren’t stronger
acids/bases, but anything stronger when put
into water, they will react with water to form
hydronium/hydroxide
Solvation
• Water has the ability to solvate both cations
and anions
• Spectator ions…
Lewis Acid and Base
• Lewis Acid- electron acceptors
• Lewis Base- electron donator
• Ex.
Heterolysis of Bonds to Carbon
• Leads to one of two ions:
• Carbocation
• Carbanion
• Carbocations are electron deficient, aka a
Lewis Acid
Important Definitions
• Electrophiles- reagents which in their
reactions seek extra electrons that will give
them a stable valence shell. Electron deficient
• Nucleophiles- reagents that seek a proton or
some other positive center. Electron rich
Strengths of Acid/Bases: Ka and pKa
• Strong acids such as HCl and H2SO4 dissociate
completely in water
• Weaker acids such as Acetic Acid do not
proceed to completion
• 0.1M solution at 25oC only about 1% of the
Acetic Acid molecules ionize.
The Acid Constant, Ka
• Because the dissociation is at equilibrium, we
can describe it with an expression for the
equilibrium constant
• For dilute solutions, the concentration of
water is considered constant so we can
rearrange the equation and establish a new
constant.
• Generic Reaction:
• Generic Expression:
Relationship in Expresson
• Products are in the numerator
• The more products that form, the stronger the
acid
• So the Larger the Ka, the Stronger the acid
• The Smaller the Ka, the weaker the acid
• As a general rule, if Ka>10, the acid completely
dissociates in water
pKa
• Since Ka can be very large or very small, it is
usually expressed as a negative logarithm, pKa
pKa = - log Ka
• This is like expressing the hydronium
concentration as pH!
• pKa is inversely proportionate to strength of
acid
Table 3.1 page 115
• The range of the table is in the order of 1062
• Everything above hydronium and below water
are approximated
• Remember, pKa is a logrithmic scale, so one
unit change is equal to a power of 10!
Water as a Weak Acid
Predicting Base Strength
• One Rule- The stronger the acid, the weaker
the conjugate base.
• We can relate the strength of a base to the
pKa of its conjugate base.
• The larger the pKa of the conjugate acid, the
stronger the base.
Acid-Base Reactions
• Reactions always favor the formation of the
weaker acid/base pair
• The reason for this is that the outcome is
determined by the position of the equilibrium
• The reaction is under equilibrium control
which always favors the formation of the most
stable (lowest potential energy) species
Using Acid/Base reactions for
Solubility
• Water insoluble compounds will react to form
water soluble ions
Relationship between Structure and
Acidity
• Two main factors to consider
1) Strength of A-H bond
2) Stability of anion (conj base)
Strength of the Acid-H bond
• As we move down a group, the strength of the
H-A bond decreases
– Due to weaker overlap with 1s orbital of Hydrogen
– Weaker bond = stronger acid
• Acidity increases as we move from left to right
– Due to electronegativity
Stability of Conjugate Base
• Hybridization effect- the more s character, the
closer to the positive charge nucleus the
negative charge of the conjugate base will be,
thus more stable
• Inductive effect- the polarization of sigma
bonds to delocalize, thus stabilize, the
conjugate base anion.
– It decreases as distance increases
Energy Review
• Two types of Energy:
– Kinetic- energy of motion
– Potential- stored energy
• Molecules posses P.E.
– The more P.E. a molecule posses, the less stable it is
• We don’t know absolute P.E. (no absolute stability)
– Instead we take about relative stability
Energy Review
• P.E. decreases as covalent bonds form
• Can express this in Enthalpies, of heat
contents
• The difference in the relative enthalpies of
reactants and products in a chemical change
called Enthalpy change is symbolized by ΔHo
• ΔHo for exothermic reactions is negative
• ΔHo for endothermic reactions is positive
Energy Review
• So, in exothermic reactions, atoms have
smaller enthalpies as products than as
reactants
• Opposite is true for endothermic reactions
Relationship between Keq and
Standard Free-Energy Change, ΔGo
• Equation:
• A negative ΔGo means reactions favor
products when equilibrium is reached because
that means Keq > 1
• ΔGo < -13 kJ/mol means the reactions goes to
completion (>99% of reactants go to products)
Energy Review
• A positive ΔGo means reaction favors reactants
when equilibrium is reached because Keq < 1
• ΔGo has two components related by the
following equation:
Energy Review
• Enthalpy change is associated with changing in
bonding
• If stronger bonds are formed in products than
were in reactants, ΔHo is negative (exothermic
reaction)
• A negative ΔHo contributes to making ΔGo
negative, thus favors products
• For ionizing of acid, the more negative the ΔHo
the stronger the acid
Energy Review
• Entropy- changes in the relative order of a system
• The more random a system is, the greater the
entropy
• + ΔSo = more ordered to less ordered
• - ΔSo = less ordered to more ordered
• Since there is a minus sign, a + ΔSo makes ΔGo
more negative, which favors products
Acidity of Carboxylic Acids
• Carboxylic Acids vs alcohols
• Carboxylic Acids are more acidic due to
– Resonance effect
– Inductive effect
Resonance Effect
• The resulting ion is stabilized through
resonance (delocalization of charge)
Inductive Effect
• Polarization of the sigma bonds weakens the
oxygen-hydrogen bond and helps stabilize the
conjugate base by delocalizing the negative
charge on the conjugate base.
Effect of Solvent on Acidity
• Protic Solvents- solvents which have a
hydrogen bonded to an electronegative atom
such as Oxygen or Nitrogen
• Protic solvents can solvate the acid and
conjugate base to stabilize them, but not
equally.
Organic Compounds as Bases
• If a compound has an atom with unshared
electrons, it is potentially a base
• Proton transfer reactions like this are often
the first step in many reactions of compounds
containing oxygen and nitrogen
Strength of Acids Summary
Pi bonds as bases
• Much like lone pairs can be basic, Pi bonds can
be basic as well
Mechanism Intro
• Mechanism- a description of events that take
place on a molecular level as reactants
become products.
Acid/Bases in Non-aqueous Solutions
• Earlier we described the Leveling Effect.
• Water will always give a proton to any base
stronger than hydroxide
• So it is impossible to use any base stronger
than hydroxide in water
• We can use stronger bases, however, if we use
a solvent that is a weaker acid than water.
Acid/Bases in Non-aqueous Solutions
• For example, we can use Sodium Amide in
hexane, diethyl ether, or liquid ammonia
• They are all weak enough acids so they won’t
give a proton to the amide
• Ex.
Alcohols as Solvents
• Alcohols are often used as solvents because
they dissolve less polar organic compounds
• Plus, we can use Alkoxides as a base.
• Ex.
Alkyl Metal reagents
• Alkyl Metals, such as alkyl lithiums, are some
of the strongest bases we have.
• The bond is considered covalent but react as if
it is ionic.
D and T Labeling
• Some Hydrogens can be exchanged for
isotopes such as Deuterium and Tritium.
• Reasons for labeling:
– Deuterium is NMR inactive
– Tritium is radioactive
• Ex.
First Synthesis Problem!!
• Convert
H3C
C
C
H
to
H3C
C
C
T