Chapter 3
Structure
and
Bonding
Acids and
Bases
Acids & Bases/
Organic Chemistry
Dr. Ron Rusay
Fall 2012
Models of Acids and Bases
•Arrhenius: Acids produce H+ & bases
produce OH ion in aqueous solutions .
•Brønsted-Lowry: Acids are H+ donors &
bases are proton acceptors.
•HCl + H2O  Cl + H3O+
acid base
Comprehensive Tutorial
Acid & Base Principles in Organic Chemistry
Highly recommended viewing
http://chemconnections.org/organic/Movies%20Organic/acidsbases.mov
Acid-Base Equilibrium
Conjugate Acid/Base Pairs
HA(aq) + H2O(l)  H3O+(aq) + A(aq)
acid
base
conj
acid
conj
base
•conjugate acid: formed when the proton is
transferred to the base.
•conjugate base: everything that remains of
the acid molecule after a proton is lost.
http://chemconnections.org/organic/Movies%20Org%20Flash/
ConjugateAcidBaseActivity.swf
QUESTION: 3.1
Aniline, C6H5NH2, was isolated in the 1800s and
began immediate use in the dye industry.
What is the formula of the conjugate acid of
this base?
A.
B.
C.
D.
E.
C6H5NH2+
C6H5NH3+
C6H5NH–
C6H5NH+
C6H5NH2
Weak Acids
• Weak acids are only partially ionized in solution.
H3O+(aq) + A-(aq)
HA(aq) + H2O(l)
HA(aq)
[H 3O  ][A- ]
Ka 
[HA]
H+(aq) + A-(aq)
or
[H  ][A- ]
Ka 
[HA]
• Ka is the acid dissociation constant.
Organic Acids & Bases
• Organic acids are weak acids, eg. Acetic
acid.
• However, there can be substantial differences
in their relative strengths. What could you use
to compare relative acidities?
• Organic bases are weak bases and relate to
ammonia.
• However, there can be substantial differences
in their relative strengths. What could you use
to compare relative basicity?
Conjugates
Ka x Kb = ?
What do pKa and pKb refer to?
pKa + pKb = ?
Remember: pH + pOH = pKw
Strengths of Bases from pKa Values
Values for pKb are not
commonly found in
the reference
literature.
Therefore, a
comparison of
relative basicity is
made by using the
pKa values of the
corresponding
conjugate acids.
Which is the stronger acid?
Which is the stronger base?
QUESTION: 3.2
Use information on the table above to determine which of the
following bases would have the weakest conjugate acid:
OC6H5–; C2H3O2–;
A.
B.
C.
D.
OC6H5–
C 2 H 3 O2 –
OCl–
NH3
OCl–;
NH3
QUESTION: 3.3
Use information on the table above to determine the order of
increasing base strength for the following bases:
OC6H5–; C2H3O2–; ClO2–;
A.
B.
C.
D.
OC6H5– < NH3 < C2H3O2– < ClO2–
C2H3O2– < ClO2– < NH3 < OC6H5–
ClO2– < C2H3O2– < NH3 < OC6H5–
NH3 < OC6H5– < C2H3O2– < ClO2–
NH3
Acid and Base Equilibria
The equilibrium favors the weaker of the acid vs. its
conjugate acid or base vs. its conjugate. Weak
wins!
http://chemconnections.org/organic/Movies%20Org%20Flash/acid-base-eq.swf
QUESTION: 3.4
Consider the following equilibria. Identify the weaker
of the two: acid vs. its conjugate acid in each
reaction. Which reactions favor formation of product?
O
O
1.
2.
+ NH3
CH3 C
pKa=4.7
OH
+ NH3
OH
4.
+ NH4
pKa=9
O
O
O
CH3 C
3.
CH3 C
pKa=36
HCl + CH3OH
Kpa=15.5
HCl + CH3OH
pKa-=–7
CH3 CCH + NH2
5.
pKa =26
6.
CH3 CH3 +
pKa=50
CH3 COH2
+ NH2
pKa=–6
A. All do: 1-6
B. 1, 4, 5
H–Cl–H + CH
3O
pKa < –15
C. 2, 3, 6
Cl
D. 1, 5
+ CH3 OH2
pKa=–2.5
CH3 CC
+
E. None do.
NH
3
Kap=36
NH2
CH3 CH2
+ NH3
pKa=36
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-12.html#Worksheets
Organic Acids & Bases
• Organic molecules in context can be considered as
behaving relatively as weak acids or weak bases.
• Formal Charge is important in considering which.
• Knowing the Formal Charge allows a prediction.
• (+) positive atoms behave acid-like, (-) negative
atoms behave base-like.
• This can be used in predicting how molecules will
react--- or don’t react, and the products of reactions.
Formal Charge / Acids & Bases
Electrophiles / Nucleophiles / Reactivity
..
CH3
H3C
: O:
N
CH3
CH3
H 3C
N
..
CH 3
H O:
::
CH 3
..
N
H3 C : :
CH 3
: O:
H3 C
..
: O:
CH3
::
CH3
C:
H3C
H3C
..
H
H3C O H
H O H
::
CH3
H
:O
..
..
CH2
C
CH3
CH3
CH 3
H3C
O:
CH3
H3C
: O:
..
H3 C O :
::
H3 C
CH3
CH2
Worksheet 1
http://chemconnections.org/organic/chem226/226assign-12.html#Worksheets
Structure and Acid-Base Properties
Important factors that effect acidity in binary
compounds, eg HCl (having only two
elements):
• Bond Length (shorter = stronger bonds;
lower acidity)
• Bond Strength (longer = weaker bonds;
higher acidity: more dissociation, more
protons [hydronium ions] in solution)
• Bond Polarity (smaller e.n. differences favor
higher acidities)
Electronegativity, Charge Density, Bond
Polarity and Acid-Base Properties
 Explain
which is the stronger acid: HBr or HF.
HBr is a stronger acid than HF. The greater the e.n. difference, the
more polar the bond, the shorter and stronger the bond, and since it
is harder to dissociate (break), the weaker the acid.
Strength of Oxyacids
(Three atoms: ternary vs. binary)
Induction or a ”Push-Pull” electronic effect influences the proton, which
weakens the oxygen-hydrogen bond. Chlorine (higher e.n.) pulls electrons more
than Iodine (lower e.n.)
QUESTION: 3.5
•
Rank 1.0M solutions of HBrO, HIO and HClO in order
of increasing acidity.
HBrO , Ka = 2.1 x 10-8
HIO , Ka = 2.3 x 10-11
HClO , Ka = 3.0 x 10-8
A) HBrO < HIO < HClO
B) HIO < HBrO < HClO
C) HClO < HBrO < HIO
D) HIO < HClO < HBrO
Question: 3.6
True (A) / False (B)
• HBrO4 is a weaker acid than HBrO2.
•
•
•
•
HClO2 , Ka = 1.2 x 10-2
HBrO , Ka = 2.06 x 10-8
HIO , Ka = 2.3 x 10-11
HClO , Ka = 3.0 x 10-8
QUESTION: 3.7
•
Rank 1.0M solutions of HBrO, HIO and HClO in order
of increasing pH
HBrO , Ka = 2.1 x 10-8
HIO , Ka = 2.3 x 10-11
HClO , Ka = 3.0 x 10-8
A) HBrO < HIO < HClO
B) HIO < HBrO < HClO
C) HClO < HBrO < HIO
D) HIO < HClO < HBrO
QUESTION: 3.8
• Use the concept of induction “push-pull”.
Rank the following organic acids in order of
decreasing acidity from most acidic to least.
1) Br-CH2COOH
2) I-CH2COOH
• A) 2 > 1 > 3
B) 3 > 2 > 1
• C) 2 > 3 > 1
D) 1 > 2 > 3
3) CH3COOH
QUESTION: 3.9
Rank the following acids in order of decreasing
acidity.
A) ClCH2COOH
C) Cl3CCOOH
1) A > B > C > D
2) D > C > B > A
B) Cl2CHCOOH
D) CH3COOH
3) C > B > A > D
4) B > C > A > D
Question: 3.10
True (A) / False (B)
Trichloroacetic acid, Cl3CCOOH, is more
acidic than trifluoroacetic acid, F3CCOOH.
Comparing Acid-Base Strengths
• Consider four possible factors (ARIO)* that affect the
stabilization of a formal charge.
The more effectively a conjugate base can stabilize its formal negative charge
the weaker it is as a base. Conceptually, consider that it is done by spreading
the negative charge, lowering electron density, and/or by reducing orbital size,
which increases electron proximity and attraction to the positive nucleus. These
factors make for a weaker base compared to a molecule that cannot affect the
charge distribution as well. The corollary is that its parent acid is a stronger acid.
ARIO:
 The type of Atom that has the formal charge. (A more
electronegative atom is better at stabilizing negative charge.)
 Resonance (Delocalizing the negative charge makes for a weaker
base.)
 Induction (“Push-Pull” electronic effects.)
 The type of Orbital where the charge resides:
*Professor David Klein, U. of Chicago
Approximate pKa Values of Functional Groups
Professor William Blatchley, Keene State College
Induction
Substituent Effects:
X-CH2COOH
“Electron Donating”: Positive effect on pKa, (Weaker Acid) eg. (butyric vs. acetic acid)
“Electron Withdrawing”: Negative effect on pKa, (Stronger Acid) eg. chlorine
pKa
acetic acid
(X = H-)
4.75
butyric acid
(X = CH3CH2-)
4.85
chloroacetic acid
(X = Cl-)
2.87
4-chlorobutyric acid (X = ClCH2CH2-)
4.52
3-chlorobutyric acid (X = CH3CH2Cl-)
4.05
2-chlorobutyric acid (CH3CH2CHCl-COOH)
2.88
alkyl groups are
weakly electron
donating
chlorines are
electron
withdrawing
(Note: the
proximity of
chlorine has an
important effect)
Question: 3.11
Using ARIO principles, predict which of the protons in the
following structure of acetylsalicylic acid is the most acidic.
An Organic Base in Context
Erythroxylon spp.
• It is a very valuable plant. The leaves are chewed by
indigenous tribes in the Andes to boost their energy.
• It has been used as a psycho-therapeutic, an
opthalmic anesthetic and was purportedly used in a
popular beverage many years ago that is at the heart
of a $20 billion corporation.
• However, both its base and conjugate acid are
currently controlled substances under U.S. Federal
Regulations: Title 21 secs. 329.1 & 1308.12 (1987).
• Can you name the beverage and the base?
The beverage reportedly produced using
the extract of leaves of Erythroxylon coca:
The compound: cocaine, is an organic base: Merck Index,
#2450, 11th ed.: Caution: May be habit forming….
Acid -Base Chemistry
(Physical Properties)
• m.p. 98 oC
• b.p. (very volatile
> 90 oC)
Solubility:
• Water: 1.67 x 10-3
g/mL
• CHCl3: 1.43 g/mL
• Ether: 0.29 g/mL
What structural feature makes cocaine
a base? What simple compound can
you relate it to?
“Regular” Cocaine
Conjugate Acid of Cocaine
(Physical Properties)
• m.p. >195 oC
Solubility:
•
Water: 2.5 g/mL
•
CHCl3: 0.08 g/mL
•
Ether: insoluble
What accounts for the differences in
solubilities of the base and conjugate
acid?
Comparison
(Physical Properties)
•
•
m.p. 98 oC
b.p. (very volatile
> 90 oC)
Solubility:
• Water: 1.67 x 10-3 g/mL
• CHCl3: 1.43 g/mL
• Ether: 0.29 g/mL
Crack Cocaine: (Base)
•
m.p. >195 oC
Solubility:
•
Water: 2.5 g/mL
•
CHCl3: 0.08 g/mL
•
Ether: insoluble
Regular Cocaine (Conjugate Acid)
http://en.wikipedia.org/wiki/Cocaine
Curved Arrows in Organic Reactions
• When bonds break and form, electrons move, and we can
show their electron movements with curved arrows.
• Consider the following generic acid/base reaction.
• Each double-headed, curved arrow above shows the
movement of two electrons. (A single headed arrow
represents a single electron, as in Free Radical Reactions.)
• Viewing all of the curved arrows represents the reaction
MECHANISM, which accounts for where the electrons
came from, how they moved, and where they wound up.
Acid -Base Reactions
Acid Base Reactions
QUESTION: 3.12
Aniline, C6H5NH2, was isolated in the 1800s and
began immediate use in the dye industry.
What is the formula of the species present in a
solution of aniline that is at a pH = 2?
A.
B.
C.
D.
E.
C6H5NH2+
C6H5NH3+
C6H5NH–
C6H5NH+
C6H5NH2
Which form is favored? Acid or Conjugate Base?
Solution effects: The pH of the surroundings (i.e. independent of the
acid-conjugate base pH contribution) controls which form is favored, acid or
conjugate base, and relates to the following equation.
pH  pK a  log
HA
A 

• If the pH of the surroundings is < than a compound’s
pKa, the compound will exist primarily in its acidic form.
•If the pH of the surroundings is > than a compound’s
pK
a, the compound will exist primarily in its basic form.
• NOTE: Remember that a buffer solution (surroundings)
maintains a nearly constant pH within certain parameters.
A weak acid
RCO2H,
pKa = 5.2
50% acid
50% base
99% acid
1% base
90% acid
10% base
10% acid
90% base
1% acid
99% base
QUESTION: 3.13
Aspirin has a pKa of 3.5. In the stomach,
pH ~ 2, what form of aspirin is favored?
QUESTIONS from Worksheet:
The pKa of a general anesthetic, sodium pentothal, is 7.4. If a
patient is given sodium pentothal orally instead of iv, will it put
the patient to sleep?
What information is needed to answer this fundamental
anesthesiology (“gas passer”) question?
A drug has a pKa of 7.8 and is a known teratogen. If given iv
to a pregnant woman whose blood pH is within normal levels,
will this drug cross the placenta and affect the baby?
What information is needed to answer this anesthesiology
(“gas passer”) question?
Lewis Acids and Bases
• Lewis Acid: electron pair acceptor
• Lewis Base: electron pair donor
• Example:
Al3+ + 6 O
H
H
Al
H
3+
O
H
6
Question: 3.14
Select the chemical equation that depicts
the correct movement of electrons in the
reactionof ammonia with hydrogen
chloride.
A)
B)
C)
D)
Acid-Base Reactions
Showing a reaction with arrows
CH3 SCH3 + BF3
CH3 + H2O
CH3 O
+ CH3 SH
(CH3)2 S–BF3
CH3 OH2
CH3 OH + CH3S
O
OH
CH3 CCH3 + H3O
H2O + CH3CCH3
HCC
HCC–CH2–O
+ H2C=O
CH2 =CH2 + HCl
I
+ CH3Br
CH2 –CH3 + Cl
ICH3 + Br
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-12.html#Worksheets
Draw products for the following acid/base
reactions, and label the conjugate products.
CH3
CH3
CH3
CH3
Acid-Base Reactions
Predicting Products
O
+ HCO3
CH 3C
OH
B(CH3 )3 + (CH3) 3N
NH3
+
OH
H 3C
+ MgCl2
C O
H 3C
H2SO 4 + CH3OCH3
BF3 +
O
O
+ AlCl3
CH 3C
OH
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-12.html#Worksheets
Predicting if reactions occur and the
products if they do
a.
b.
c,
HO
+ Br–CH3
HO–Br +
HO
+ Br–CH3
HO–CH3 + Br
CH
3
H–CC
+ H2 C=O
H–CC–CH 2-O
H–CC
+ H2 C=O
H–CC–O–CH2
NH2 + H–OCH3
H2N–OCH3 +
NH2 + H–OCH3
H2N–H +
H
OCH
3
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-12.html#Worksheets
Question: 3.15
Using pKa values given in the structure below, what is the product of the
following reaction (NH4+/NH3, pKa = 9.3/ pKb = 4.7)?
Consider that the reaction is with 1 mole of each reactant.
A.
B.
C.
D.
“Solvation”: Solvent Effects
• A large amount of solvent, which surrounds a substrate, can profoundly
affect the substrate’s properties.
• Consider the following solvent and ion–dipole attractions of two different
bases, which are both stabilized by the interactions.
• The tert-butoxide ion is sterically hindered, and is not as well solvated
as the ethoxide ion.
QUESTION: 3.16
Which has a lower pKa in water?
A) tert-butanol or B) ethanol
Choosing a Reaction Solvent
An appropriate SOLVENT for an acid/base reaction:
 Should be able to surround the reactants and facilitate their
collisions, increasing the rate of reaction.
 Should NOT itself react with the reactants.
Because water can act as an acid or a base, it has a LEVELING
EFFECT on strong acids and bases, which are common reactants or
intermediates in many organic reactions:
Acids stronger than H3O+ cannot be used in water.
Bases stronger than OH- cannot be used in water.
Water would not be an appropriate solvent for the following
reaction. WHY?
NaNH2 + H2O → NH3 + NaOH
Question: 3.17
Which of the following solvents would be the best choice to
use in the reaction:
A) CH3COOH
B)
C)
D)