Models of Acids and Bases
Chapter 1
Structure
and
Bonding
Acids and
Bases
Acids & Bases/
Organic Chemistry
Dr. Ron Rusay
Fall 2009
•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
Acid-Base Equilibrium
Comprehensive Tutorial
Conjugate Acid/Base Pairs
Acid & Base Principles in Organic Chemistry
Highly recommended viewing
•HA(aq) + H2O(l) → H3O+(aq) + A−(aq)
acid 1
conj
acid 2
base 2
conj
base 1
•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
Weak Acids
QUESTION: 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. C6H5 NH2+
B. C6H5 NH3+
C. C6H5 NH–
D. C6 H 5 NH+
E. C6H5 NH2
• Weak acids are only partially ionized in solution.
H3O+(aq) + A-(aq)
HA(aq) + H2O(l)
HA(aq)
Ka =
[H 3O + ][A - ]
[HA]
H+(aq) + A-(aq)
or
Ka =
[H + ][A - ]
[HA]
• Ka is the acid dissociation constant.
Conjugates
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?
Kw
K a x Kb = ?
What do pKa and pKb refer to? -log Ka or -log Kb
pKa + pKb = ? 14
Remember: pH + pOH = pKw
Which is the stronger acid?
methanol
Which is the stronger base?
methylamine
QUESTION: 2
QUESTION: 3
Use information on the table above to determine which of the
following bases would have the weakest conjugate acid:
OC6 H5– ; C2H3O 2 – ;
A.
B.
C.
D.
–
OC6H 5
C 2H 3O 2–
OCl–
NH3
OCl–;
Use information on the table above to determine the order of
increasing base strength for the following bases:
OC6 H5– ; C2H3O 2 – ; ClO2 – ;
NH3
A.
B.
C.
D.
–
OC6H 5 < NH3 < C2H3 O 2 – < ClO2–
C2H 3O2– < ClO2– < NH 3 < OC 6H5–
ClO 2– < C2 H3O2– < NH 3 < OC 6H5 –
NH3 < OC6 H5– < C2H 3O2– < ClO2 –
NH3
QUESTION: 4
Acid and Base Equilibria
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
pKa=4.7
2.
OH
• Weak is favored!
http://chemconnections.org/organic/Movies%20Org%20Flash/acid-base-eq.swf
CH3 COH2
+ NH3
OH
4.
HCl + CH3OH
pKa-=–7
CH3 C!CH +
5.
pKa =26
6.
CH3 CH3 +
A. All do: 1-6
+ NH 2
pKa=–6
pKa=36
HCl + CH3OH
pKa=15.5
3.
+ NH 4
pKa=9
O
O
O
CH3 C
The equilibrium favors the weaker of the acid vs. its
conjugate acid or base vs. its conjugate. Weak
wins!
CH3 C
+ NH3
CH3 C
1.
C. 2, 3, 6
Cl
D. 1, 5
+ CH3 OH2
pKa=–2.5
CH3 C!C
NH 2
B. 1, 4, 5
H–Cl–H + CH3O
pKa < –15
+
E. None do.
NH3
pKa=36
NH2
CH3 CH2
+ NH 3
pKa=36
pKa=50
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
Formal Charge / Acids & Bases
Electrophiles / Nucleophiles / Reactivity
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.
..
: O:
CH3
H3C
Base /
nucleophile
N
CH
CH3 3
H 3C
CH 3
CH3
..
O:
::
CH3
C:
H3C
CH2
C
CH3
+1 Acid /
electrophile
H
..
H
H3C O H
::
..
H3C
CH3
CH 3
: O:
::
: O:
CH3
H3C
H3C
H O:
H3 C
..
: O:
H O H
• This can be used in predicting how molecules will
react--- or donʼt react, and the products of reactions.
..
CH 3
CH 3
..
N
H3 C : :
-1
N
CH3
:O
..
H3 C O :
::
H3 C
CH3
CH2
Worksheet 1
http://chemconnections.org/organic/chem226/226assign-09.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)
 Select & explain which is the stronger acid:
HBr or HF.
Strength of Oxyacids
(Three atoms: ternary vs. binary)
Push-Pull electronic effects on the proton.
Therefore HBr is a stronger acid than HF.
Question: 6
QUESTION: 5
•
Rank 1.0M solutions of HBrO, HIO and HClO in order
of increasing acidity.
• HBrO4 is a weaker acid than HBrO2 .
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: 7
•
True (A) / False (B)
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
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: 8
• Use the concept of push-pull. Rank the
following organic acids in order of decreasing
acidity.
1) Br-CH2 COOH
A) HBrO < HIO < HClO
B) HIO < HBrO < HClO
C) HClO < HBrO < HIO
D) HIO < HClO < HBrO
2) I-CH2 COOH
• A) 2 > 1 > 3
B) 3 > 2 > 1
• C) 2 > 3 > 1
D) 1 > 2 > 3
3) CH3 COOH
QUESTION: 9
Rank the following acids in order of decreasing
acidity.
A) ClCH2COOH
C) Cl3 CCOOH
1) A > B > C > D
2) D > C > B > A
B) Cl2 CHCOOH
D) CH3 COOH
2) C > B > A > D
3) B > C > A > D
An Organic Base in Context
Erythroxylon spp.
• It is very valuable. 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 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?
Question: 10
True (A) / False (B)
Trichloroacetic acid, Cl3CCOOH, is more
acidic than trifluoroacetic acid, F3 CCOOH.
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)
CH3 ..
N
CO2CH3
O2 C
"Crack" Cocaine
What structural feature makes cocaine
a base? What simple compound can
you relate it to?
• m.p. 98 o C
• 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
“Regular” Cocaine
Conjugate Acid of Cocaine
(Physical Properties)
• m.p. >195 o C
CH3 + H Cl
Solubility:
N
CO2CH3
•
Water: 2.5 g/mL
•
CHCl3 : 0.08 g/mL
O2 C
•
Ether: insoluble
Cocaine Hydrochloride
What accounts for the differences in
solubilities of the base and conjugate
acid?
Acid -Base Reactions
CH3 ..
N
CH3
+
N
CO2CH3
+
H
Acid Base Reactions
H Cl
Cl
O2 C
-
CO2CH3
CH3
pK a = pH + log
CH3 ..
N
-
CO2CH3
OH -
CO2CH3
O2 C
O2 C
Cocaine Hydrochloride
The pH of a solution (its surroundings) determines
which and is related by the following equation.
H Cl
+
O2 C
Which form, Acid or its Conjugate Base?
+
N
"Crack" Cocaine
A weak acid
RCO2 H
with a pKa =
5.2
50% acid
50% base
[HA]
[A ]
!
• A compound will exist primarily in its acidic form if the
pH of the solution is < than its pK a
• A compound will exist primarily in its basic form if the
pH of the solution is > than its pKa
• NOTE: A buffer solution maintains a nearly constant
pH within certain parameters.
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?
99% acid
1% base
90% acid
10% base
1% acid
99% base
10% acid
90% base
Lewis Acids and Bases
• Lewis Acid: electron pair acceptor
• Lewis Base: electron pair donor
• Example:
Al3+
+6 O
H
Al
H
3+
H
O
H
6
Question: 11
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
Acid-Base Reactions
Showing a reaction with arrows
Predicting Products
O
CH3 SCH3 + BF3
CH3 + H2O
CH3 O
+ CH3 SH
(CH3)2 S–BF3
HC!C
+ H 2C=O
I
+ CH 3Br
NH3
CH3 OH + CH3S
OH
CH2 =CH2 + HCl
OH
B(CH3 )3 + (CH 3) 3N
CH3 OH2
O
CH3 CCH3 + H3O
+ HCO3
CH 3C
H2O + CH 3CCH3
HC!C–CH2–O
H 3C
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
+ MgCl 2
C O
H 3C
O
O
CH 3C
ICH3 + Br
OH
H2SO 4 + CH 3OCH3
BF3 +
CH2 –CH3 + Cl
+
+ AlCl3
OH
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
Predicting if reactions occur and the
products if they do
a.
b.
c,
HO
+ Br–CH 3
HO–Br +
HO
+ Br–CH 3
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
OCH3
Worksheet 5: Acids & Bases
http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets