Models of Acids and Bases
Chapter 1
Structure
and
Bonding
•Arrhenius: Acids produce H+ & bases
produce OH− ion in aqueous solutions .
•Brønsted-Lowry: Acids are H+ donors &
bases are proton acceptors.
Acids & Bases/
Organic Chemistry
Acids and
Bases
Dr. Ron Rusay
Fall 2008
•HCl + H2O → Cl− + H3O+
acid
QUESTION: 1.1a
Acid-Base Equilibrium
Conjugate Acid/Base Pairs
•HA(aq) + H2O(l) →
acid 1
H3O+(aq)
+
conj
acid 2
base 2
A−(aq)
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.
• Weak acids are only partially ionized in solution.
HA(aq)
Ka =
[H 3O + ][A - ]
[HA]
H3O+(aq) + A-(aq)
H+(aq) + A-(aq)
or
Ka =
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?
1.
2.
3.
4.
C6H5NH2+
C6H5NH3+
C6H5NH–
C6H5NH+
Organic Acids & Bases
Weak Acids
HA(aq) + H2O(l)
base
[H + ][A - ]
[HA]
• Ka is the acid dissociation constant.
• 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
Kw
K a x Kb = ?
What do pKa and pKb refer to? -log Ka or -log Kb
14
pKa + pKb = ?
Remember: pH + pOH = pKw
QUESTION: 1.2a
Which is the stronger acid?
methanol
Use information on this table to determine which of the following bases
would have the weakest conjugate acid:
OC6H5–; C2H3O2–;
Which is the stronger base?
methylamine
1.
2.
3.
4.
OCl–;
NH3
–
OC6H5
C 2H3O2–
OCl–
NH3
QUESTION: 1.3a
Acid and Base Equilibria
Consider the following equilibrium reactions. Identify the
weaker of the two: acid vs. its conjugate acid in each
reaction. Which reactions favor formation of product?
O
O
A
B
+ NH3
CH3 C
pKa=4.7
OH
The equilibrium favors the weaker of the acid vs. its
conjugate acid or base vs. its conjugate. Weak
wins!
• Weak is favored!
D
+ NH4
pK a=9
O
O
O
CH3 C
C
CH3 C
OH
+ NH3
pKa=36
HCl + CH3OH
pK a=15.5
HCl + CH3OH
p Ka-=–7
CH3 C≡CH +
E
pKa =26
F
CH3 CH3 +
pKa=50
NH 2
CH3 COH2
+ NH2
pK a=–6
2) A, C, D
H–Cl–H + CH3O
pKa < –15
3) B, C, F
Cl
4) A, E
+ CH3 OH2
pKa=–2.5
CH3 C≡C
+
NH3
pKa=36
NH2
1) All do: A-F
CH3 CH2
+ NH3
pKa=36
5) None do.
Formal Charge = # of valence e-s [1/2(# of bonding e-s ) + # of non-bonding e-s ]
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
N
CH3
CH3
For N:
= 5-[1/2(4)+4]
= -1
Base
H3C
N
..
N
H3 C : :
CH 3
CH3
..
O:
::
CH3
C:
H 3C
CH2
C
CH3
..
H
H 3C O H
::
..
H3C
CH3
CH 3
: O:
::
: O:
CH3
H3C
H3C
H O:
H3C
..
: O:
H O H
• This can be used in predicting how molecules will
react--- or don’t react, and the products of reactions.
..
CH3
CH 3
CH 3
:O
..
H3 C O:
::
H3C
H
For O:
= 6-[1/2(6)+2]
= +1
CH3
CH2
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.
Acid
Question: 1.5a
QUESTION: 1.4a
•
• HBrO4 is a weaker acid than HBrO2.
HBrO , K a = 2.1 x 10-8
HIO , K a = 2.3 x 10-11
HClO , Ka = 3.0 x 10-8
•
•
•
•
1) HBrO < HIO < HClO
2) HIO < HBrO < HClO
3) HClO < HBrO < HIO
4) HIO < HClO < HBrO
QUESTION: 1.6a
•
True (A) / False (B)
Rank 1.0M solutions of HBrO, HIO and HClO in order
of increasing acidity.
Rank 1.0M solutions of HBrO, HIO and HClO in order
of increasing pH
1) HBrO < HIO < HClO
2) HIO < HBrO < HClO
3) HClO < HBrO < HIO
4) HIO < HClO < HBrO
QUESTION: 1.8a
Rank the following acids in order of decreasing
acidity.
1) A > B > C > D
2) D > C > B > A
QUESTION: 1.7a
• Use the concept of push-pull. Rank the
following organic acids in order of decreasing
acidity.
A) Br-CH2COOH
HBrO , K a = 2.1 x 10-8
HIO , Ka = 2.3 x 10-11
HClO , Ka = 3.0 x 10-8
A) ClCH2COOH
C) Cl3CCOOH
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
B) Cl2CHCOOH
D) CH3COOH
2) C > B > A > D
3) B > C > A > D
• 1) A > B > C
B) I-CH2COOH
2) C > B > A
C) CH3COOH
3) B > C > A
Question: 1.9a
True (A) / False (B)
Trichloroacetic acid, Cl3CCOOH, is more
acidic than trifluoroacetic acid, F3CCOOH.
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?
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….
“Regular” Cocaine
Acid -Base Chemistry
(Physical Properties)
CH3 ..
N
CO2CH3
O2 C
• m.p. 98 oC
• b.p. (very volatile
> 90 oC)
Solubility:
• Water: 1.67 x 10-3
g/mL
"Crack" Cocaine
• 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?
Conjugate Acid of Cocaine
(Physical Properties)
• m.p. >195 oC
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
+
O2 C
+
N
CO2CH3
H
Cl
Acid Base Reactions
H Cl
-
CO2CH3
O2 C
Cocaine Hydrochloride
CH3
+
N
H Cl
CH3 ..
N
-
CO2CH3
+
O2 C
OH -
CO2CH3
O2 C
"Crack" Cocaine
Which form, Acid or its Conjugate Base?
The pH of the solution (surroundings) determines it.
pK a = pH + log
A weak
acid with a
pKa = 5.2
[HA]
[A ]
−
• A compound will exist primarily in its acidic form if the
pH is < than its pKa
• A compound will exist primarily in its basic form if the
pH 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 common “gas
passer” question?
Lewis Acids and Bases
• Lewis Acid: electron pair acceptor
• Lewis Base: electron pair donor
• Example:
A drug has a pKa of 7.8 and is a known teratogen. If given iv
to a pregnant woman whose blood pH is 7.4, will this drug
cross the placenta and affect the baby?
Al3+
+6 O
H
Al
O
H
What information is needed to answer this common “gas
passer” question?
3+
H
H
6
Question: 1.10a
Select the chemical equation that depicts
the correct movement of electrons in the
reaction of 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)2 S–BF 3
OH
H 3C
CH3 CCH3 + H3O
H2O + CH 3CCH3
HC≡C
HC≡C–CH2–O
CH2 =CH2 + HCl
I
+ CH3Br
ICH3 + Br
b.
c,
+ Br–CH3
HO–Br +
HO
+ Br–CH3
HO–CH3 + Br
CH3
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
O
O
CH 3C
+ AlCl3
OH
OH
+ MgCl2
C O
H 3C
BF3 +
CH2 –CH3 + Cl
HO
+
H2SO 4 + CH 3OCH3
Predicting if reactions occur and the
products if they do
a.
NH3
CH3 OH + CH3S
O
+ H 2C=O
OH
B(CH3 )3 + (CH 3) 3N
CH3 OH2
+ CH3 SH
+ HCO 3
CH 3C