acids & bases/ organic chemistry
DESCRIPTION
Chapter 1 Structure and Bonding Acids and Bases. Acids & Bases/ Organic Chemistry. Dr. Ron Rusay Fall 2009. 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. - PowerPoint PPT PresentationTRANSCRIPT
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Acids & Bases/ Organic Chemistry
Dr. Ron Rusay
Fall 2009
Chapter 1
Structure and
Bonding
Acids and Bases
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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
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Comprehensive TutorialAcid & Base Principles in Organic Chemistry
Highly recommended viewing
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Acid-Base EquilibriumConjugate Acid/Base Pairs
•HA(aq) + H2O(l) H3O+(aq) + A(aq)conj conj
acid 1 base 2 acid 2 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
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QUESTION: 1Aniline, 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. C6H5NH2+
B. C6H5NH3+
C. C6H5NH–
D. C6H5NH+
E. C6H5NH2
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Weak AcidsWeak Acids
• Weak acids are only partially ionized in solution.
or
• Ka is the acid dissociation constant.
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
HA(aq) H+(aq) + A-(aq)
[HA]
]][AO[H -3
aK
[HA]]][A[H -
aK
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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?
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Conjugates
Ka x Kb = ?
What do pKa and pKb refer to?pKa + pKb = ?
Kw
-log Ka or -log Kb
14
Remember: pH + pOH = pKw
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Which is the stronger acid?
Which is the stronger base?
methanol
methylamine
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QUESTION: 2
Use information on the table above to determine which of the following bases would have the weakest conjugate acid:
OC6H5–; C2H3O2
–; OCl–; NH3
A. OC6H5–
B. C2H3O2–
C. OCl– D. NH3
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QUESTION: 3
Use information on the table above to determine the order of increasing base strength for the following bases:
OC6H5–; C2H3O2
–; ClO2–; NH3
A. OC6H5– < NH3 < C2H3O2
– < ClO2–
B. C2H3O2– < ClO2
– < NH3 < OC6H5–
C. ClO2– < C2H3O2
– < NH3 < OC6H5–
D. NH3 < OC6H5– < C2H3O2
– < ClO2–
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Acid and Base Equilibria
• Weak is favored!
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
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QUESTION: 4
CH3CCH + NH2
pKa=26
CH3CC + NH3
pKa=36
CH3CH3 + NH2
pKa=50
CH3CH2 + NH3
CH3COH
O
+ NH3 CH3CO
O
+ NH4
HCl + CH3OH pKa=15.5
H–Cl–H + CH3O
HCl + CH3OH Cl + CH3OH2
pKa=36
pKa=4.7
pKa-=–7 pKa=–2.5
pKa=9
CH3COH
O
+ NH3
O
+ NH2
pKa=36 pKa=–6
CH3COH2
pKa < –15
Consider the following equilibria. Identify the weaker of the two: acid vs. its conjugate acid in each reaction. Which reactions favor formation of product?
1.
2.
3.
4.
5.
6.
A. All do: 1-6
B. 1, 4, 5
C. 2, 3, 6
D. 1, 5
E. None do.
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
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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.
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Formal Charge / Acids & BasesElectrophiles / Nucleophiles / Reactivity
CH3
NCH3CH3
CH3
CH3
NCH3
..
CH3 NCH3
CH3
O..
: :
:CH3
O
O
: :..
::::
:H O
..
::
CH3
CH3
CH3 C
CH3
CH3
CH3 C:CH3
CH3
CH3
O: :..
:H O H
H
:CH3 O H
CH3
..
CH3
O
CH2
: :..
:CH3 O
..
::
CH3
O
CH3
H:
CH2
-1
+1
Base /nucleophile
Acid / electrophile
Worksheet 1http://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
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Structure and Acid-Base PropertiesImportant 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.
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Therefore HBr is a stronger acid than HF.
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Strength of Oxyacids(Three atoms: ternary vs. binary)
Push-Pull electronic effects on the proton.
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QUESTION: 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
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Question: 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
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QUESTION: 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
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QUESTION: 8• Use the concept of push-pull. Rank the
following organic acids in order of decreasing acidity.
1) Br-CH2COOH 2) I-CH2COOH 3) CH3COOH
• A) 2 > 1 > 3 B) 3 > 2 > 1
• C) 2 > 3 > 1 D) 1 > 2 > 3
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QUESTION: 9Rank the following acids in order of decreasing
acidity.A) ClCH2COOH B) Cl2CHCOOH
C) Cl3CCOOH D) CH3COOH
1) A > B > C > D 2) C > B > A > D
2) D > C > B > A 3) B > C > A > D
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Question: 10 True (A) / False (B)
Trichloroacetic acid, Cl3CCOOH, is more acidic than trifluoroacetic acid, F3CCOOH.
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An Organic Base in ContextErythroxylon 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?
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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….
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Acid -Base Chemistry(Physical Properties)
NCO2CH3
O2C
CH3..
"Crack" Cocaine
• 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?
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“Regular” Cocaine
Conjugate Acid of Cocaine(Physical Properties)
CH3 NCO2CH3
O2C
H Cl -
Cocaine Hydrochloride
+• 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?
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Acid -Base Reactions N
CO2CH3
O2C
CH3
+ H Cl
..
CH3N
CO2CH3
O2C
H Cl -
Cocaine Hydrochloride
+
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Acid Base Reactions CH3
NCO2CH3
O2C
H Cl -+
+ OH -
..
"Crack" Cocaine
CH3
NCO2CH3
O2C
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Which form, Acid or its Conjugate Base?
• A compound will exist primarily in its acidic form if the pH of the solution is < than its pKa
• 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.
A
HAlogpHp aK
The pH of a solution (its surroundings) determines which and is related by the following equation.
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A weak acid RCO2Hwith a pKa = 5.2
99% acid1% base
90% acid10% base
50% acid50% base
10% acid90% base
1% acid99% base
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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?
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Lewis Acids and Bases
• Lewis Acid: electron pair acceptor
• Lewis Base: electron pair donor
• Example:
Al3+ + 6H
OH
Al OH
H6
3+
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Question: 11
Select the chemical equation that depicts the correct movement of electrons in the reactionof ammonia with hydrogen chloride.
A) B)
C) D)
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Acid-Base ReactionsShowing a reaction with arrows
CH3SCH3 + BF3 (CH3)2S–BF3
CH3 + H2O
O
CH3CCH3 + H3O
HCC + H2C=O HCC–CH2–O
I + CH3Br ICH3 + Br
CH3O + CH3SH CH3OH + CH3S
H2O + CH3CCH3
OH
CH2=CH2 + HCl CH2–CH3 + Cl
CH3 OH2
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
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Acid-Base ReactionsPredicting Products
O
CH3COH
+ HCO3
O
CH3COH
+ AlCl3
B(CH3)3 + (CH3)3N
+ OH
C OH3C
H3C+ MgCl2
H2SO4 + CH3OCH3
BF3 + O
NH3
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
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Predicting if reactions occur and the products if they do
a. HO + Br–CH3 HO–Br + CH3
HO–CH3 + Br
b. H–CC + H2C=O H–CC–CH2-O
H–CC–O–CH2
NH2 + H–OCH3 H2N–OCH3 + H
H2N–H + OCH3
HO + Br–CH3
c,
H–CC + H2C=O
NH2 + H–OCH3
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets