chapter 1 models of acids and bases -...
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![Page 1: Chapter 1 Models of Acids and Bases - ChemConnectionschemconnections.org/.../Presentations/Chap1-09/Ch01-acids-ques-09.pdf · Models of Acids and Bases ... positive atoms behave acid-like,](https://reader034.vdocuments.site/reader034/viewer/2022042513/5ab927987f8b9ad13d8d5c67/html5/thumbnails/1.jpg)
Acids & Bases/Organic Chemistry
Dr. Ron RusayFall 2009
Chapter 1
Structureand
Bonding
Acids andBases
Models of Acids and Bases
•Arrhenius: Acids produce H+ & basesproduce OH− ion in aqueous solutions .•Brønsted-Lowry: Acids are H+ donors &bases are proton acceptors.
•HCl + H2O → Cl− + H3O+
acid base
Comprehensive TutorialAcid & Base Principles in Organic Chemistry
Highly recommended viewing
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 istransferred to the base.•conjugate base: everything that remains ofthe acid molecule after a proton is lost.
http://chemconnections.org//organic/Movies%20Org%20Flash/ConjugateAcidBaseActivity.swf
QUESTION: 1Aniline, C6H5NH2, was isolated in the 1800s and
began immediate use in the dye industry.What is the formula of the conjugate acid ofthis base?
A. C6H5NH2+
B. C6H5NH3+
C. C6H5NH–
D. C6H5NH+
E. C6H5NH2
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 useto compare relative acidities?
• Organic bases are weak bases and relate toammonia.
• However, there can be substantial differencesin their relative strengths. What could you useto compare relative basicity?
Conjugates
Ka x Kb = ?What do pKa and pKb refer to?
pKa + pKb = ?
Kw
-log Ka or -log Kb 14
Remember: pH + pOH = pKw
Which is the stronger acid?
Which is the stronger base?
methanol
methylamine
QUESTION: 2
Use information on the table above to determine which of thefollowing bases would have the weakest conjugate acid:
OC6H5–; C2H3O2
–; OCl–; NH3
A. OC6H5–
B. C2H3O2–
C. OCl–D. NH3
QUESTION: 3
Use information on the table above to determine the order ofincreasing 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. itsconjugate acid or base vs. its conjugate. Weakwins!
http://chemconnections.org/organic/Movies%20Org%20Flash/acid-base-eq.swf
QUESTION: 4
CH3C!CH + NH2
pKa=26
CH3C!C + NH3
pKa=36
CH3CH3 + NH2
pKa=50
CH3CH2 + NH3
CH3C
OH
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
CH3C
OH
O
+ NH3
O
+ NH2
pKa=36 pKa=–6
CH3COH2
pKa < –15
Consider the following equilibria. Identify the weakerof the two: acid vs. its conjugate acid in eachreaction. 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
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 willreact--- or donʼt react, and the products of reactions.
Formal Charge / Acids & BasesElectrophiles / Nucleophiles / Reactivity
CH3
NCH
3CH
3
CH3
CH3
NCH
3
..
CH3
NCH
3
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
Structure and Acid-Base PropertiesImportant factors that effect acidity in binary
compounds, eg HCl (having only twoelements):
• Bond Length (shorter = stronger bonds;lower acidity)
• Bond Strength (longer = weaker bonds;higher acidity: more dissociation, moreprotons [hydronium ions] in solution)
• Bond Polarity (smaller e.n. differences favorhigher acidities)
Select & explain which is the stronger acid:HBr or HF.
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Therefore HBr is a stronger acid than HF.
Strength of Oxyacids(Three atoms: ternary vs. binary)
Push-Pull electronic effects on the proton.
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
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
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
QUESTION: 8• Use the concept of push-pull. Rank the
following organic acids in order of decreasingacidity.
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) Cl2CHCOOHC) Cl3CCOOH D) CH3COOH
1) A > B > C > D 2) C > B > A > D2) D > C > B > A 3) B > C > A > D
Question: 10 True (A) / False (B)
Trichloroacetic acid, Cl3CCOOH, is moreacidic than trifluoroacetic acid, F3CCOOH.
An Organic Base in ContextErythroxylon spp.
• It is very valuable. The leaves are chewed byindigenous tribes in the Andes to boost their energy.
• It has been used as a psycho-therapeutic, anopthalmic anesthetic and was purportedly used in apopular beverage that is at the heart of a $20 billioncorporation.
• However, both its base and conjugate acid arecurrently controlled substances under U.S. FederalRegulations: Title 21 secs. 329.1 & 1308.12 (1987).
• Can you name the beverage and the base?
The beverage reportedly produced usingthe 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)
NCO
2CH
3
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 cocainea base? What simple compound canyou relate it to?
“Regular” Cocaine Conjugate Acid of Cocaine
(Physical Properties)
CH3 NCO2CH3
O2C
H Cl -
Cocaine Hydrochloride
+• m.p. >195 oCSolubility:• Water: 2.5 g/mL
• CHCl3: 0.08 g/mL
• Ether: insoluble
What accounts for the differences insolubilities of the base and conjugateacid?
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Acid -Base Reactions
NCO
2CH
3
O2C
CH3
+ H Cl
..
CH3
NCO2CH3
O2C
H Cl -
Cocaine Hydrochloride
+
Acid Base Reactions
CH3
NCO
2CH
3
O2C
H Cl -
+
+ OH
-
..
"Crack" Cocaine
CH3
NCO
2CH
3
O2C
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) determineswhich and is related by the following equation.
A weak acidRCO2Hwith a pKa =5.2
99% acid1% base
90% acid10% base
50% acid50% base
10% acid90% base
1% acid99% base
QUESTIONS from Worksheet:
The pKa of a general anesthetic, sodium pentothal, is 7.4. If apatient is given sodium pentothal orally instead of iv, will it putthe patient to sleep?
What information is needed to answer this fundamentalanesthesiology (“gas passer”) question?
A drug has a pKa of 7.8 and is a known teratogen. If given ivto 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
H
O
H
Al O
H
H6
3+
![Page 7: Chapter 1 Models of Acids and Bases - ChemConnectionschemconnections.org/.../Presentations/Chap1-09/Ch01-acids-ques-09.pdf · Models of Acids and Bases ... positive atoms behave acid-like,](https://reader034.vdocuments.site/reader034/viewer/2022042513/5ab927987f8b9ad13d8d5c67/html5/thumbnails/7.jpg)
Question: 11
Select the chemical equation that depictsthe correct movement of electrons in thereactionof ammonia with hydrogenchloride.
A) B)
C) D)
Acid-Base ReactionsShowing a reaction with arrows
CH3SCH3 + BF3 (CH3)2S–BF3
CH3 + H2O
O
CH3CCH3 + H3O
HC!C + H2C=O HC!C–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
Acid-Base ReactionsPredicting Products
O
CH3COH
+ HCO3
O
CH3COH
+ AlCl3
B(CH3)3 + (CH3)3N
+ OH
C O
H3C
H3C
+ MgCl2
H2SO4 + CH3OCH3
BF3 + O
NH3
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets
![Page 8: Chapter 1 Models of Acids and Bases - ChemConnectionschemconnections.org/.../Presentations/Chap1-09/Ch01-acids-ques-09.pdf · Models of Acids and Bases ... positive atoms behave acid-like,](https://reader034.vdocuments.site/reader034/viewer/2022042513/5ab927987f8b9ad13d8d5c67/html5/thumbnails/8.jpg)
Predicting if reactions occur and theproducts if they do
a. HO + Br–CH3 HO–Br + CH3
HO–CH3 + Br
b. H–C!C + H2C=O H–C!C–CH2-O
H–C!C–O–CH2
NH2 + H–OCH3 H2N–OCH3 + H
H2N–H + OCH3
HO + Br–CH3
c,
H–C!C + H2C=O
NH2 + H–OCH3
Worksheet 5: Acids & Baseshttp://chemconnections.org/organic/chem226/226assign-09.html#Worksheets