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Developing Critical Thinking Problems for Organic
Chemistry
Ray A. Gross, Jr.
Prince George’s Community College
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January 10, 2007
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Thoughts About Students
• They do not spend enough time working problems.
• They need to be taught (learn) how to reason.
• One way to force them to reason is to give them problems they have not seen before.
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Nature of the Problems
• Focused on developing reasoning skills• Solvable by applying course content • The amount of content is limited
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Kinds of Problems Available
• Spectroscopy
• Synthesis
• Degradation*
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Concept of Degradation
A compound of unknown structure is broken down into smaller pieces, which can be reassembled in a logical way to determine the structure.
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Example of Ozonolysis
+CH3CH2CCH3
O
CH3CH2CH
O
Xunknownhydrocarbon
(1) O3
(2) Zn/H+
remove oxygen
CH3CH2CCH3+ CH3CH2CH
anathons
CH3CH2CCH3
HCCH2CH3
analyte
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O3
(R)
C7H14C4H8O C3H6Oketone aldehyde
+
+
C4H8C3H6
anathons
analyte
Cleavage of a double bond yields two carbonyl compounds,aldehydes or ketones.
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Some Aldehydes may be identified by their molecular formulas.
C
O
H HCH2O = C
H H
molecularformula
structuralformula
anathon
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=
molecularformula
structuralformula
anathon
C2H4OC
O
CH3 HC
CH3 H
Some Aldehydes may be identified by their molecular formulas.
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Consider the following reaction.
C3H6
O3
(R)CH2O + C2H4O
Given only molecular formulas.
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Anathons are like building blocks.
C3H6
O3
(R)CH2O + C2H4O
CH H
O+
CH3CH
O
CH H CH3CH
+
CH
HC
H
CH3
propene
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+CH3CH2CCH3
O
CH3CH2CH
O
(1) O3
(2) Ag2O
(1) O3
(2) Zn/H+
CH3CH2CCH3
O
CH3CH2COH
O
+
ketone
ketone
aldehyde
acidCH3CH2C CHCH2CH3
CH3
Ketones remain ketonesAldehydes are oxidized to acids
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(1) O3
(2) Ag2O
(1) O3
(2) Zn/H+
+C7H14
C4H8O C3H6O2
C4H8O C3H6O+
Same reactions using molecular formulas.
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Identify aldehydes and ketones.
(1) O3
(2) Ag2O
(1) O3
(2) Zn/H+
+C7H14
C4H8O C3H6O2
C4H8O C3H6O+
no change
ketone
plus 1 O
aldehyde
Result: A four-carbon ketone and a three-carbon aldehyde.
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Place C atoms in logical locations to find anathons.
Connection of anathons reveals the unknown’s structure.
C7H14C4H8O C3H6O
ketone aldehyde
anathons
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Concept
• Degradation of an unknown leads to products.
• The structure of each degradation product can be determined directly or indirectly from the two kinds of ozonolysis.
• The products can then be converted into anathons, which are connected in a logical way to produce the starting compound.
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Producing Solvable Problems
• Find structures that are deducible from the two kinds of ozonolysis reactions.
• Convert them into anathons.• Join anathons to make unknowns.
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Structures from Molecular Formulas
CH2O CO2 H C H
O
C2H4O C2H4O2 CH3CH
O
C3H4O C3H4OO
(R) (O) R product
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Remove Oxygen to form Anathons
CH2O CO2 H C H
O
C2H4O C2H4O2 CH3CH
O
C3H4O C3H4OO
(R) (O) R product Anathon
H C H
CH3CH
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Join Anathons to Create Unknowns
Anathon
H C H
CH3CH
H2C CHCH3
H2C
CH3CH
Unknown
The process ensures solvable structures are produced.
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Internal Anathons
• When a molecule is cleaved in two places, the middle compound produces an internal anathon.
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Example of an Internal Anathon
H2C CHCH2CH=CHCH3
O3 (O)
O3 (R) CH2O + C3H4O2 + C2H4O
CO2 + C3H4O4 + C2H4O2
HCH HCCH2CH CH3CH
terminal anathon
terminal anathon
internal anathon
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Carbon-Carbon Triple Bonds
• Ozonolysis by either workup gives two carboxylic acids.
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Cleavage of a Triple Bond
CH3C CCH2CH3
CH3CO2H + CH3CH2CO2H
CH3CO2H + CH3CH2CO2HO3 (O)
O3 (R)C5H8
A triple bond in the analyte leads to two new hydrogen atoms and four new oxygen atoms.
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O3 (R)C5H8 C2H4O2 C3H6O2+
2 new H atoms = triple bond two caboxylic acids
CH3C CCH2CH3
Anathons from triple bonds have three bonding sites, andthose from double bonds have two.
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Anathons Readily Determined by their Ozonolysis Products
• 11 Terminal Anathons
• 11 Interior Anathons
• 3 Either Terminal or Interior
306 unique unknown hydrocarbons can be producedfrom the 22 anathons. All of them are solvable by the techniques shown.
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Example of Terminal/Interior Anathon
terminal anathon
A secondterminal anathon is bonded here.
interior anathon
Threeterminal anathons are bonded as indicated.
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Examples of the Unknowns
CH3CH2CH CHCH2CH C(CH3)2
CH3CH2CH C CCH3
CH2CH3
CHC CCH2CH3
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Problems Created that:
• Students have never seen before• Require a minimum amount of course content• Require students to apply logic and reasoning
(synthesis, analysis and evaluation)• Increase in complexity over the semester• Allow the instructor to give each student a
unique problem each week
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The approach described might be of use in other chemistry courses or disciplines.
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Problems of this type are available in the following references.
Chemical Educator 2006, 11, 372-377.
Journal of Chemical Education 2006, 83, 604-609.