In the Classroom
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Using Building-Block Puzzles To Practice Drawing Organic Mechanisms Ender Erdik Department of Chemistry, Ankara University, Besevler, Ankara 06100 Turkey;
[email protected] Reaction mechanisms in organic chemistry are indispensable because they provide a framework on which a vast range of reactions can be predicted, employing chemical principles and limits (1). In organic chemistry courses, students learn a given reaction more easily if its mechanism has been presented in an earlier reaction. However, success in writing feasible mechanisms for organic reactions depends on the extent of the student’s reasoning ability and critical thinking, as well as his or her preparation. After a one-year organic chemistry course, many students still have difficulty in writing a mechanism because they write reactants (starting materials), transition states, and products, but do not consider the bond changes and electron flow in the mechanism (2). However, if a student understands the binding of organic groups and functional groups in the transition states, he or she will recognize that a reasonable reaction mechanism can be written. As a part of my effort to teach organic reaction mechanisms, I provide examples (3, 4) to help the students become familiar with the concepts of molecular structure and reactivity so they can find the types of nucleophiles, electrophiles, or radicals (5) in the reactants, determine the electron flow in the transition states, and then, by critical thinking, devise possible mechanistic schemes leading to products. I created a thought-provoking, pencil-and-paper activity to aid students in writing organic reaction mechanisms. In this activity, organic groups and functional groups that
constitute the formulas of organic and inorganic reactants, ionic intermediates, and products are presented as building blocks. To discover the mechanism, the atoms and atom groups must be placed correctly in the puzzle so that they bind properly to form the right formulas of the reactants, intermediates, and products in the reactions. I designed the puzzles in the text (reaction A) and in the Supplemental MaterialW (reactions B and C) to test the ability of students in writing the mechanisms of (i) reactions of ketone-derived carbanions (ketone enolates) with electrophiles and (ii) reactions of ketones with carbanions. Reactions A and B are typical examples for (i), and reactions C and A are examples for (ii). Acetone is used as the ketone in all reactions. An undergraduate student who is successful in organic chemistry at the sophomore level is expected to write reactions with reactants other than acetone and the corresponding intermediates and products of these reactions in the building-block design. This activity is instructional for students, in addition to being fun. Building-Block Design for Reaction of AcetoneDerived Carbanion with an Electrophile Students are given the puzzle diagram and the building blocks (Figure 1). They are told that acetone reacts with an electrophile to produce a product via an aldol reaction. The
Puzzle
Building Blocks
CH3
ⴚ CH3
9
CH2
CH 2
1
ⴚ
5
CH2
1
ⴚ ⴚ
C
O
C
3
OH 4
1 ⴚ
OH
C
Heat
H 1
2
2
6
O 6
ⴚ
ⴚ
Figure 1. Reaction of acetone with an electrophilic reagent: (left) puzzle diagram and (right) building blocks.
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In the Classroom
CH3 ⴚ
ⴚ
OH
CH3
CH3
CH2
C
CH3
O
C
O
C
CH3
ⴚ
C
CH3 CH3
O
O CH2
ⴚ
H
H
OH
CH3
OH CH3
ⴚ
C
CH3
C
CH3 Heat
OH
CH3
CH2
ⴚ
C
C CH
OH CH3
O
O
ⴚ
H
OH CH3
C
O
Figure 2. Solution to the puzzle shown in Figure 1.
students are also given the following information: • The number of times each block is used in the puzzle is given on the lower right side of the block. • Aqueous hydroxide ion is used to produce the carbanion from acetone, that is, acetone enolate. • Acetone, acetone enolate, and the other reactive intermediates maintain their geometry throughout the reaction schemes.
The solution to the puzzle is given in Figure 2. This activity can be used in-class or as homework. It can also be used as a test.
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Supplemental Material
Two additional reactions and instructor’s notes are available in this issue of JCE Online. Literature Cited 1. Sykes, P. A Guidebook to Mechanisms in Organic Chemistry, 5th ed.; Longman: New York, 1982. 2. Scudder, P. H. Electron Flow in Organic Chemistry; Wiley: New York, 1992. 3. Groutas, W. C. Organic Reaction Mechanisms. Selected Problems and Solutions; Wiley: New York, 2000, 4. Miller, A. Writing Reaction Mechanisms in Organic Chemistry; Academic Press: New York, 1992. 5. Grossman, R. B. The Art of Writing Reasonable Organic Reaction Mechanisms; Springer: New York, 1999; p 100.
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