JOHN J. ALEXANDER University of Cincinnati Cincinnati. 45221
Organic Laboratory
photochemistry can be rationalized on the basis of one species: the zwitterion. Students must recognize t h a t the protonated zwitterion is more likely to react with a n external nucleophile (solvent) t h a n with the internal center of negative charge on t h e zwitterion. Moreover, a n answer to this question requires some facility with electron pushing.
R o n a l d S. Lenox Wabash College Crawfordsuille, Indiana 47933 Lahorntory courses in organic chemistry at the sophomore h r l may appn)pr~;~tely emphasize draw in^ scientific or ju~~itw conclusions from experimental observations. ~ h i question i tests both the student's ability t o do this as well as his or her knowledge of ideal and non-ideal behavior of liquids.
Question
Question:
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A chemist finds three bottles marked X. Y and Z.. resnectively, in the laboratory, each of which contains a liquid. T h e boiling point of all three liquids is 134OC a t ~ r e v a i l i n elahoratory pressure.
How can one rationalize the difference in the photochemistry of Santonin (compound (I)) when this compound is photolyzed in neutral solvent a s ethanol contrasted with its photolysis in acidic media a s aqueous acetic acid? Give structures and reaction mechanisms to rationalize the difference; a few words are also needed.
(a) The chemist mixes some of the contents of bottle X and bottle Y. Upon performing a distillation, he discovers that the initial boiling point of the mixture is 120'. Are liquids X and Y identical compounds? Why? (h) After mixing liquids X and Z, the chemist finds that upon distillation the entire mixture boils at 134'C. Can X and Z be identical compounds? Can they be different compounds? (c) Made increasingly curious by the above results, the chemist distills a mixture of Y andZand finds the boilingpoint of this mixture to be 134% What statements can the chemist now make about the identities of liquids X, Y and Z? Answer
Obviously, X and Y are not identieal. X and Y could be miscible and form a minimum boiling point azeotrope. The two liquids could also be immiscible and the lowered boiling paint is the result of a two ohase distillation. ~hX , and % ,nu\ or mat nor be identical. If rhry are dilirrent rompounds, the hding pvint of the mixture wvuld he 131°C if the twr, vmnpuunds formed an deal mixture and Haoult's 1 . d ~i~ dwywl. IIwever, i t i.i sppnrent that rompwnd Znnd compound Y are not the iamp nmpound If they wew, [he bdin:: pmnr ufn mirturec,~X and j! would be the.iamens that ohserved for X and Y ~ I ~ I I Y ' , . (c) We already know (see (b) above) that Y and Zare not identical compounds. The fact that a mixture of Y and Z boils at 1 3 4 T tells us that X and Z are not identical compounds; if they were, the boiling point would be 120°C. Thus, the chemist can conclude that he has three different liquids. (a)
Answer
In the photochemistry of dienones, the Zwitterion Z was postulated in 1961by Zimmerman and Schuster' to be involved.They suggested that in acidic media thezwitterion is protonated. Thepinacolic type (or cycl0propy1carbinyl)rearrangement shown leading to (11) is of the type studied by Roberts in ground state chemistry.
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Zimmerman, H. E., and Sehuster, D. I., J . Amer. Chem. Soc., 83, 4486 (1961).
Organic Photochemistry H. E. Zimmerman
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Uniuersity of Wisconsin Madison, Wisconsin 53706 Organic photochemistry is a t o ~ i cintroduced with increasing frequency into the sophomore course. T h e following question, which undermaduates a t Wisconsin have been able to handle, tests their ability to remember that most of dienone
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