the microscale laboratory Discussion The reactions shown in Figure 1are carried out in this experiment. The thermal decomposition of dicyclopentadiene introduces the theme of electrocvclic reactions, i n t h i s case a [2+41 cycloreversion. The reaction is + (CH&H20)2CHN(CH3)2 CHN(CH3)2 2CH3CHzOH particularly easy to carry out because of the low enerm requirements. The condensation reaction of the diethylacetal with cyclopentadiene illustrates the ease with which a proton may be removed from cyclopentadiene as the reagent is also a base and functions catalytically The presence of the anti-ammatic thiophene 1,l-dioxide 3 can be demonstrated in solution by UV spectrometry (2). I t reacts via a remarkable thermally allowed [6+41 cycloaddition. ~ r e s u m a b l v to eive initialfi the unisoiated iitermediate pictured. This is followed by cheletropic elimination of SO2 and elimination of dimethylamine driven by the 4 formation of t h e stable aromatic system of azulene. Figure 1. The series of reactions carried out in this experiment. The beautiful wlor is due to a low-lying excited singlet state Collect the blue band in a weighed vial, evaporate the (LUMO), compared to the isomeric naphthalene a s pentane, and weigh the product. The azulene may be reillustrated in Figure 2(3). crystallized from 95% ethanol; the melting point is 9&99 Literature Clted 'C. The yield varies with a maximum of 48% obtained so far. If a UV-vis spectrometer is available, the visible spectrum will show the following absorbtions ( in petroleum = 541 nm;log em, = 2.34; 558 ether or cyclohexane): (2.43); 580 (2,541; 603 (2.46); 632 (2.48); 661 (2.12); 697 Improvements in the Ultra-Micro (2.07).
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Boiling-Point Technique Gerald Rausch and John Tonnis University of Wiswnsin-Lacrosse Lacrosse, WI 54601
Naphthalene
Azulene
Figure 2. Molecular orbital diagram for naphthalene and azulene. A264
Journal of Chemical Education
Of all the microscale techniques, the ultra-micro boiling point is usually the most difficult for students to master. Students frequently pull 10-20 capillaries before fabricating a bell of appropriate size and straightness to enter a melting point capillary tube. In addition, the bells may be so short that they have a tendency to climb out of the liquid when heat is applied. The use of commercial micmpipets solves both problems and isvery economical. A 10-pL pipet can be cut easily into three segments that can be flame sealed to provide three bulbs (Drumond "microcaps" VWR Scientific, Catalog #53440-103). These sealed micropipet segments provide both ideal diameter and length for liquid samples of about 5uL.
