overhecld projector demon~trcltion~ A Model for Valence Shell Electron-Pair Repulsion Theory Bruce R. Prall Marian College Fond du Lac. Wi 54935
Valence shell electron-pair r e ~ u l s i o n(VSEPR) theory enables one to predict t h e arra&ment of atoms covalently bonded in apolyatomicmolecule. VSEPR theory is based on the hypothesis that electron pairs associated with the central atom of a covalent molecule will arrange themselves so as to minimize electrostatic repulsion. ~ i i i m u mrepulsion results when the various electron pairs assume positions of greatest poasihle separation from eichother. Since likemagnetic poles also arrange themselves so as to minimize their renuliion. ~~-~~~ . thev ~" serve-as excellent models to demonstrate ~ E P theory. R The followine demonstrations utilize this concept to demonstrate the geometric shapes of planar complexes and molecules. &~
edited by DORISKOLB Bradley University Peoria, IL 61625
hybridized complexes, sueh as [PtC4I2- and [Cu(NH&12+.The other is achieved by carefully placing one of the magnekork units in the center of the trigonal planar arrangement as described in procedure (B). Procedure (D) Randomly placing five or more cork-magnet units on the water in the culture dish will also result in the cork-magnet units orienting themselves in svmmetrical arrangements. These orientations, however, ma; not serve as relevant models of chemical svstems. The two-dimensional nature of the liquid surface limits the demonstration to molecular arrangements that lie within a single plane.
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Materials Overhead projector Culture dish (2W-mm diameter X 80-mm height) available from Fisher Scientific (or a 2-L beaker or any other round clear glass container) Six no. 10 one-hole corks (hole diameter %in.) Sixmagnetic stirringbars of approximatelyequal magneticstrength (S/~sx 1% in.) Glycerine Preparation Fill the cultwe dish with water to a depth of approximately 6.0 cm. and center it on the staee of the overhead nroiedor. Next place elvberine~ on one of,the mainetic stirrine aid insert it snuelv - .~ " ~ ~ bars:~ ~ into the bottom of acork until half of it ;sexpnred. Repeat the process fur the remaining five mapets and corks, making certain that thesame poleofeach magnet isstirking out the bottom of the cork. ~
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Procedure (A) Randomly place two of the cork-magnet units on the water in the culture dish with the protruding magnet pointing down. The two like magnetic fields repeleachother, resulting in the two corks orientine themselves at o ~ w s i t sides e of the culture dish. This linear orientation is observed insp-hybridized molecules and complexes such as HgCh and [Ag(NH3)21t. Procedure (6) Randomly place three of the cork-magnet units on the water in the culture dish with the protruding part of the magnet pointing down as indicated in procedure (A).The three Like magnetic fields repel each other, resulting in the three corks orienting themselves in the shape of a triangle. This trigonal planar orientation is observed in sp2-hybridized molecules and complexes sueh as [HgCI3]- and BFs. Procedure ( C ) Randomlv dace four of the cork-magnet units on the water in the culture dish with the protruding part the magnet pointing down aa indicated in ormedure (A). fields , ~In ~this , svstem the maenetic " repel each other, resulting in one of two possible symmetrical orientations, one being the square planar arrangement exhibited by dsp2-
Reaction of Bromine with Hydrocarbons on the Overhead, Real or Simulated Sally Solomon, Mlchael Gregory, Sandeep Padmanabhan, and Kurt Smlth Drexel University Phiiadeiphia, PA 19104
An effective way to introduce the structure of benzene is to have students predict the chemical ~ron .e r t i e ofs benzene from considering-its molecular formula. T o do this, the lecturer performs a demonstration on the overhead nroiector in which bromine is added first to cyclohexane a& c;clohexene, then to benzene. Unfortunately, without adequate ventilation this demonstration is not practical. Here we describe a simulation that looks like the addition of bromine to hvdrocarbons but is not.' ~~~~
~ ~ Procedure The bromine is simulated using a mixture of food colorings, the nonreacting liquid hydrocarbons with water and the cyclohexene with bleach. A reasonable simulation of bromine can be prepared by adding 7 drops of red food coloring and 3 drops of yellow food coloring to 10 mL water. If you try to make the color deeper and more "brominelike", the decolorization hy bleach will be too slow. Bottles laheled "cyclohexane" and "benzene" are filled with water. The bottle of "cyclohexene" contains sodium hypochlorite solution, ordinary bleach. To perform the demonstration place three beakers on the overhead projector on a blank transparency slide. Near each beaker write the molecular formula of the hydrocarbon it contains, leaving room for writing the reactions if desired. Include the structural formulas of cyclohexane and cyclohexene, but not benzene (since this is what students are supposed to be thinking about.) Place 20 mL of "simulated cyclohexane" (water), "simulated cyclohexene" (bleach),and "simulated benzene" (water)in three beakers. Discuss the reaction of cyclohexane with bromine, then add a few drops of the "bromine" solution. Students note that the reddish color persists. As the drops of food dye are added to cyclobenene, the red color vanishes, appearing as it would if the dye really were bromine and the liquid cyclohexene. Since the absorptionof food coloring by
of
' Solomon, Sally "Overhead Projector Demonstrations"; present-
ed at the 199th Natlonal ACS Meeting. Boston, MA, April 1990.
Volume 67
Number 11 November 1990
981
