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 t o predict t h e arr&ment of atoms covalently honded in a ~ o l v a t o m imolecule. c VSEPR theory is based on the hypothe&that electron pairs associated &th the central atom of a covalent molecule will arrange themselves so as t o minimize electrostatic repulsion. ~ i i i m u mrepulsion results when t h e 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 t o demonstrate V ~ E P Rtheory. T h e followine demonstrations utilize this concept to demonstrate the geometric shapes of planar complexes and molecules.
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 stirring hars of approximately equal magneticstrength (S/~sx 1% in.) Glycerine Preparation Fill the culture dish with water to a depth of approximately 6.0 cm, and center it on the stage of the overhead projector. Next place glycerine on one of the magnetic stirring hars, and insert it snugly into the bottom of a cork until half of it is exposed. Repeat the process for the remaining five magnets and corks, making certain that the same pole of each magnet is sticking out the bottom of the cork. Procedure (A) Randomly place two of the cork-magnet units on the water in the culture dish with the protruding magnet pointing down. The twolike magnetic fields repeleachorher, resulting in the twn corks orientine themselvesat o ~ ~ o s isides t e of the culturedish. This linear orientation is observed ssp-hybridized molecules and complexes such as HgCh and [Ag(NH<. 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 spz-hybridized molecules and complexes such as [HgC$]- 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). . ~In~this . svstem the maenetic fields repel each other, resulting in one of two possihle symmetrical orientations, one being thesquare planar arrangement exhibited by dsp2-
edited by DORISKOLB Bradley University Peoria, IL 61625
hybridized complexes, such as [PtClr12-and [CUINH-~)~]~'. The other is achieved by carefully placing one of the magnet-rork 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. T h e two-dimensional nature of the liquid surface limits the demonstration to molecular arrangements t h a t lie within a single plane.
Reaction of Bromine with Hydrocarbons on the Overhead, Real or Simulated Sally Solomon, Michael Gregory, Sandeep Padmanabhan, and Kurt Smlth Drexel University Phibdelphla, PA 19104 An effective way t o introduce the structure of benzene is t o have students predict t h e chemical properties of benzene from considering its molecular formula. T o d o this, the lecturer performs a demonstration on the overhead nroiector in which hromine is added first t o cyclohexane a& c;clohexene, then t o benzene. Unfortunatelv, without adeauate ventilation this demonstration is not practical. wk describe a simulation that looks like the addition of hromine to hvdrocarbons b u t is not.'
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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 cyclohexene, the red color vanishes, appearing as it would if the dye really were bromine and the liquid cyclohexene. Since the absorption of food coloring by
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Solomon, Sally "Overhead Projector Demonstrations"; presented at the 199th National ACS Meeting. Boston, MA, April 1990. Volume 67
Number 11 November 1990
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bleach is slower than the instantaneous absorption of bromine by cyclohexene, you may have to swirl this beaker a hit to remove the color rapidly. If you try to add too much "bromine" to the bleach solution, i t will turn vellowish. When asked what the" exoect to h a ~ ~ when e n brominiis added to benzene. most stude& noticine ~~~~~~e rhr scarrity of hydrogen in the henzene formula, predict that henrrnr will nlw alrjorb bromine. N'hen they ssrr thnt rhe red r r h persists, students are surprised and puzzled. Yon can tell them that
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the 19th-century chemists were also baffled about why benzene did not exhibit the high reactivity of other polyenes. Students introduced to the structure of benzene in this way are eager to hear an explanation for what they saw. We have done this demonstration with the actual substances and the simulated ones. Both are effective. If you use the simulation, it is recommended that you tell the truth about whet you really did soon after the henzene structure discussion.
