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Marlooro~gnSchool 250 S Ros~moreA \ e n ~ e LO$ Ange cs C A 90004
Cis, Trans Isomerism and Polarity of Molecules C. V. Krishnan East lslip High School, Redman Street, lslip Terrace, NY 11752 One areaof concern for a science teacher is to find suitable exueriments on chemical bondine that demonstrate different aspects of bonding. Most school laboratory manuals give onlv one or two exoeriments like "solution and molecular pol&ity7' and "solubility of different salts in different solvents". In covalent bonding, a pair of electrons is shared by two atoms of the same element or different elements. If the two atoms have different tendencies to attract electrons, that is, if they differ in electronegativity, a polar covalent bond is formed. The atom with the greater electronegativity attracts the electron pair more strongly and will have a partial negative (A_) charge. The atom with the smaller electronegativity will have a partial positive (6+) charge. In a symmetrical molecule like CHAor CCla. the center of oositive charee and the center of negative charge coincide. Such molecules have polar bonds but have no net dipole moments. If the partial positive and negative charges are not cancelled out by symmetrv, . the molecule will be a dioole and it will have a dipole moment. The dipole moment, w, is a phvsical property and can be mea.;ured experimentally. It isdefined as the Goduct of the magnitudr of the charge, 0 , and the distance, d, hetween the centers of charge,
very little from their normal straight downward path. In this exoeriment. two liouids of identical molecular formula are uskd to show that iifferences in the spatial arrangements of atoms can cause molecular polarity.
where p is in Debye units, D, q is in electrostatic units, and d is in centimeters. Since the distance involved has direction, dipole moments are vectors. The direction is usually indicaed by an arrow from the partial positive charge to the oartial neeative charge. One experiment commonly used to indicate qualitatively the polar or nonpolar nature of liquids like water, alcohols, chloroform, hexane, and carbon tetrachloride is to bring a chareed rubber rod (or a chareed elass rod) near the flowine liquk and observe the deviatgn f;om the regular downwar; path of the flow of liquid. Polar liquids will he deflected towards the charged rod whereas nonpolar liquids deviate
trans-1,2-dichloroethene.
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Materials Burets, pipets, pipet bulb, cis-, and trans-l,2-dicbloroethene from Aldrich Chemical Company, rubber rod and fur or glass rod and silk cloth. Procedure Fill a 50-mlburet (or 50-mlpipet with hulb attached)with cis-19dichloroethene. Rub the dry rubber rod with fur so that the rubber rod acquires an excess of electrons at the expense of the fur. Rubbing the glass rod with a dry silk cloth makes electron transfer from glass rod to silk. Open the stopcock of the buret (or remove the hulb from the pipette) and allow the liquid to drain into a wide-mouthed beaker. While draining, bring the charged rod near the flow of the liquid and obsewe the path of the liquid flow. Repeat the experiment usine trans-1.2-dichloroetbene. Resuns The direction of the flow of cis-1.2-dichloroethene will change and will be deflected towards the charged rod. There will be practically no deviation from the normal flow for
Dlscurwlon Isomers are substances with the same molecular formula, and are of two types, structural isomers and stereo isomers. Structural isomers have different structural framework and the bonding arrangements for the component atoms or groups are different. Cisand trans isomers are stereo isomers that have the same atomic linkages but different spatial arrangement of atoms and groups.Tbey are also called geometric isomers and the arrangements in space are referred to as configurations. The cis isomer has likegroups on the same
Volume 63
Number 7 July 1986
80 1
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side of the molecule, whereas, in the trans isomer like groups are on opposite sides of the molecule. Double-bonded carbon atoms are nearer to each other than when joined by a single bond. Free rotation around the hond joining the two carbon atoms requires a considerable amount of energy (about 65 kcal) and is not possible a t ordinary temperatures. This restriction in the rotation of one carbon atom with respect to another by the presence of a double bond makes it possible to have cis and trans isomers. Cis and trans isomers have widely differentphysical properties as shown in the examples below.' These differences allow us to distinguish between cis and trans isomers rather easily. For example, the cis isomers have measurable dipole moments. Measured dipole moments often agree with calculated dipole moments obtained by the vector sum of individual hond dipole moments. These hond moments cancel each other in trans isomer but add to give a net dipole moment in cis isomer.
mp = -80 OC bp = 60'C ,L = 1.85 D
mp = -50 OC bp = 4 8 T ,L= O D
cis-
trans-
1,2-dichloroethenes
mp = -53 'C hp = 110 *C p = 1.35 D
mp = -6 'C bp = 108 "C @=OD
as-
trans-
1,2-dibramoethenes
602
Journal of Chemical Education
Among the simple cis and trans isomers available, the most suitable ones are the ones used in the present experiment. The cis- and trans-1.2-dibromoethenes are available only as a mixture and the trans-1,2-diiodoethene is a solid a t room temperature. Since the cis- and trans-2-butenes have low boiling points and their cis compounds have low dipole moments.. thev - are also not suitable for the qualitative experiments described in this paper. The cis- and trans-2pentenes also are not suitable because of their low boiling points. The experiment suggested here is quite simple. The solvents can be stored and repeatedly used. The concept of polarities in molecules due to differences in spatial arrangements of atoms is clearly demonstrated in this experiment. However, the extent of discussion on dipole moments and stereo isomers should be restricted de~endineon the abilities of students.
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Hazard Warnlng This is only a demonstration to be carried out by teachers in a properly ventilated space. T h e materials a r e quite volatile and should be kept away from flame. The materials should be recovered after theexperiment and stored in bottles for further use. The toxicities of cis- and trans-12dichloroethenes are not known clearly.
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Morrison.R.T.; Boyd, R. N. In "Organic Chemistry".3rd ed.: Allyn and Bacon: Boston, 1976: p 153.
