Steo 4. Now if (a1 to (hl . . to (cl . . in B involves a clockwise rotation then B is of the ~ c m f i ~ u r a t i oand n , the original molecule A must therefore be S.
This method avoids the use of Avorinde's structure I and is less likely to cause confusion in the understanding and use of the Fischer prolection. An ohvioua extrnsion of Step 3 lies in further switchhg any one pair of suhstituents (exciuding (d)) in structure B to generate a projection A' having the same configuration as A, and then to apply Step 4 to A' to give the R or S designation of A directly.
Newman projection can be more easily drawn from structure 4 than from the corresponding Fischer projection, thus facilitating, for example, the determination of the structure of the major alkene product obtained in the E2 elimination of 4. Llterature Clted 1. MoW~~O", Robert T.;Boyd. Robert N. Olganie Cko Boston, 1983, pp 138-139.
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John W. Buntlng Unlvenity of Toromo Twonto, Ontario M5S 1Al
w o r d o n F. Hatnbly Bryan College Dayton. TN 37321
Canada
Assigning Absolute Conflguratlon: Another Vlew
One More Vlew on Asslgnlng Absolute Conflguratlons To the Editor:
To the Editor:
Ayorinde reported a gimmick for assigning absolute configuration using Fiseher projections thus supposedly helping the student who has trouble visualizing in three dimensions [1983,60,928]. I have taught a similar method for years, and I have become increasingly dissatisfied with it. Should we be teachine a two-dimensional eimmick when stereochemistrv is so vital to organic chemistry? Furthermore, the ~isch; projection method is often very difficult to apply, without the use of models, when the chiral carbon is part of a ring, suchas in 1;few students in their first organic courseare able to perform the mental gymnastics requiredto draw aFischer projection of 1. I use the Cahn-Ingold-Prelog method (I): priorities 1to 4 are assigned in order of decreasing atomic number ("we're number 1" is easy to relate to the largest atomic number) and the molecule is viewed opposite the lowest priority group.
Initially many students will experience difficulty in applying this method, but they master i t relatively quickly and learn some stereochemistrv in the vrocess, and. accordinelv. 1 prefer this method. In using the method (.)n2.one views'the chiral carbon from behind which mas be difficult for some. Alternatively, when the lowest priorfty group is in front, as in 2, I show that interchanging the H and C1 gives the enantiomer 1, whose configuration is readily assigned as S; accordingly, 2 is R. This method can he utilized to draw multichiral-carhon molecules, such as (2R,3S)-2-hromo-3-methylpentane. A nossible structure 3 is drawn. with the lowest orioritv " erouns behind to facilitate assignment, which is r e a i l y determined to be 2S,3S. A pair of suhstituents a t chiral carbon 2 is then switched to give the correct answer 4 (2R,3S).
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F. 0. Ayorinde has given a method for helping students assign the absolute configuration at a chiral center, given a Fischer projection [1983, 60, 9281. The original suggestion for handling this matter, put forward by Cahn, Ingold, and Prelog ( I ) is worth quoting here: Transform the given Fischer projection by an even number of exchanges of pairs of groups, so that d occupies the bottom position, and, on the transformed projection, read the pattern as R or S. a-b-e I have always taught the use of this "double swap" method, and i t seems to have served auite well. In connection with this topic ;curious semantic error was made (in the same paper) by the distinguished trio. I quote: The suggested indications for asymmetry leading, under the seauence and conversion rules. to s right- and left-handed oattern.. are capiral italir H and S , respectively, where R deriws from the Latin rerrus, meaning "right", and S f n m sinister.
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Thev were correct with sinister as the source of S. but the Latin rectus means "straight"-dexter is Latin for "right", in the directional sense. I t is obvious that the use of D from dexter was out of the question and some other letter had to be used. T o extricate themselves from this awkward dilemma they actually made a linguistic leap from Latin to English (or German?) and so we have R for the opposite of S. The etvmoloeical relationshin between rectus and the use of the ddrived English "right" (in the directional sense) I leave to those readers who. like mvself. like to curl un with an unabridged dictionary whkn such crisks arise! The use of bilineual terminoloev. as in the above case. is not rare in science. A recent example I ran across (science 1983,222, 1233) is in the assignment to a new mammalian taxon the name Dinnetherium. This interesting term is derived from the Navajo dinne, meaning "the (Navajo) people" and therion, Greek for "wild beast". Only the most narrowminded purist could object tosuch an exotic combination for the name of a creature that may turn out to be one of our remote ancestors.
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DavldTodd Worcester Polytechnic Institute Worcester. MA 01609
In my experience, students have more difficulty in doing this type of problem with Fischer projections. A correct
732
Journal of Chemical Education
Literature Clted 1. Cahn. R.S.: Ingold, C. K.:Prelw, V.Ermn'enfia 1956.12.81
