Determination of Chiral Molecule Configuration Using the f 1,2,5 Rule When describing the particular configurationabout a chiralearhon in a molecule, theprefixesR and S are utilized.This determination involves s two sten orocess.
termination easier. But, neither mental transfers nor physical models are necessary. The +1,2,5 method (read, plus or minus one, two, five method) described below allows one to determine the configuration rapidly. There are four unique groups to a chiral center. Since each group has a designated priority, each center is defined by a unique sequence of four numbers. There are 24 possible sequences. Half of these can he rotated 180' in the plane of the oaoer to obtain the other twelve. The answer to anv ~ossibleconfieuration lies in the use of the 11.2.5 rule. One need onlv i a d e ~the groups on the paper according to prioritiand then follow the rule below. Add the two nurnhers (priority numbers) alony the horii..,ntal axis. If the lourr id the r w is ~n the loft, suhrmct c the I wneht,nubtrart r~ from this sum the prinrny number ar the rupof the vrrtiralaxi~.If the lower ( ~ f t h ~ t ~ . ~ n i~i om f n m thissum r h ~ ~ e q t ~ ~ n ~ e n t l mthe h eIx,ttomot rat th~\.ertiralsxi.;.Ifrheanswerkeither + I , - I . r 2 o r + 5 , therhiml center has the R configuration. Any other answer means the configuration is S. An example serves to illustrate. Assume one needs the configuration of the following chiral molecule. The priorities have been determined as shown. Now, apply the rule. The sum across the horizontal axis is 2 3 = 5. The lower of the two numbers is a 2 and is on the left. Subtract the upper vertical axis number to get 5 - 1 = 4. The answer "4" corresponds to an S configuration.
+
2
4
3
i
Now, suppose the next figure corresponds to a rotation of the previous example by 1809. Again, adding the horizontal axis gives 5. The lower of the two numbers is now on the right, so the lower vertical axis number is subtracted from this sum to get 5 - 1 = 4. The answer again is "4". This chiral center has the S configuration. As is expected, a rotation of 180° gives the same molecule and configuration. 4
1
Consider now the mirror image of the previous example:
3
(1)2+3=5 (2)5-4=1 (3) Equals R configuration,
Now a practical example:
CH,-C-C
I
H
O 'H
4
(1)3+2=5 2 .(2)5-4=1 ( 3 ) Equals R configuration
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So, a simple mathematical tool can be utilized to counter the problems of projection of a two-dimensional figure into a three-dimensional model. By simply memorizing the sequence i1.2.5, one knows it corresponds to the R configuration. This in essence is the 1l,2,5 rule.
Richard Adams Dietzel
Volume 56,
umber 7. July 1979 1
451
