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Why is this stereocenter by the Br an R? I thought it was S? For problem b in the following image:

Stereochemistry answers

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    $\begingroup$ It is S. That image is confused. Image, go home. You're drunk, image. $\endgroup$ Mar 31, 2016 at 8:00
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    $\begingroup$ The descriptors for the first structure are also wrong. The name for a. should read (2S,3S,4S)-4-amino-3-(1-ethyl-1H-pyrrol-2-yl)pentan-2-ol. $\endgroup$
    – user7951
    Mar 31, 2016 at 10:55

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Both structure (a) and structure (b) have misassigned chirality centers. There should be only one $R$ assignment over all four structures. The correct assignments are as follows:

enter image description here

Structure (a)

The misassigned chirality center is the one in the center. The carbon atom with the OH group has the highest priority (one C-O bond beats any number of C-C or C-N bonds). The second highest priority is the pyrrole ring (one C-N bond and one C=C bond beats one C-N, one C-C, and one C-H). The carbon atom with the primary amine is the third priority. The groups are arranged counter-clockwise and the lowest priority (H) is in the back.

enter image description here

Structure (b)

The misassigned chirality center is the one with the bromo group. The bromine atom is the highest priority. The carbon atom bonded to chlorine is second, and the carbon atom bonded to fluorine is third. The groups are arranged clockwise, but the lowest priority (H) is forward.

enter image description here

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