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I came across the compound dl-stilbene dibromide, and I am confused by exactly what this means. I thought it might have meant a racemic mixture but there is only one chemical structure given.

dl-stilbene dibromide

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That picture is showing relative stereochemistry. Essentially, it is just saying that, in the shown confirmation, the hydrogens will be on one side and the bromines will be on the other, but half of the mixture will have the bromines coming out and the other half will have the bromines going back.

If the S or R labels had been shown in the molecule (or they had used just a d- or l- prefix) the picture would have been showing absolute stereochemistry. By saying dl-stilbene dibromide, they are saying that it is a racemic mixture of the compound shown and its enantiomer, but not any diastereomers.

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Here's a snippet with surroundings from the book (Experimental Organic Chemistry – A Miniscale and Microscale Approach), for the context:


Fig.1 Book snippet


Should we accept the name “stilbene dibromide” (which is almost as bad as would be e.g. “ethylene hydrochloride” for chloroethane (ethyl chloride)), there are some other problems.

49 is defined in the text as a diastereomer, but the reaction scheme it's named “dl-Stilbene dibromide”, denoting a racemic mixture (i.e. of both enantiomers), therefore should apply to entire 49 structure including the “(+ enantiomer)” text. (Preferred notation is “and enantiomer” text, or including structure of the other enantiomer, without ‘+’ sign, but that's for the standalone context, here we have a reaction scheme.)

It's true that sometimes the constant-width bold/hashed bond is used to denote relative stereochemistry, together and in contrast with wedge bold/hashed bonds for absolute stereochemistry. This mixed convention is not supported by IUPAC, even it's described as “obscure” in Graphical representation of stereochemical configuration (2006):

ST-0.8 Mixtures of diastereoisomers

There are a variety of obscure conventions for depicting arbitrary mixtures of diastereoisomers. The most common (but still obscure) of these conventions says that bold and hashed wedges should be used for absolute configuration, unwedged bold and unwedged hashed lines represent relative configuration and racemic character, while hollow wedges and dashed lines represent relative configuration (…)

However this is not the case for this book, according to the following example depicting a single enantiomeric product:


Fig.2 Example snippet with reaction with single enantiomeric product


It should be also emphasised that dl- stereodescriptor usage is strongly discouraged ((E)- and meso- are OK), the recommended one for racemates is rac-.

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