I want to draw a scheme that indicates the difference between the products formed in a reaction with a molecule that will form one thing when it has methyl or hydrogen substitution, but another thing when it has any substituent more bulky than isopropyl.

Is there any abbreviation (like "R" or "X") for "bulky"?

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    $\begingroup$ In discussion about Felkin-Anh etc. $\mathrm{R_L}$ or just $\mathrm{L}$ are sometimes used. $\endgroup$ – orthocresol Jul 12 '16 at 14:54

I've never come across one, but mostly those I come across will have "R" or "X", but somewhere in the reaction scheme specify what R is, like:

R = Me, Et, Pr, iPr...

So in your case, you could specify something like:

R = iPr or bulkier

  • $\begingroup$ I thought the series was Methyl, Ethyl, Butyl, Futyl :) $\endgroup$ – porphyrin Jul 13 '16 at 8:22
  • $\begingroup$ Nice one~ @porphyrin $\endgroup$ – IT Tsoi Jul 13 '16 at 11:43

I've seen numerous textbooks utilizing $\ce{R^*}$ ("R-star" notation) for a bulky group, and also even $\ce{R^{**}}$ ("R-star-star" notation) for "extremely"/"mega" bulky groups. From my understanding, star "*" symbol is supposed to resemble branched structure of the substituent.

Illustration on usage from [1, pp. 85--86]:

In place of a bulky substituent, we selected the chemically very inert tri-tert-butylsilyl group $\ce{Si^tBu3}$, called supersilyl and symbolized by $\ce{R^*}$. [...] In addition, the even more sterically crowded disupersilylsilyl group $\ce{R'} = \ce{SiHR^*2}$ and the extremely bulky methyldisupersilyl group $\ce{R^{**}} = \ce{SiMeR^*2}$, which we jokingly call “megasilyl”.

Note: An asterisk in the right superscript position next to the chemical element symbol also denotes electrical or nuclear excitation.


  1. Silicon Chemistry: From the Atom to Extended Systems; Jutzi, P., Schubert, U., Eds.; Wiley-VCH: Weinheim; Cambridge, 2003. ISBN 978-3-527-30647-3.

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