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The letter $\ce{R}$ appears frequently in chemical structure illustrations. For instance, when I Google keratin, I find Wikipedia's Spanish article about it.

In the image illustrating part of the chemical structure, I see elements like $\ce{H, C, O, N, and S}$ - which I recognize - but I also see $\ce{R}$, which I do not. What does $\ce{R}$ stand for in this context?

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Depending on the context, the letter R can stand for several things. From our friends teaching organic chemistry at UCLA:

R group: An abbreviation for any group in which a carbon or hydrogen atom is attached to the rest of the molecule. Sometimes used more loosely, to include other elements such as halogens, oxygen, or nitrogen. R is an abbreviation for radical, when the term radical applied to a portion of a complete molecule (not necessarily a free radical), such as a methyl group. Should not be confused with R (the gas constant), R (the one-letter abbreviation for the amino acid arginine) or R (a designation of absolute configuration).

In the image from the Wikipedia article you have referenced, the letter R is as defined in the first section of the above quote - that is, it stands for "any group in which a carbon or hydrogen atom is attached to the rest of the molecule."

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    $\begingroup$ I thought that $\ce{R}$ originates from German "der Rest" (English "residue"), and the word "radical" was just an adaptation. I wonder if native German speakers could comment on this. $\endgroup$
    – andselisk
    Aug 4, 2017 at 7:10
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    $\begingroup$ @andselisk That's how German teachers and textbooks use it, yes. Your translation is wrong however, residue is "Rückstand" in German (e.g. from a distillation). The two words can be synonymous, but not here. "der Rest" better translates as rest of the molecule, R. $\endgroup$
    – Karl
    Jan 8, 2023 at 12:30

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