| MadSci Network: Chemistry |
|
Why do we use the common name ethylene glycol when the correct
nomenclature
is 1,2-ethanediol? Doesn't the 'ethylene' imply the presence of a double
bond
when in fact only single bonds (associated with -anes) exist? Is this
misleading? Kind regards Chris Actually, no, the name is not misleading. "Ethylene glycol" is a systematic name, just not part of the IUPAC system of nomenclature. Let's analyze it. Ethylene. You may have heard of "methylene." That's the IUPAC name for the compound CH2, or for the side chain =CH2. It means one carbon with two hydrogens and two dangling bonds, as opposed to "methyl" (CH3), which is one carbon with three hydrogens and one dangling bond, or "methyne" (CH), which is one carbon with one hydrogen and three dangling bonds. These names refer not only to molecular subunits, but to actual (highly-reactive) compounds. Any straight-chain alkane is a string of methylenes capped on the ends, H (CH2)xH.But IUPAC took over "methylene" from the old-style nomenclature system. In that system, "methylene" could be used to refer to a one-carbon compound with two identical substituents, such as methylene chloride CH2Cl2, or to a string, such as tetramethylenediamine H2N(CH2) 4NH2. In the same way, "ethylene" could refer to the compound CH2=CH2, or to any -CH2CH2- unit with two dangling bonds, as in ethylene chloride ClCH2CH2Cl. In the IUPAC system "ethylene" as a structural unit would be called "1,2- ethanediyl." Notice that, under the old system, "ethylene chloride" refers uniquely to 1,2-dichloroethane! 1,1-dichloroethane Cl2CHCH3 would be called "ethylidene chloride" where "ethylidene" refers to "1,1- ethanediyl." Glycol "Glycol" is a generic name for any 1,2-diol. We occasionally still speak of "glycolization" reactions such asTherefore "ethylene glycol" is the old-style, systematic name for HOCH2CH2OH, and is just as unambiguous as "1,2-ethanediol." In fact, it's not only unambiguous, it's redundant because both "ethylene" and "glycol" refer to the same substitution pattern! Why do we use "ethylene glycol" instead of "1,2- ethanediol"? That's a harder question...I put it down to TRADITION. Without our traditions, our lives would be as shaky as... as a fiddler on the roof! Besides, doesn't "ethylene glycol" come more trippingly off the tongue than "1,2-ethanediol"?Incidentally, ethylene glycol, a sweet-tasting substance traditionally used in antifreeze, is quite toxic. It has mostly been replaced by the less-toxic propylene glycol. (See, I told you we'd come back to this!Some useful websites:
|
Try the links in the MadSci Library for more information on Chemistry.