Here’s a cute little structure:
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| (a) |
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Drawn like this, (a) looks like a little house and, indeed, is known as a housane. Alexander Senning called this structure “the poor man’s housane” [1, p. 77] while referring to pentaprismane as “the rich man’s housane” [1, p. 78]. Of course, there is a systematic name too. To arrive to it, we need to get acquainted with a naming system known as von Baeyer nomenclature. The method of naming of bicyclic hydrocarbons was described by Adolf von Baeyer in 1900 [2].
First, let’s number the carbon atoms thus:
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| (a) |
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Note that the atoms 1 and 4 are tertiary carbon atoms while the atoms 2, 3 and 5 are secondary carbons. Or, using the graph theory language, we can say that in the graph (a) the degrees of vertices 1 and 4 are 3 while the degrees of the rest of vertices are 2.
There are three possible paths between the vertices 1 and 4, viz. 1–4; 1–5–4; and 1–2–3–4. von Baeyer called these paths Brücken (“bridges”) and assigned them the numbers corresponding to the numbers of intervening carbon atoms. Thus, ‘0’ for the direct link 1–4; ‘1’ for 1–5–4; and ‘2’ for 1–2–3–4. von Baeyer referred to these three numbers as “characteristic”. Modern IUPACese employs the word bridge in von Baeyer sense while the tertiary carbon atoms connected by the bridge are called bridgeheads* [3].
Next, von Baeyer suggested to name a bicyclic saturated hydrocarbon containing n carbon atoms ‘bicyclonane’. Thus (a) is ‘bicyclopentane’. Finally, this name is combined with characteristic numbers, thus yielding ‘bicyclo-[0,1,2]-pentane’.
The modern version of von Baeyer system cites the characteristic numbers in the reverse order to that of the original paper by von Baeyer (that is, from the highest to the lowest), separates them by full stops, and does not use the dashes; the [x.y.z] bit is referred to as “bridge descriptor” [4]. So the modern systematic name of (a) would be ‘bicyclo[2.1.0]pentane’. However, these differences are not important. You can arrive to the structure (a) no matter what is the order of citation in the bridge descriptor. For all I know, it could be [1.2.0] or [2.0.1]. Nor should we worry about numbering of the atoms because no locants enter the systematic name.
The situation gets more complicated when we move to the bridged systems with three or more rings. The original von Baeyer method was extended to tricyclic systems by Buchner and Weigand [5] and further developed by IUPAC [3 and references therein]. Consider the structure (b):
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| (b) |
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We need to number the atoms again, but this time the way we do it is important. First, we have to identify the main ring thus [3]:
The main ring is selected so as to include as many skeletal atoms of the polycyclic compound as possible.
After that, we number the atoms
from a bridgehead atom via the longest path to the second bridgehead atom; numbering of atoms continues round the main ring; and then the main bridge atoms are numbered starting from the lower numbered bridgehead atom.
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| (b) |
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The main ring is 1–2–3–4–5–6–7–8–1. Between the two bridgehead atoms 1 and 5 there are three bridges: 1–2–3–4–5 (‘3’), 5–6–7–8–1 (also ‘3’) and 1–9–5 (‘1’). Fair enough. But then there is another bridge between 3–10–7 (also ‘1’). For this second bridge, we have to indicate its bridgehead atoms. To do that, we use the superscript locants ‘3,7’, which explains why do we need a particular numbering order. The complete bridge descriptor is ‘[3.3.1.13,7]’. Since the structure contains three cycles and ten carbon atoms, its full name is ‘tricyclo[3.3.1.13,7]decane’.
Compared with alkanes and cycloakanes, von Baeyer names do not immediately bring to mind the corresponding structures. Even the simplest von Baeyer name will make you reach for pen and paper. Another problem is pronounceability. Try saying pentane, cyclopentane and bicyclo[2.1.0]pentane aloud and you’ll know what I mean. The names like tricyclo[3.3.1.13,7]decane are worse still since now the bridge descriptor contains two (semantically different!) kinds of numbers. It never was a good idea, if you ask me, and it shouldn’t come as a surprise that in many databases these numbers got jumbled together, as in ‘tricyclo[3.3.1.13,7]decane’.
To name heteropolycyclic molecules such as (c), we can combine von Baeyer system with skeletal replacement:
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| (c) |
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Continuing in this fashion, von Baeyer system could be taken to ridiculous heights, such as 2,4,6,8,9,10-hexaoxa-1,3,5,7-tetrasilatricyclo[3.3.1.13,7]decane for (d):
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| (d) |
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Alternatively, the same structure could be given a much shorter name, tricyclo[3.3.1.13,7]tetrasiloxane, using the method “for ring systems consisting of repeating units” [6, p. 100].
| * | “Bridgehead” is an English calque of the French military term tête de pont. |
References
- Senning, A. The Etymology of Chemical Names: Tradition and Convenience vs. Rationality in Chemical Nomenclature. De Gruyter, Berlin—Boston, 2019.
- Baeyer, A. (1900) Systematik und Nomenclatur bicyclischer Kohienwasserstoffe. Berichte der Deutschen Chemischen Gesellschaft 33, 3771—3775.
- Moss, G.P. (1999) Extension and revision of the von Baeyer system for naming polycyclic compounds (including bicyclic compounds) (IUPAC Recommendations 1999). Pure and Applied Chemistry 71, 513—529.
- Hellwich, K.-H., Hartshorn, R.M., Yerin, A., Damhus, T. and Hutton, A.T. (2020). Brief guide to the nomenclature of organic chemistry (IUPAC Technical Report). Pure and Applied Chemistry 92, 527—539.
- Buchner, E. and Weigand, W. (1913) Bornylen und Diazoessigester. [Nebst einer Nomenklatur tricyclischer Kohlenstoff‐Ringsysteme nach Adolf von Baeyer]. Berichte der Deutschen Chemischen Gesellschaft 46, 2108—2117.
- Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005.
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