What’s the difference between the structures (a) and (b)?
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| (a) | (b) |
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Why, isn’t it obvious: in the former structure, the two methyl groups are found on the same side of the double bond while in the latter they are on the opposing sides*. Both compounds can be named but-2-ene. However, to distinguish between (a) and (b), they are decribed respectively as cis-but-2-ene and trans-but-2-ene. The corresponding stereodescriptors are derived from the Latin prepositions cis, “(on) this side of”, and trāns, “across”. Perhaps unsurprisingly, such molecules are called cis-trans isomers. The earlier term geometric isomers is “strongly discouraged” by IUPAC [2] but remains in wide use.
We have already seen an example of cis-trans isomerism when looking at diamminedichloridoplatinum(II). In coordination chemistry, cis-trans isomers are usually found in square planar (SP-4) complexes of the form [Ma2b2]. Similarly, organic cis-trans isomers such as (a) and (b) show tetragonal planar geometry of the form [abC=Cab].
What if we substitute one of the hydrogens at the double bond in but-2-ene with another group? Then we’ll have a structure of the form [abC=Cbc] and a different system to describe its configuration will be needed.
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| (c) | (d) |
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The system recommended by IUPAC is the E/Z convention [3, P-91.2.1.2.1]. To take a well-deserved break from Greek and Latin, the descriptors ‘E’ and ‘Z’ are derived from the German words entgegen, “against” and zusammen, “together”. Like the R/S and C/A conventions, the E/Z convention makes use of Cahn–Ingold–Prelog (CIP) sequence rules to assign the priorities to the atoms or groups.
Let’s see how it works on the example of the charmingly named angelic acid (c)†. Its structure looks like that of trans-but-2-ene (b) substituted with carboxy group. Carboxy group is senior to methyl; on the other end of the double bond, we have methyl group that is senior to hydrogen. So, the higher-priority group at C-2 (carboxy) is cis to the higher-priority group at C-3 (methyl) and the full systematic name of (c) is (2Z)-2-methylbut-2-enoic acid. On the contrary, tiglic acid (d) will be named (2E)-2-methylbut-2-enoic acid.
We can employ the E/Z convention even when the cis/trans notation is perfectly adequate. In fact, the E/Z names are preferred by IUPAC: (2Z)-but-2-ene (a) and (2E)-but-2-ene (b).
The descriptors ‘E’ and ‘Z’ can also be used with non-carbon double bonds [3, P-93.4.2.1.3].
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| (e) | (f) |
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Have a look at cis-azobenzene (e) and its derivative, cis-azoxybenzene (f). The lone pair of electrons is considered to have zero atomic number. The higher-priority groups (phenyls) in (e) are cis (zusammen!) to each other and so the systematic name is (Z)-diphenyldiazene. Now applying the CIP rules to (f), we’ll see that the higher-priority group at the first nitrogen atom is oxo (O > C) which is trans (entgegen!) to the higher-priority group at the second nitrogen, viz. phenyl (C > lone pair). Therefore, the systematic name of (f) will be (E)-diphenyldiazene oxide. IUPAC does not recommend ‘cis’ and ‘trans’ for double bonds linked to heteroatoms [3, P-93.4.2.1.3].
Another type of cis-trans isomerism is found in ring systems [3, P-93.5.1.2]. Observe the isomers of 1,4-dimethylcyclohexane:
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| (g) | (h) |
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In the structure (g), the two methyl groups are on the same side of the plane of a ring‡, so this isomer is called cis-1,4-dimethylcyclohexane. In (h), the methyls are on opposite sides of the plane, thus trans-1,4-dimethylcyclohexane.
As in case of the double bond, there is no free rotation between the atoms in the ring. However, the saturated carbon atoms in the rings have tetrahedral geometry, as opposed to trigonal planar one in alkenes. Which could mean... more stereoisomers!
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-cyclohexa-3,5-diene-1,2-diol.png) |
-cyclohexa-3,5-diene-1,2-diol.png) |
| (i) | (j) | (k) |
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There are three possible stereoisomers of cyclohexa-3,5-diene-1,2-diol: one cis (i) and two trans, (j) and (k), the latter pair being enantiomers. For them just ‘trans’ is not a good enough descriptor: we need the R/S convention. If we don’t know which of the two trans isomers we are talking about, we can employ the descriptor ‘rel’ that indicates the relative configuration of one chiral centre with respect to (any) other chiral centre [3, P-93.1.2.1]. So rel-(1R,2R)-cyclohexa-3,5-diene-1,2-diol means “(1R,2R)-cyclohexa-3,5-diene-1,2-diol or its enantiomer”. We also can describe (i) as (1R,2S)-cyclohexa-3,5-diene-1,2-diol, but why should we do that if cis-cyclohexa-3,5-diene-1,2-diol is shorter and as unambiguous?
| * | If it’s not intuitively clear what the “same side” or “opposite side” of a bond mean, we can use the concept of “a reference plane identifiable as common among stereoisomers”. For double bonds, “the reference plane contains the doubly bonded atoms and is perpendicular to the plane containing these atoms and those directly attached to them” [1, E-2.1]. For example, the reference plane is perpendicular to the plane of the diagrams (a)—(f). |
| † | Angelic acid is called so because it was first isolated from the roots of angelica (Angelica archangelica). Its salts and esters are known as angelates. |
| ‡ | Here, “the reference plane is that in which the ring skeleton lies or to which it approximates” [1, E-2.1]. |
References
- Cross, L.C. and Klyne, W. (1976) Nomenclature of Organic Chemistry. Section E: Stereochemistry (Recommendations 1974). Pure and Applied Chemistry 45, 11—30.
- Moss, G.P. (1996) Basic terminology of stereochemistry (IUPAC Recommendations 1996). Pure and Applied Chemistry 68, 2193—2222.
- Favre, H.A. and Powell, W.H. Nomenclature of Organic Chemistry: IUPAC Recommendations 2013 and Preferred IUPAC Names. Royal Society of Chemistry, Cambridge, 2014.
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