|WikiProject Chemistry||(Rated C-class, Mid-importance)|
Other types of conjugation
In addition to the concept of ordinary conjugation, which is the most important aspect of electron delocalization in organic chemistry, there are also the concepts of (1) hyperconjugation, (2) homoconjugation and (3) spiroconjugation. It seems that hyperconjugation has been properly addressed, albeit briefly, with the link to the hyperconjugation page from this current page about conjugated systems.
I think it would be worthwhile to add a page about homoconjugation from which the reference H. Duerr and R. Gleiter, Angew. Chemie Int. Ed. Eng. 17, 559, (1978) may be useful.
I think it would also be worthwhile to add a page about spiroconjugation the concept of which was first introduced simultaneously by R. Hoffmann, A. Imamura and G. Zeiss, J. Am. Chem. Soc., 89, 5215, (1967) and by H.E. Simmons and T. Fukunaga, J. Am. Chem. Soc., 89, 5208, (1967).
Later in 1994 a polyspiroquinoid polymer was proposed that exhibited spiroconjugation delocalized in 1-dimension, as described in M.J. Bucknum and R. Hoffmann, J. Am. Chem. Soc., 116, 11456, (1994). Bucknum et al. later showed in 2004 that the polyspiroquinoid substructure of his and Hoffmann's hypothetical carbon allotrope glitter was the basis for spiroconjugation extended and delocalized into 2-dimensions (spirographite substructure) and into fully 3-dimensions (glitter structure) as described in M.J. Bucknum and E.A. Castro, J. Math. Chem., 36(4), 381, (2004). This paper explained the metallic status calculated for the glitter allotrope of carbon on the basis of the concept of endo-spiroconjugation.
In 2005 Bucknum et al. showed compelling evidence for the existence of the 3D spiroconjugated glitter allotrope of carbon in a comparison of its calculated diffraction pattern with that of the fairly commonly observed, but heretofore uncharacterized form of carbon known as n-diamond, see M.J. Bucknum and E.A. Castro, Mol. Phys., 103(20), 2707, (2005) for details.
The latter report of the synthesis of the spiroconjugated 3D glitter carbon allotrope provides an apt comparison to the corrfesponding 2D graphene carbon sheet allotrope which possesses ordinary conjugation in 2D. The respective 2D (graphene) and 3D (glitter) resonance structures are diagrammed in this paper.
Cis or Trans?
In the beta-carotine example shown, the double bonds are all cis. In Conjugated linoleic acid, the bonds are trans. Does cis / trans alignment have an effect on the conjugation? David.Throop 18:42, 28 November 2006 (UTC)
- It may have an effect (stability, UV absorbance, etc) but does not affect the designation of something as being "conjugated". DMacks 18:54, 28 November 2006 (UTC)
- The reason that possible alignments are only cis/trans, rather then having the third bond go out of the page is that due to conjugation three such bonds tend to be in the same plane. The cis and trans alignments have different conjugation-energies, and in this sense they effect conjugation. Cederal 14:41, 17 December 2006 (UTC)
"It is important to note that merely possessing alternating double and single bonds is not necessarily enough for a system to be conjugated. Some cyclic hydrocarbons (such as cyclooctatetraene) do indeed possess alternating single and double bonds. Although the molecule may appear planar looking only at its chemical structure, the molecule is not actually, and typically adopts a "tub shaped" conformation."
Wouldn't "boat-shaped" be a better description? Isn't that the term used in most organic text books?
"By partial rotations about the carbon-carbon single bonds of the ring, the chair conformation can assume another shape called the "boat" conformation."1
12004. Solomons, T. W. Graham & Fryhle, Craig B. Organic Chemistry 8th Ed. John Wiley & Sons Inc. Hoboken, NJ. p. 164
- Boat shaped is the more familiar term, though it would have to be a flat bottomed boat with no sides. I've seen boat shaped used to describe hexagonal cyclic compounds but is it also used for compounds with 8 atoms forming the ring? It looks a lot less like a boat without the triangular ends, though it doesn't look a lot like a tub either. Richard001 05:54, 6 May 2007 (UTC)
- Richard001 is right, "boat" is used to refer to six-membered saturated rings. Tub-shaped is correct, and has been used in this sense since at least 1952.
- O. Bastiansen, L. Hedberg and K. Hedberg, J. Chem. Phys., 27, 1311 (1957); W.B. Person, G. C. Pimantel and K.S. Pitzer, J. Am. Chem. Soc. 74, 3437 (1952).
- Htruane (talk) 23:57, 14 December 2012 (UTC)
- Richard001 is right, "boat" is used to refer to six-membered saturated rings. Tub-shaped is correct, and has been used in this sense since at least 1952.
Conjugated systems and energy required for excited state
Why do conjugated systems require less energy to reach the excited state? That's what I came here to find, but there's no mention of the reason behind this trend. Thinking about the delocalized nature of the electrons, it seems plausible to say that they have a weaker attachment to their ground states and thus require only a little energy to reach the excited state. Hopefully someone can expand upon this in the article. Richard001 05:58, 6 May 2007 (UTC)
Doubly conjugated molecules?
In normal (planar) conjuaged molecules, each atom in the system possesses a p-orbital, as the article says. What about linear molecules, such as cumulenes and polyynes, in which each atom in the chain possesses two p-orbitals instead of one? Is there a special name for this sort of thing? If so, what is it? Stonemason89 (talk) 20:55, 11 September 2008 (UTC)
- The conjugated system still involves only one p orbital on each atom. What the other orbitals are doing and how the atoms are hybridized? Not so relevant...those orbitals are pointing in different directions, so there's little/no overlap with those in the conjugated system. It's just like how a triple bond has two essentially independent pi systems. In 1,2,3-butatriene, the Δ1,2 and Δ3,4 bonds are conjugated, involving those p atomic orbitals that compose the double bonds. The Δ2,3 bond is composed of different p atomic orbitals on those sp-hybridized than the other double bonds, so it is perpendicular to the other two double bonds. Therefore, this pi system is not in conjugation with the others. DMacks (talk) 21:17, 11 September 2008 (UTC)
 This is an obscuration, not a clarification – I asked to explain a consequence relation, not the foundations of quantum chemistry. Or maybe my competence in English is insufficient to use the verb to imply? Incnis Mrsi (talk) 16:44, 13 April 2012 (UTC)
- I'm sorry that my note is not helpful. Probably I did not understand your original question, so perhaps you could explain more on this talk page what is the problem. Dirac66 (talk) 19:18, 13 April 2012 (UTC)
- We define a homoconjugation as
|“||an overlap of two π-systems separated by a non-conjugating group, such as CH2.||”|
- So, overlap ∧ separation ↔ homoconjugation, and there is no reason to believe that separation → overlap . Then, we have:
|“||For example, the molecule CH2=CH–CH2–CH=CH2 (1,4-pentadiene) is homoconjugated because the two C=C double bonds (which are π-systems because each double bond contains one π bond) are separated by one CH2 group.||”|
- By construction, these two π-systems are separated. Hence, overlap ↔ homoconjugation. But, assuming both sides are true, which of two is the antecedent and which is the consequence? Do we check experimentally that 1,4-pentadiene is a conjugated system (i.e. develops an overlap) and therefore is a homoconjugation? If so, it should be expressed like "the molecule CH2=CH–CH2–CH=CH2, known to be a conjugated system, is an example of homoconjugation…". Or the article suggests that the –C=CH–CH2–CH=C– always leads to a homoconjugation? In this case, it is not only "because double bonds are separated", but also for some theoretical reason (which is missing) for such systems to develop an overlap . Incnis Mrsi (talk) 19:53, 13 April 2012 (UTC)
You can't be logical about these things, this is chemistry, not physics (joke)! For a system to be considered conjugated (i.e. for the electrons to delocalise over more than two nuclei), the C=C double bonds normally need to be adjacent (C=C–C=C) or separated (C=C–X–C=C) by an atom or group with its own pi system (e.g. X = NH or O). Sometimes you observe experimentally the effects of delocalised electrons even when the two C=C's are separated by an atom or group without a pi system (e.g. X = CH2).
The usual explanation for all this is as follows: pi electron delocalisation requires good overlap (constructive interference) of valence p orbitals. Two p orbitals constitute a pi bond in each C=C. The two pi bonds in C=C–C=C or C=C–X–C=C must be similar enough in size and energy, close enough in space, and correctly oriented. Where X = NH or O, a further p orbital is present in the form of a lone pair on X, and this overlaps with both C=C pi bonds simultaneously. In the case of X = CH2, the two C=C units are spatially separated without a p orbital or other pi system in between. Nevertheless, some overlap of the two pi bonds is apparently sometimes possible. I'd guess the overlap is poor, the interaction weaker and the electron delocalisation less pronounced. --Ben (talk) 21:07, 13 April 2012 (UTC)
- Interestingly, we have a fairly detailed homoaromaticity page but little content and no separate page for homoconjugation. I wonder how much of homoAr could be factored out into a more general one on the homoconjugation topic? Or should the homoconjugation→Conjugated system redirect be changed to point to homoaromaticity instead? DMacks (talk) 21:32, 13 April 2012 (UTC)
(edit conflict) To answer your question of whether (i) the structure C=C–CH2–C=C or (ii) homoconjugation is the antecedent, it is the structure that permits homoconjugation. I imagine not all molecules containing the fragment C=C–CH2–C=C exhibit homoconjugation. For example, the structure of the rest of the molecule might constrain the two planar R2C=CR2 fragments to lie perpendicular to one another, preventing any overlap of pi systems (two p orbitals have zero net overlap if they are orthogonal).
|Structure of fragment||Degree of conjugation|
|C=C–CH2–C=C||homoconjugated or non-conjugated|
- OK, I think I understand what is missing. It is not sufficient to classify all C=C-C=C molecules as conjugated and all C=C-C-C=C as homoconjugated, which would reduce these terms to a question of nomenclature. Instead we have to summarize the physical (spectroscopic) and/or chemical (reactivity) evidence that there is a real interaction, which may of course be true in some C=C-C-C=C molecules and not in others.
- For conjugated molecules we do have a section on pigments which effectively presents some spectroscopic evidence, although it should be made more explicit that this is evidence for conjugation and not just a consequence (and practical application). And we can add UV evidence for shorter chains, as well as chemical reactions which depend on conjugation.
- For homoconjugated molecules the interactions are smaller, but we should summarize the evidence which does exist. I am not an organic chemist so will leave this to others. But assuming that there is some evidence for homoconjugation in molecules which are not actually homoaromatic, I would leave homoconjugation in this article and just refer to homoaromaticity as a related concept. Dirac66 (talk) 22:48, 13 April 2012 (UTC)
I am totally confused by this. It is confusing to include homoconjugation here, IMHO. Either too much or not enough. I am confused enough by systems like c=c=c-c=c and c=c=c-c=c=c → conjugated? How about C≡C-C≡C ? A double bond is not the only type of C.C pi bond. How about CN-c=c ? OCN-c=c ? NC-c=c ? (etc.) This article gives no guidance. As far as "tub shaped" - 40 years a chemist, don't recall ever hearing the term. Too bad there aren't terms like "concave" that would better describe the shape, huh? You really have to make up your mind. Is conjugation an experimental fact or a theoretical fact? If experimental, how is it determined/measured? If theoretical, how is the determination made that there is enough "pi" character to the intervening single bonds to characterize the system as conjugated? Given that most modern calculations are DFT based, is the use of Molecular Orbitals appropriate?188.8.131.52 (talk) 02:36, 23 June 2013 (UTC)
Alternating Single and Double Bonds?
I am a bit concerned about describing conjugation in terms of single and double bonds.
I thought the whole point about conjugation is that these are NOT single and double bonds, but an intermediate form, and each bond is equal to all the others (rather than some being single and some double).
In addition, atomic orbital terminology is used (e.g. p-orbital) when it might be better to use molecular orbital terminology. In fact the article seems unsure as to which to use, as there are phrases discussing such things as "pi electrons" and "p orbitals" in the same sentence. Marchino61 (talk) 05:54, 1 September 2013 (UTC)
- It is true that the bonds are intermediate for aromatic molecules such as benzene, which has two conjugated structures of equal importance, so that the true wave function has six equivalent CC bonds which are intermediate between single and double bonds. I have now included this point in the article. For butadiene, however, there is a unique conjugated structure with C=C and C-C bonds which approximately represent reality, and the effects of conjugation constitute a correction so that the C-C bond is stronger and shorter than a normal C-C bond. Dirac66 (talk) 21:00, 23 July 2015 (UTC)
I can't agree with that - the whole point is that the electrons are delocalised and belong to no one bond in particular (as the article correctly states). Writing the molecule as containing single and double bonds is a) what people did before conjugation was understood, and now b) a simplification brought on by the fact that some means of drawing these compounds is necessary. Chemists recognise that this is just a simplified model for what is actually going on. A better way is two show all the contributing resonance structures that make up the system.
- Actually the total wave function of any polyatomic molecule can be written in terms of delocalized MO's, but for many molecules (such as alkanes) the dominant Lewis structure is a quite good approximation to reality without adding any minor structures. The question is how important are the minor structures in conjugated systems such as butadiene, especially since they are ionic structures (as shown in your link to ChemVista). It is clear that they are more important in butadiene than in ethane, but also that they are less important than in benzene. We could mention the ionic structures in the article, but I would prefer to have an additional source such as a quantum-mechanical study which gives a quantitative idea of their importance. Dirac66 (talk) 14:31, 26 July 2015 (UTC)
The comment(s) below were originally left at several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section., and are posted here for posterity. Following
|The discussion of spiroconjugation within the larger context of conjugation in organic chemistry is well-placed. The discovery of an organic molecule spiroconjugated in 3D as the glitter network in the 21st century, is comparable in significance to the discovery of 2D conjugation in the ordinary graphene grid in the 20th century.Hexagonite 13:17, 28 September 2007 (UTC)|
Last edited at 13:17, 28 September 2007 (UTC). Substituted at 12:10, 29 April 2016 (UTC)