[Winga]

If a compound contain alternative single and triple bonds, e.g. octa-2,4,6-triyne, is it a conjugated system?

[xiankai]

to the best of my knowledge, all members of a homologous group that have unsaturated bonds, mainly have only one unsaturated bond, C=C or triple bond. the rest are single bonds by default. im not sure if having 2 unsaturated bonds may result in quadruple or quintuple bonds  

btw... alternating bonds... sounds like a polymer to me, with the monomer having just 1 triple bond and a few other single bonds.

[Winga]

I think it will become something like allenes, cumulated C=C double bonds (C=C=C).

[Donaldson Tan]

http://en.wikipedia.org/wiki/Conjugated_system

conjugated system strictly refers to alternating single C-C and C=C bonds.

to change from alkyne to allene requires the carbon atom to rehybridise (from sp to sp2). the new sp2 state has less s character than the previous state, so i don't think it would be energetically favourable to rearrange itself from alkyne to allene. for polymers, it would also to change the overall shape of the polymer for this transition in hybridisation state. This requires lots of energy.

[Winga]

to change from alkyne to allene requires the carbon atom to rehybridise (from sp to sp2).

You mean the hybridisation of allene's carbons is sp2 or they rehybridise from sp to sp2 and then from sp2 back to sp?

Is the hybridisation of allene's carbons sp hybridised?

[Donaldson Tan]

oh yeah. i presumed C=C means sp2 hybridised, ie. I erred. you are right that it remains sp.

Regarding the formation of allene from alkyne, it would means that 1C=C and 1 C=C is converted to 2C=C. I included the average bond energy below:

C=C = 820 kJ/mol (196 kcal/mol), C=C = 611 kJ/mol (146 kcal/mol), C-C = 368 kJ/mol (88 kcal/mol)
dH = +(820+368) - 2(611) = -34kJ/mol

dH is negative to break 1C=C and 1C-C to yield 2 C=C. Rearrangement to form C=C would be energetically favourble.

[Winga]

So, the resonance is favourable, but it is not a conjugated system according to the definition?

[Winga]

Wait, I think there is a problem in the calculation.

octa-2,4,6-triyne
C-C=C-C=C-C=C-C <=> C-C⁺=C=C=C^--C=C-C

dH = 2(820) + 368 - 3(611) = +175 kJ/mol

[movies]

Yes, the system would be conjugated.  The energetics will never be favorable to form the conjugate resonance structure because then it would just exist like that, right?  The key to understanding conjugation is that it is capable of stabilizing other high-energy intermediates, for example allylic cations which are stabilized by resonance.

One caveat to conjugation is that the orbitals have to be aligned properly to allow for orbital overlap.  There are examples of adjacent double-bonds which cannot conjugate to one another because they are conformationally restricted.

[Donaldson Tan]

One caveat to conjugation is that the orbitals have to be aligned properly to allow for orbital overlap.  There are examples of adjacent double-bonds which cannot conjugate to one another because they are conformationally restricted.

how can the carbon atoms be conformationally restricted?

[Winga]

In this case, the p-orbitals are not parallel to each other but perpendicular.

[Donaldson Tan]

what talking you? in C=C, each pi bond is located in different plane. the first pi bond is due to overlapping of 2pz orbitals, and the second pi bond is due to overlap of 2py orbitals.

[movies]

how can the carbon atoms be conformationally restricted?

Not in this particular case, but there are molecules where this is the case.  Take cyclooctatetraene, for example, it adopts a conformation where each pi-bond is perpendicular to the two adjacent to it to avoid being anti-aromatic.