Depends what the use is. C4 is longest tahr is gaseous so C5 is a long chain. The solids start to form somewhere at C20. The question is like how long is a rope? When is the rope a long one.
Тhe existence of all substances, including substances (ropes) of carbon atoms at a temperature of 298 K and a pressure of 1 atm is determined by a negative change in the Gibbs standard energy of its formation. If it is positive, then this rope can not exist.
According to R. Zbinden, the linear hydrocarbon chain consisting of 7-8 carbon atoms may be considered as long enough because the contribution of the end-groups becomes negligibly small
There is no exact answer, since short and long, small and big are only applicable within a given setting.
Your question can be unequivocally answered when you ask for the superlative. What is the shortest chain? What is the longest chain?
The shortest carbon chain consist of two connected C-atoms, thus ethane, ethane, ethyne.
The longest alkan, octanonacontahectane, links 198 C atoms.[1]
The longest synthesized polyene chain connects 80 C atoms with 27 double bonds.[2] The longest saturated fatty acid assembles 192 C-atoms (dononacontahectanoic acid).[3] The longest synthesized conjugated unsaturated fatty acid joins 45 C atoms.[4] Structure and spectra of a conjugated polyenal with 230 C atoms and 91 double bonds have been calculated.[4]
You can also unequivocally answer the question by relying on the comparative: A C120-chain or even a C3-chain is longer than a C2-chain.
What is a long chain? You define yourself “long” in the context of your topic.
[1] JK Hobbs, MJ Hill, A Keller, PJ Barham
Experimentally determined temperature-concentration phase diagrams of monodisperse alkanes with chains containing between 100 and 200 carbons
Journal of Polymer Science, Part B: Polymer Physics 1999, 37, 3188 [2] M Zeeshan, HR Sliwka, V Partali, A Martinez
The longest polyene
Organic Letters 2012, 14, 5496 [3] GM Brooke, S Burnett, S Mohammed, D Proctor, MC Whiting,
Versatile process for the syntheses of very long chain alkanes, functionalised derivatives and some branched chain hydrocarbons
J. Chem. Soc. Perkin Trans. 1 1996, 1635
[4] A Martinez, M Zeeshan, A Zaidi, HR Sliwka, KR Naqvi, V Partali
On infinitenes - Reliable calculation of λ∞ and molecular modeling of lemniscate structured carotenoids
Computational & Theoretical Chemistry 2018, 1125, 133
usually, long chain and short chain descriptions are relative terms. Depends on the frame of reference that one has to consider the length of a C-chain. Otherwise, my colleagues above have tried to put it in better perspectives
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