While modern day discoveries and characterizations of allotropes are reported, the definition of allotropy remains ambiguous as the question of what constitutes an allotrope is under debate. Lagow et al. reported the synthesis of a terminally capped linear acetylenic carbon with alternating single and triple binds, claiming it to be a stable sp carbon allotrope (1994), a subject of debate as the classification of acetylenic carbon as an allotrope continues to be determined, and the stability of such a compound is in question. The proposed structure and stability of a linear sp carbon of such a proposed length and with alternating single and triple bonds is disputed by Hirsch et al. Thus, the stability of long-chain carbon allotropes and the characterization of the linear sp carbon synthesized by Lagow et al., given its terminal end design, remains in question as to the significance of the claim.
The IUPAC Red Book defines allotropes as "different structural modifications of [an] element," with allotropic transition considered the "transition of a pure element, at a defined temperature and pressure, from one crystal structure to another which contains the same atoms but which has different properties" (Golden Age, p.2). This definition excludes ozone and oxygen, and does not discuss the classification of carbon allotropes, which can be examined on the basis of the hybridization of their valence orbitals. More modern definitions include the classification of tin where one allotrope is a covalently bonded solid and the other a metal, and differentiate between allotropic molecules, such as dioxygen, infinite covalent solids, like diamond, and infinite, covalently bonded layers with weak intermolecular forces, as seen with graphite. There are also materials that crystallize where the covalent bonding between the elements is unchanged, which are termed polymorphs rather than true allotropes.
The report that a series of acetylenic carbon species with sp hybridized carbon atom linear chains and stabilized terminal groups has added to the debate over the definition of allotropes because of their incorporation of heteroatoms at their terminal ends, as well as the experimental determination of their overall stability. For reference, acetylene is a molecular structure where carbon is bound to two other atoms via two double bonds or one single and one triple bond, with linear hybridization (180o between bonds):
The controversy arises because, while they do not directly fit the definition of allotropes, non-molecular allotropes like diamond and graphite will terminate with other elements (so no longer a pure element) to become stable. It is argued that the stoichiometry of the terminating groups of the acetylenic carbon species is defined as compared to that for diamond or graphite, so are less considered to belong to the allotropes. Also, allotropes have generally been isolated through reducing conditions rather than synthesized, as is the case with the acetylenic carbon species.
The linear carbon allotrope with sp hybridization proposed by Lagow et al. was synthesized using non-reactive terminal end groups for stabilization, with claims that the chain length was a minimum of 300 carbon atoms long. They synthesized a (t-Bu) C8(t-Bu) model, producing a crystalline structure of alternating long and short bonds, demonstrated to be thermally stable at 130oC and at pressures up to 60 kbar. In a second synthesis carbon was produced as a by-product. Through modification of this process they were able to prepare acetylene carbon chains with phenyl caps, with stability shown to 130oC. During the study, they also determined that terminating the carbon chains with delocalized electrons rather than end groups showed instability, and the addition of free radicals to the end caps suppresses fullerenes. Lagow et al. concede that the question of characterization of their synthesized carbon compound as a carbon allotrope arises as the fullerenes are the only molecular allotropes absent of end groups, with diamond and graphite containing terminal groups. Hirsch et al., however, dispute the discovery based on theoretical evidence, contrary to Lagow's claims based on observation.
Hirsch et al. reported on the isolation and characterization of new rod-shaped homologues 1-5, as well as the following homologue, C18N2, extrapolating increases in chain length to predict the spectroscopic characterization of the hypothetical carbyne sp-C?. It was found during stability experiments that polyenes become less stable with increases in chain…