than the propargylic C–H bond of propyne (382 kcal/mol). The 13C NMR spectrum of allenes is characterized by the signal of the sp-hybridized carbon atom, resonating at a characteristic 200-220 ppm. In contrast, the sp2-hybridized carbon atoms resonate around 80 ppm in a region typical for alkyne and nitrile carbon atoms, while the protons of a CH2 group of a terminal allene resonate at around 4.5 ppm — somewhat upfield of a typical vinylic proton. Allenes possess a rich cycloaddition chemistry, including both [4+2] and [2+2] modes of addition, as well as undergoing formal cycloaddition processes catalyzed by transition metals. Allenes also serve as substrates for transition metal catalyzed hydrofunctionalization reactions. Synthesis Although allenes often require specialized syntheses, the parent allene, propadiene is produced industrially on a large scale as an equilibrium mixture with methylacetylene: H2C=C=CH2 <=> H3C-C#CH This mixture, known as MAPP gas, is commercially available. At 298 K, the ΔG° of this reaction is –1.9 kcal/mol, corresponding to Keq = 24.7. The first allene to be synthesized was penta-2,3-dienedioic acid, which was prepared by Burton and Pechmann in 1887. However, the structure was only correctly identified in 1954. Laboratory methods for the formation of allenes include: from geminal dihalocyclopropanes and organolithium compounds (or metallic sodium or magnesium) in the Skattebøl rearrangement (Doering–LaFlamme allene synthesis) via rearrangement of cyclopropylidene carbenes/carbenoids from reaction of certain terminal alkynes with formaldehyde, copper(I) bromide, and added base (Crabbé–Ma allene synthesis) from propargylic halides by SN2′ displacement by an organocuprate from dehydrohalogenation of certain dihalides from reaction of a triphenylphosphinyl ester with an acid halide, a Wittig reaction accompanied by dehydrohalogenation from propargylic alcohols via the Myers allene synthesis protocol—a stereospecific process from metalation of allene or substituted allenes with BuLi and reaction with electrophiles (RX, R3SiX, D2O, etc.) The chemistry of allenes has been reviewed in a number of books and journal articles. Some key approaches towards allenes are outlined in the following scheme: One of the older methods is the Skattebøl rearrangement (also called the Doering–Moore–Skattebøl or Doering–LaFlamme rearrangement), in which a gem-dihalocyclopropane 3 is treated with an organolithium compound (or dissolving metal) and the presumed intermediate rearranges into an allene either directly or via carbene-like species. Notably, even strained allenes can be generated by this procedure. Modifications involving leaving groups of different nature are also known. Arguably, the most convenient modern method of allene synthesis is by sigmatropic rearrangement of propargylic