History and Synthesis
The possibility of the existence of a hydrocarbon with the C10H16 formula and diamond-like structure of the molecule was suggested by H. Decker at a conference in 1924. Decker called this molecule decaterpene and was surprised that it had not been synthesized yet.
The first attempt of laboratory synthesis was made by German chemist Hans Meerwein in 1924 using reaction of formaldehyde with diethyl malonate in the presence of piperidine. Instead of adamantane, Meerwein obtained 1,3,5,7-tetracarbomethoxybicyclononane-2,6-dione. This compound was later named Meerwein's ester and used in the syntheses of adamantane and its derivatives. Later, another German chemist D. Bottger tried to obtain adamantane using Meerwein's ester as precursor. However, the product, tricyclo- decane ring system, was again an adamantane derivative.
Other researchers attempted to synthesize adamantane using phloroglucinol and derivatives of cyclohexanone but also without success.
Adamantane was first synthesized by Vladimir Prelog in 1941 from Meerwein's ester. The process was impractical as it contained five stages (simplified in the image below) and had a yield of about 0.16%. However, it was sometimes used to synthesize certain derivatives of adamantane.
Prelog's method was refined in 1956. The decarboxylation yield was increased by the addition of the Heinsdecker pathway (11%), and the Hoffman reaction (24%) that raised the total yield to 6.5%. The process was still too complex, and a more convenient method was accidentally found by Paul von Ragué Schleyer in 1957: dicyclopentadiene was first hydrogenated in the presence of a catalyst (e.g. platinum dioxide) and then transformed into adamantane using a Lewis acid (e.g. aluminium chloride) as another catalyst. This method increased the yield to 30–40% and provided an affordable source of adamantane; it therefore stimulated characterization of adamantane and is still used in the laboratory practice. The adamantane synthesis yield was later increased to 60% and nowadays, adamantane is an affordable chemical compound with a cost of the order $1/gram.
All the above methods yield adamantane in the form of polycrystalline powder. Using this powder, single crystals can be grown from the melt, solution or vapor phase (e.g. with the Bridgman–Stockbarger technique). Melt growth result in the worst crystalline quality with a mosaic spread in the X-ray reflection of about 1°. Best crystals are obtained from the liquid phase, but the growth is inpracticably slow – several months for a 5–10 mm crystal. Growth from the vapor phase is a reasonable compromise in terms of speed and quality. Adamantane is sublimated in a quartz tube placed in a furnace, which is equipped with several heaters maintaining a certain temperature gradient (about 10 °C/cm for adamantane) along the tube. Crystallization starts at one end of the tube which is kept near the freezing point of adamantane. Slow cooling of the tube, while maintaining the temperature gradient, gradually shifts the melting zone (rate ~2 mm/hour) producing a single-crystal boule.
Read more about this topic: Adamantane
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