Adamantane

Natural occurrence

Adamantane was first isolated from petroleum by the Czech chemists S. Landa, V. Machacek, and M. Mzourek. They used fractional distillation of petroleum. They could produce only a few milligrams of adamantane, but noticed its high boiling and melting points. Because of the (assumed) similarity of its structure to that of diamond, the new compound was named adamantane.

Petroleum remains a source of adamantane; the content varies from between 0.0001% and 0.03% depending on the oil field and is too low for commercial production.

Petroleum contains more than thirty derivatives of adamantane. Their isolation from a complex mixture of hydrocarbons is possible due to their high melting point and the ability to distill with water vapor and form stable adducts with thiourea.

Physical properties

Pure adamantane is a colorless, crystalline solid with a characteristic camphor smell. It is practically insoluble in water, but readily soluble in nonpolar organic solvents. Adamantane has an unusually high melting point for a hydrocarbon. At 270 °C, its melting point is much higher than other hydrocarbons with the same molecular weight, such as camphene (45 °C), limonene (−74 °C), ocimene (50 °C), terpinene (60 °C) or twistane (164 °C), or than a linear C₁₀H₂₂ hydrocarbon decane (−28 °C). However, adamantane slowly sublimes even at room temperature. Adamantane can be distilled with water vapor.

Structure

As deduced by electron diffraction and X-ray crystallography, the molecule has T_d symmetry. The carbon–carbon bond lengths are 1.54 Å, almost identical to that of diamond. The carbon–hydrogen distances are 1.112 Å.

At ambient conditions, adamantane crystallizes in a face-centered cubic structure (space group Fm3m, a = 9.426 ± 0.008 Å, four molecules in the unit cell) containing orientationally disordered adamantane molecules. This structure transforms into an ordered, primitive, tetragonal phase (a = 6.641 Å, c = 8.875 Å) with two molecules per cell, either upon cooling to 208 K or pressurizing to above 0.5 GPa.

This phase transition is of the first order; it is accompanied by an anomaly in the heat capacity, elastic, and other properties. In particular, whereas adamantane molecules freely rotate in the cubic phase, they are frozen in the tetragonal one; the density increases stepwise from 1.08 to 1.18 g/cm³, and the entropy changes by a significant amount of 1594 J/(mol·K).

Hardness

Elastic constants of adamantane were measured using large (centimeter-sized) single crystals and the ultrasonic echo technique. The principal value of the elasticity tensor, C₁₁, was deduced as 7.52, 8.20, and 6.17 GPa for the <110>, <111>, and <100> crystalline directions. For comparison, the corresponding values for crystalline diamond are 1161, 1174, and 1123 GPa. The arrangement of carbon atoms is the same in adamantane and diamond; however, in the adamantane solid, molecules do not form a covalent lattice as in diamond, but interact through weak van der Waals forces. As a result, adamantane crystals are very soft and plastic.

Spectroscopy

The nuclear magnetic resonance (NMR) spectrum of adamantane consists of two poorly resolved signals, which correspond to sites 1 and 2 (see picture below). The ¹H and ¹³C NMR chemical shifts are respectively 1.873 and 1.756 ppm and are 28.46 and 37.85 ppm. The simplicity of these spectra is consistent with high molecular symmetry.

Mass spectra of adamantane and its derivatives are rather characteristic. The main peak at m/z = 136 corresponds to the C
₁₀H⁺
₁₆ ion. Its fragmentation results in weaker signals as m/z = 93, 80, 79, 67, 41 and 39.

The infrared absorption spectrum of adamantane is relatively simple because of the high symmetry of the molecule. The main absorption bands and their assignment are given in the table:

^* Legends correspond to types of oscillations: δ – deformation, ν – stretching, ρ and ω – out of plane deformation vibrations of CH₂ groups.

Optical activity

Adamantane derivatives with different substituents at every nodal carbon sites are chiral. Such optical activity was described in adamantane in 1969 with the four different substituents being hydrogen, bromine, methyl, and carboxyl. The values of specific rotation are small and are usually within 1°.

Nomenclature

Using the rules of systematic nomenclature, adamantane is called tricyclo[3.3.1.1^3,7]decane. However, IUPAC recommends using the name "adamantane".

The adamantane molecule is composed of only carbon and hydrogen and has T_d symmetry. Therefore, its 16 hydrogen and 10 carbon atoms can be described by only two sites, which are labeled in the figure as 1 (4 equivalent sites) and 2 (6 equivalent sites).

Structural relatives of adamantane are noradamantane and homoadamantane, which respectively contain one less and one more CH₂ link than the adamantane.

The functional group derived from adamantane is adamantyl, formally named as 1-adamantyl or 2-adamantyl depending on which site is connected to the parent molecule. Adamantyl groups are a bulky pendant group used to improve the thermal and mechanical properties of polymers.

Chemical properties

The chemistry of adamantane is dominated by the stability of the tertiary adamantanium cation, C₁₀H⁺
₁₅, itself isolable when treating 1‑fluoro­adamantane with SbF₅.

Radical reactions of adamantane also occur preferentially at the bridgehead due to steric hindrance. For example, sulfoxidation gives exclusively 1-adamantanesulfonic acid.

Bridgehead reactions

Adamantanium's stability eases S_N,1 substitution for 1‑adamantane derivatives. Thus 1‑bromo­adamantane hydrolyzes in acid to 1‑hydroxy­adamantane, and the latter fluorolyzes to 1‑fluoro­adamantane. 1‑Hydroxyadamantane also alkylates activated arenes without a catalyst, and the drug amantadine can be prepared in a Ritter reaction with acetonitrile:

With unsubstituted adamantane, strong Brønsted acids and other comparable electrophiles abstract a hydride equivalent from a bridgehead. For example, nitrosyl salts or an applied voltage oxidize adamantane to the cation intermediate. Under most conditions, the resulting adamantanium cation then attacks nearby nucleophiles, for a net substitution. Ozonation gives the alcohol:

Adamantane also reacts with less exotic reagents.

Various brominators, including molecular bromine, react via an ionic pathway, and similar reactions are possible with gaseous fluorine (but chlorination is likely a radical reaction). The reaction products depend on the conditions, especially the presence and type of catalysis. Boiling adamantane with bromine results in a monosubstituted adamantane, 1-bromadamantane. Multiple substitution is achieved with a Lewis acid:

The rate of bromination is accelerated upon addition of Lewis acids and is unchanged by irradiation or addition of free radicals.

Adamantane undergoes Koch-Haaf carboxylation with formic acid to give 1-adamantanecarboxylic acid...

...and a Scholl-type arylation with benzene and acidic clay.

Nitration of adamantane is a difficult reaction characterized by moderate yields. Direct nitration occurs best in glacial acetic acid.

Substitution at bridging carbons

Adamantane oxidizes in concentrated sulfuric acid to 2‑adamantanone:

Even though the end product is unsubstituted at the bridgeheads, they are mechanistically involved. A bridgehead cation forms initially before undergoing intermolecular hydride transfer.

Cations formed at a bridging carbon appear to equilibrate with a protoadamantyl cation: a ring expansion/contraction exchanges the roles of a neighboring bridgehead and the cationic center. Indeed, if an adamantane bridgehead carbon is substituted to stabilize a positive charge, then cation formation along the bridges causes a semipinacol rearrangement. Such processes can be reversed to substitute at multiple bridging carbons at simultaneously.

The carbonyl group of adamantanone is less reactive than most cyclic carbonyls, since (by symmetry) any adduct suffers from axial strain. Nevertheless, sufficiently reactive reagents (e.g. alkyllithiums instead of Grignard reagents) allow further functionalization at the bridging site. For example, adamantanone is the starting compound for 2-adamantanecarbonitrile and 2-methyl-adamantane.

Uses

Adamantane itself enjoys few applications since it is merely an unfunctionalized hydrocarbon. It is used in some dry etching masks and polymer formulations.

In solid-state NMR spectroscopy, adamantane is a common standard for chemical shift referencing.

In dye lasers, adamantane may be used to extend the life of the gain medium; it cannot be photoionized under atmosphere because its absorption bands lie in the vacuum-ultraviolet region of the spectrum. Photoionization energies have been determined for adamantane as well as for several bigger diamondoids.

In medicine

All medical applications known so far involve not pure adamantane, but its derivatives. The first adamantane derivative used as a drug was amantadine – first (1967) as an antiviral drug against various strains of influenza and then to treat Parkinson's disease. Other drugs among adamantane derivatives include adapalene, adapromine, bromantane (bromantan), carmantadine, chlodantane (chlodantan), dopamantine, gludantan (gludantane), hemantane (hymantane), idramantone (kemantane), memantine, nitromemantine, rimantadine, saxagliptin, somantadine, tromantadine, and vildagliptin. Polymers of adamantane have been patented as antiviral agents against HIV.

Influenza virus strains have developed drug resistance to amantadine and rimantadine, which are not effective against prevalent strains as of 2016.

In designer drugs

Adamantane was recently identified as a key structural subunit in several synthetic cannabinoid designer drugs, namely AB-001 and SDB-001.

Spacecraft propellant

Adamantane is an attractive candidate for propellant in Hall-effect thrusters because it ionizes easily, can be stored in solid form rather than a heavy pressure tank, and is relatively nontoxic.

Potential technological applications

Some alkyl derivatives of adamantane have been used as a working fluid in hydraulic systems. Adamantane-based polymers might find application for coatings of touchscreens, and there are prospects for using adamantane and its homologues in nanotechnology. For example, the soft cage-like structure of adamantane solid allows incorporation of guest molecules, which can be released inside the human body upon breaking the matrix. Adamantane could be used as molecular building blocks for self-assembly of molecular crystals.

Adamantane analogues

Many molecules and ions adopt adamantane-like cage structures. Those include phosphorus trioxide P₄O₆, arsenic trioxide As₄O₆, phosphorus pentoxide P₄O₁₀ = (PO)₄O₆, phosphorus pentasulfide P₄S₁₀ = (PS)₄S₆, and hexamethylenetetramine C₆N₄H₁₂ = N₄(CH₂)₆. Particularly notorious is tetramethylenedisulfotetramine, often shortened to "tetramine", a rodenticide banned in most countries for extreme toxicity to humans. The silicon analogue of adamantane, sila-adamantane, was synthesized in 2005. Arsenicin A is a naturally occurring organoarsenic chemical isolated from the New Caledonian sea sponge Echinochalina bargibanti and is the first known heterocycle to contain multiple arsenic atoms.

Conjoining adamantane cages produces higher diamondoids, such as diamantane (C₁₄H₂₀ – two fused adamantane cages), triamantane (C₁₈H₂₄), tetramantane (C₂₂H₂₈), pentamantane (C₂₆H₃₂), hexamantane (C₂₆H₃₀), etc. Their synthesis is similar to that of adamantane and like adamantane, they can also be extracted from petroleum, though at even much smaller yields.

The related dication of 1,3-didehydroadamantane also has elevated stability due to the phenomenon called "three-dimensional aromaticity" or homoaromaticity. This four-center two-electron bond involves one pair of electrons delocalized among the four bridgehead atoms: