organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, annulynes or dehydroannulenes are conjugated monocyclic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

s with alternating single and

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

s in addition to at least one

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

. They are related to

annulene Annulenes are monocyclic hydrocarbons that contain the maximum number of non-cumulated or conjugated double bonds (' mancude'). They have the general formula C''n''H''n'' (when ''n'' is an even number) or C''n''H''n''+1 (when ''n'' is an odd num ...

s, which only have alternating single and double bonds. The smallest member of this class is nnulyne but is never observed because the molecule carries too much angle strain. The next member is nnulyne or benzyne which is a

reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...

well known in

. nnulyne is known to exist but quickly dimerizes or trimerizes; the compound has been trapped as its

radical anion In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a ...

and observed by

EPR spectroscopy Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spin ...

. 0nnulyne, like nnulyne, only exists in theory. 2nnulyne has been observed in 2005 by Stevenson et al. in solution by

NMR spectroscopy Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...

room temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...

. Reaction of 1,5-hexadiyne and

potassium tert-butoxide Potassium ''tert''-butoxide (or potassium ''t''-butoxide) is a chemical compound with the formula CH3)3COKsub>''n'' (abbr. KOtBu). This colourless solid is a strong base (pKa of conjugate acid is 17 in H2O), which is useful in organic syn ...

was reported to yield two isomers 5,9-di-''trans''- 2annulyne and 3,11-di-''trans''- 2nnulyne in a 1:1 ratio. The proposed reaction sequence involved an unspecified electron transfer reaction. A third single isomer 3,9-di-''trans''- 2nnulyne could be obtained in three steps from

hexabromocyclododecane Hexabromocyclododecane (HBCD or HBCDD) is a brominated flame retardant. It consists of twelve carbon, eighteen hydrogen, and six bromine atoms tied to the ring. Its primary application is in extruded (XPS) and expanded (EPS) polystyrene foam used ...

. Unlike other annulynes, the 2nnulyne isomers were found to be very stable and did not self-condense. These annulynes reacted with

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

metal to form

s and dianions. The NMR

chemical shift In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of ...

s of two internal protons of the dianion were negative and attributed to an

aromatic ring current An aromatic ring current is an effect observed in aromatic molecules such as benzene and naphthalene. If a magnetic field is directed perpendicular to the plane of the aromatic system, a ring current is induced in the delocalized π electrons o ...

. The external proton next to the triple bond had a chemical shift of nearly 14, attributed to the positive charge of the potassium cation coordinated to it. Two new 2nnulyne isomers were reported in 2008 by the same group. The Stevenson findings were challenged by Christl and Hopf in 2009. They were unable to conceive a plausible mechanism and argued that based on the available spectroscopic data the products formed were in fact two linear 1,3-hexadien-5-yne isomers and not macrocycles. Computational analysis of this compound suggests valence isomerization to

biphenyl Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...

is very exothermic but also with a high kinetic barrier. 4nnulyne was reported in 1962 by Jackman et al.

Other annulynes

Dehydroannulenes with more than one triple bond were pioneered by Franz Sondheimer whose research group reported bisdehydro 2nnulene in 1962 and 1,5,9-tridehydro 2nnulene in 1966. A dehydrobenzo 4nnulene was reported in 2001 by Boydston and Haley

Applied research areas

Certain two-dimensional carbon networks containing a repeating dehydroannulene motif have been investigated for potential optoelectronics applications. ''Building Symmetric Two-Dimensional Two-Photon Materials'' Ajit Bhaskar, Ramakrishna Guda, Michael M. Haley, and Theodore Goodson III J. Am. Chem. Soc., 2006, 128 (43), pp 13972–13973

References

{{authority control Annulenes Cycloalkynes