Propellane
Encyclopedia
In organic chemistry
, propellane is any member of a class of polycyclic
hydrocarbon
s, whose carbon
skeleton consists of three rings of carbon
atoms sharing a common carbon-carbon covalent bond
. The name derives from a supposed resemblance of the molecule to a propeller
: namely, the rings would be the propeller's blades, and the shared C-C bond would be its axis.
Propellanes with small cycles are highly strained and unstable, and are easily turned into polymer
s with interesting structures. Partly for these reasons, they have been the object of much research.
In the literature, the bond shared by the three cycles is usually called the "bridge
"; the shared carbon atoms are then the "bridgehead
s". The notation [x.y.z]propellane means the member of the family whose rings have x, y, and z carbons, not counting the two bridgeheads; or x+2, y+2, and z+2 carbons, counting them. The chemical formula is therefore C2+x+y+zH2(x+y+z). The minimum value for x, y, and z is 1, meaning a 3-carbon ring.
There is no structural ordering between the rings, so, for example, [1.3.2]propellane is the same substance as [3.2.1]propellane. Therefore it is customary to sort the indices in decreasing order, x ≥ y ≥ z.
or [2.2.2]propellane
, the two carbons at the ends of the axial bond will be highly strained, and their bonds may even assume an inverted tetrahedral geometry.
The resulting steric strain causes such compounds to be unstable and highly reactive. The axial C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons.
Surprisingly, the most strained member [1.1.1] is far more stable than the other small ring members ([2.1.1], [2.2.1], [2.2.2], [3.2.1], [3.1.1], and [4.1.1]).
with two active centers, and then joining these radicals in a linear chain. For the propellanes with small cycles (such as [1.1.1], [3.2.1], or 1,3-dihydroadamantane), this process is easily achieved, yielding either simple polymers or alternating copolymers. For example, [1.1.1]propellane yields spontaneoulsy an interesting rigid polymer called staffane
; and [3.2.1]propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units [-C8H12-] alternate with [-O-O-] groups.
described in 1978 a general method which should yield [n.3.3]propellanes for any n greater than 2.
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
, propellane is any member of a class of polycyclic
Polycyclic compound
In organic chemistry, a polycyclic compound is a cyclic compound with more than one hydrocarbon loop or ring structures . In general, the term includes all polycyclic aromatic compounds, including the polycyclic aromatic hydrocarbons, the heterocyclic aromatic compounds containing sulfur,...
hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s, whose carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
skeleton consists of three rings of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms sharing a common carbon-carbon covalent bond
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...
. The name derives from a supposed resemblance of the molecule to a propeller
Propeller
A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid is accelerated behind the blade. Propeller dynamics can be modeled by both Bernoulli's...
: namely, the rings would be the propeller's blades, and the shared C-C bond would be its axis.
Propellanes with small cycles are highly strained and unstable, and are easily turned into polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s with interesting structures. Partly for these reasons, they have been the object of much research.
In the literature, the bond shared by the three cycles is usually called the "bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
"; the shared carbon atoms are then the "bridgehead
Bridgehead
A bridgehead is a High Middle Ages military term, which antedating the invention of cannons was in the original meaning expressly a referent term to the military fortification that protects the end of a bridge...
s". The notation [x.y.z]propellane means the member of the family whose rings have x, y, and z carbons, not counting the two bridgeheads; or x+2, y+2, and z+2 carbons, counting them. The chemical formula is therefore C2+x+y+zH2(x+y+z). The minimum value for x, y, and z is 1, meaning a 3-carbon ring.
There is no structural ordering between the rings, so, for example, [1.3.2]propellane is the same substance as [3.2.1]propellane. Therefore it is customary to sort the indices in decreasing order, x ≥ y ≥ z.
General properties
Data can be found in Osmont et al., Energy and Fuels, Vol. 22, pp. 2241-2257 (2008).Strain
In the propellanes with small cycles, such as [1.1.1]propellane1.1.1-Propellane
[1.1.1]Propellane is an organic compound, the simplest member of the propellane family. It is a hydrocarbon with formula C5H6 or C23. The molecular structure consists of three rings of three carbon atoms each, sharing one C-C bond....
or [2.2.2]propellane
2.2.2-Propellane
[2.2.2]Propellane, formally tricyclo[2.2.2.01,4]octane is an organic compound, a member of the propellane family. It is a hydrocarbon with formula C8H12, or C23. Its molecule has three rings with four carbon atoms each, sharing one C-C bond....
, the two carbons at the ends of the axial bond will be highly strained, and their bonds may even assume an inverted tetrahedral geometry.
The resulting steric strain causes such compounds to be unstable and highly reactive. The axial C-C bond is easily broken (even spontaneously) to yield less-strained bicyclic or even monocyclic hydrocarbons.
Surprisingly, the most strained member [1.1.1] is far more stable than the other small ring members ([2.1.1], [2.2.1], [2.2.2], [3.2.1], [3.1.1], and [4.1.1]).
Polymerization
In principle, any propellane can be polymerized by breaking the axial C-C bond to yield a radicalRadical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
with two active centers, and then joining these radicals in a linear chain. For the propellanes with small cycles (such as [1.1.1], [3.2.1], or 1,3-dihydroadamantane), this process is easily achieved, yielding either simple polymers or alternating copolymers. For example, [1.1.1]propellane yields spontaneoulsy an interesting rigid polymer called staffane
Staffane
A staffane or [n]staffane is an organic compound, a polycyclic hydrocarbon with molecular structure H-[-C≡3≡C-]n-H, for some integer n ≥ 1. The chemical formula is therefore C5nH6n+2...
; and [3.2.1]propellane combines spontaneously with oxygen at room temperature to give a copolymer where the bridge-opened propellane units [-C8H12-] alternate with [-O-O-] groups.
Synthesis
The smaller-cycle propellanes are difficult to synthesize because of their strain. Larger membes are more easily obtained. R. Weber and J. CookJames M. Cook
James Merrill Cook was an American businessman, banker and politician.-Life:...
described in 1978 a general method which should yield [n.3.3]propellanes for any n greater than 2.
True propellanes
- [1.1.1]propellane1.1.1-Propellane[1.1.1]Propellane is an organic compound, the simplest member of the propellane family. It is a hydrocarbon with formula C5H6 or C23. The molecular structure consists of three rings of three carbon atoms each, sharing one C-C bond....
, C5H6 (K. Wiberg and F. Walker, 1982). It is a highly strained molecule: the two central carbons have an inverted tetrahedron geometry, and each of the three cycles is the notoriously strained cyclopropaneCyclopropaneCyclopropane is a cycloalkane molecule with the molecular formula C3H6, consisting of three carbon atoms linked to each other to form a ring, with each carbon atom bearing two hydrogen atoms...
ring. The length of the central bondBond length- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
is only 160 pm. It is an unstable product that undergoes thermal isomerization to 3-methylenecylobutene at 114 °C,and spontaneously reacts with acetic acidAcetic acidAcetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
to form a methylenecyclobutane esterEsterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
.
- [2.1.1]propellane, C6H8 (K. Wiberg, F. Walker, W. Pratt, and J. Michl, ????). This compound was detected by infrared spectroscopyInfrared spectroscopyInfrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
at 30 KKelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
but has not been isolated as a stable molecule at room temperature (as of 2003). It is believed to polymerize above 50 K. The bonds of the shared carbons have an inverted tetrahedral geometry; the compound's strain energy was estimated as 106 kcal/mol.
- [2.2.1]propellane, C7H10 (F Walker, K. Wiberg, and J. Michl, 1982). Obtained gas-phase dehalogenation with alkali metalAlkali metalThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
atoms. Stable only in frozen gas matrix below 50K; oligomerizes or polymerizes at higher temperatures. The strain energy released by breaking the axial bond was estimated as 75 kcal/mol.
- [3.1.1]propellane, C7H10. Isolable.
- [3.2.1]propellane or tricyclo[3.2.1.01,5]octane, C8H12 (K. Wiberg and G. Burgmaier, 1969). Isolable. Has inverted tetrahedral geometry at the shared carbons. Estimated strain energy of 60 kcal/mol. Remarkably resistant to thermolysis; polymerizes in diphenyl etherDiphenyl etherDiphenyl ether is the organic compound with the formula O2. The molecule is subject to reactions typical of other phenyl rings, including hydroxylation, nitration, halogenation, sulfonation, and Friedel-Crafts alkylation or acylation...
solution with halflife of about 20 hours at 195°. It reacts spontaneously with oxygen at room temperature to give a copolymer with -O-O- bridges.
- [4.1.1]propellane, C8H12. (D. Hamon, V. Trennery, 1981) Isolable.
- [2.2.2]propellane2.2.2-Propellane[2.2.2]Propellane, formally tricyclo[2.2.2.01,4]octane is an organic compound, a member of the propellane family. It is a hydrocarbon with formula C8H12, or C23. Its molecule has three rings with four carbon atoms each, sharing one C-C bond....
or tricyclo[2.2.2.01,4]octane, C8H12 (P. EatonPhilip EatonPhilip E. Eaton is a Professor Emeritus of Chemistry at the University of Chicago. He and his fellow researchers were the first to synthesize the "impossible" cubane molecule in 1964....
and G. Temme, 1973). This propellane is unstable, too, due to the three cyclobutaneCyclobutaneCyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...
-like rings and the highly distorted bond angles (three of them nearly 90°, the other three nearly 120°) at the axial carbons. Its strain energyStrain energyIn a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
is estimated to be 93 kcal/mol (390 kJ/mol).
- [3.3.3]propellane, C11H18 (R. Weber and J. Cook, 1978). It is a stable solid that melts at 130 °C.
- [4.3.3]propellane, C12H20 (R. Weber and J. Cook, 1978). A stable solid that melts at 100-101°C.
- [6.3.3]propellane, C14H24 (R. Weber and J. Cook, 1978). An oily liquid that boils at 275-277°C.
- [10.3.3]propellane, C18H32 (S. Yang and J. Cook, 1976). A stable solid that sublimes at 33-34°C.
Propellane derivatives
- 1,3-dehydroadamantane1,3-Dehydroadamantane1,3-Dehydroadamantane, formally tetracyclo[3.3.1.13,7.01,3]decane, is an organic compound with formula C10H14, which can be obtained from adamantane by removal of two hydrogen atoms to create an internal bond...
, C10H14 (R. Pincock and E.Torupka, 1969). This compound is formally derived from adamantaneAdamantaneAdamantane is a colorless, crystalline chemical compound with a camphor-like odor. With a formula C10H16, it is a cycloalkane and also the simplest diamondoid. Adamantane molecules consist of three cyclohexane rings arranged in the "armchair" configuration. It is unique in that it is both rigid...
by removing two hydrogens and adding an internal bond. It can be viewed as [1.3.3]propellane (whose axis would be the new bond), with an extra methyleneMethyleneMethylene is a chemical species in which a carbon atom is bonded to two hydrogen atoms. Three different possibilities present themselves:* the -CH2- substituent group: e.g., dichloromethane ....
connecting its two larger "propeller blades". It is unstable and reactive and can be polymerized.