Cyclopropane

Trimethylene

Colorless gas with a smell petroletherähnlichem

Gaseous

-127.62 ° C

-32.86 ° C

Very poor in water ( 502 mg · l -1)

0

1.3799 (-42 ° C)

Risk

Not set

53.3 kJ / mol

Template: Infobox chemical / molecular formula search available

Cyclopropane is a colorless, gaseous cycloalkane, the first link in this class of compounds.

Discovery

When trying the method of Wurtz reaction ( Wurtz synthesis) on α, ω - extend dihaloalkanes, discovered August friend in 1882 is a hydrocarbon with the formula C3H6, which he called trimethylene.

Production and representation

Gustavson 1887 was a simpler, safer synthesis by used for dehalogenation instead of the previously usual sodium metal zinc dust in aqueous ethanol. The preparation for this reaction principle is likely to be the cheapest even today; It was optimized and also with 1,3- dichloropropane or 1-bromo- accessible 3-chloropropane.

Properties

Physical Properties

The molecule belongs to the symmetry group D3h. A structure determined by electron diffraction yielded a distance of the carbon atoms of 151.0 pm. For the C -H bond length, a value of 108.9 pm was determined, and for the HCH angle of 115.1 °.

The three carbon atoms of the cyclopropane necessarily lie in one plane. In the classical, ie non- quantum chemical Baeyer- model therefore has a high angle strain is to be expected, as well as torsional strain ( Pitzer strain) by the ecliptic position of the hydrogen atoms. For the strain energy 12-13 kcal mol-1 were calculated. The structural data show, however, that must be special bond conditions during cyclopropane. Simple models, such as the Baeyer- model are therefore not appropriate.

With the concept of strictly localized sigma bonds cyclopropane can not be understood. In the small molecule occur interactions of electrons, which play no role in the larger cycloalkanes. Therefore cyclopropane differs in physical and chemical properties strongly dependent on other cycloalkanes.

Quantum chemical calculations by ab initio methods ( see Article chemo computer science ) have shown that the electron population have high values ​​outside the edges of the hypothetical triangle, so the line through which one the centers of the C atom nuclei can think of connected to.

The physical chemist Theodor Förster was probably to be the first that the cyclopropane the " Valenzrichtung " from the " Binding Position ", that is, the C- C bond in the classical formula differs.

Charles Coulson and WE Moffitt developed this orbital model, which was later named after them on.

The valence orbitals of sp3 -hybridized carbon atom form angles of 109.4 ° and have 25 % s character and 75 % p character. Two of them are used for the bonds to the hydrogen atoms. In cyclopropane the remaining orbitals of three CH2 fragments had to be combined. The area of ​​the overlap is small and then, but not in the connecting line between the carbon atoms.

The overlap is stronger if one uses hybrid orbitals with higher p- component. The optimum, revealed an orbital, which only has 17% s character for the C -C bond, whereas 33 % s - character for the C -H bonds, could be so called sp5 orbital.

This leads to the picture " curved " C -C bonds (English bent bonds) that have been casually referred to as " banana bonds ". The valence angles ( interior angle ) of the C -C -C bonds amounted to 104 °.

The model explains why the C -C bonds are shorter than in cyclohexane and straight-chain saturated hydrocarbons ( alkanes ) and still weaker. The higher s- character of the C -H bonds affects the force constant of the C -H bond ( infrared and Raman spectrum), the chemical shift and coupling constants in NMR spectra, and the increased CH acidity of the hydrocarbon.

A second orbital model was developed by Arthur Donald Walsh. The Walsh model goes from sp2 hybrid orbitals of the CH2 group; the " bent " bonds are formed by overlap then pure p orbitals.

The known at aromatics σ - bonds were used in 1986 by Michael JS Dewar for the non-aromatics cyclopropane as an explanation that the strain energy from a theoretical extrapolation is far higher than the actual 27.5 kcal / mol.

Chemical Properties

In comparison with higher cycloalkanes cyclopropane shows a specific pattern of reactivity: often, a C- C bond is broken. Association reactions are characteristic. Explanations have been given, among others, the concepts " strain energy " and " Walsh " model.

When heating cyclopropane, which was labeled with deuterium atoms, observed is called a " geometric isomerization " cis -1 ,2- Dideuteriocyclopropan formed in an equilibrium reaction of trans-1 ,2- Dideuteriocyclopropan.

Thus, the deuterium atoms change their position (Greek topos ), so you can refer to this process as topomerization of the molecule. A concerted mechanism was discussed.

At about 500 ° C is converted to propene cyclopropane. This Konstitutionsisomerisierung examined in the range from 469.6 to 518.6 ° C, requires an activation energy of about 65 kcal mol -1. As an intermediate step, a " trimethylene diradical " (propane -1 ,3- diyl) was postulated.

On a palladium catalyst cyclopropane reacts with hydrogen to propane. With hydrogen bromide ( HBr), it reacts to form 1- bromopropane, which also shows the " olefinic character." Chlorine gas also reacts with addition to 1,3- dichloropropane.

Use

In the past it has been applied in a 15-30 % mixture with oxygen as an anesthetic, which apart today because of the disadvantages ( explosive vapors, poor controllability of anesthesia, cardiac and hepatic toxicity ).

Safety

Cyclopropane forms like all lower hydrocarbons with air flammable mixtures. The explosion range is between 2.4 vol % (40 g/m3) as the lower explosive limit ( LEL) and 10.4 % by volume (385 g/m3 ) and upper explosive limit (UEL ). The marginal gap width was determined to be 0.91 mm. The result is thus a mapping in the explosion group IIA. With a minimum ignition energy of 0.17 mJ vapor - air mixtures are extremely flammable. The ignition temperature is 495 ° C. The fabric falls within the temperature class T1.

Derivatives and substituents

Many derivatives are produced by chemical synthesis, eg with alkyl group as the balance you are in the petroleum industry, examines cancer research and other scientific areas.

The biosynthesis is catalyzed by cyclopropane synthase, so that mycolic acids produced which are essential to ensure that tuberculosis can be treated only with difficulty with hydrophobic antibiotics. The enzyme itself is a methyl, the cis -or trans- cyclopropane rings incorporating the unsaturated fatty acid there by S-adenosylmethionine.