Cyclopentadiene
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| General | ||||||||||||||||
| Surname | Cyclopentadiene | |||||||||||||||
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| Molecular formula | C 5 H 6 | |||||||||||||||
| Brief description |
colorless liquid with a terpene to camphor-like odor, becoming increasingly darker through polymerization |
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| properties | ||||||||||||||||
| Molar mass | 66.10 g mol −1 | |||||||||||||||
| Physical state |
liquid |
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| density |
0.80 g cm −3 |
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| Melting point |
−97 ° C |
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| boiling point |
40 ° C |
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| Vapor pressure |
479 h Pa (20 ° C) |
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| solubility |
practically insoluble in water |
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| Refractive index |
1.4440 (20 ° C) |
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| MAK |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . Refractive index: Na-D line , 20 ° C | ||||||||||||||||
Cyclopentadiene is a colorless liquid. It belongs to the substance group of dienes to.
Cyclopentadiene is found in coal tar . It is the precursor of the cyclopentadienyl - anion , which as a ligand ( Cp ) in metallocenes ( sandwich compounds ) in the organometallic chemistry is widespread. The best-known example is the iron complex ferrocene .
Chlorinated products of cyclopentadiene are used as insecticides ; from hexachlorocyclopentadiene , for example, dieldrin , aldrin , chlordane and heptachlor are produced.
Extraction and presentation
Cyclopentadiene can be obtained from dicyclopentadiene by thermal cleavage (retro- Diels-Alder reaction ). In practice, it is distilled from its dimer in the presence of a catalyst such as iron powder . However, it must be consumed quickly as it dimerizes back to dicyclopentadiene . It will keep for some time in the freezer.
properties
Cyclopentadiene has a strong tendency to dimerize spontaneously. At 20 ° C, a Diels-Alder reaction forms the endo - dicyclopentadiene and at 100 ° C also the exo -dimer.
endo (left) and exo (right) form of the dimer of cyclopentadiene
Cyclopentadiene is insoluble in water and readily soluble in ethanol , benzene and ether . The flash point is 25 ° C, the ignition temperature is 640 ° C. The hazard potential of cyclopentadiene has not yet been adequately tested. The odor threshold is 5.1–5.2 mg · m −3 .
Cyclopentadienyl anion
Due to the resonance stabilization of the anion, cyclopentadiene can be easily deprotonated on the methylene group by bases such as n -butyllithium . Direct metalation with alkali metals is also possible:
- Representation of the cyclopentadienyl anion
With a pK s value of 16.0 cyclopentadiene is one of the most acidic hydrocarbons. The cyclopentadienyl anion (also: cyclopentadienide anion, abbreviation: Cp) consists of a single negatively charged, aromatic five-membered ring system. It forms η 5 - complexes with metal cations , for example, metallocenes , sandwich or triple-decker complexes . As the η 5 ligand, the cyclopentadienyl anion provides a complex with six valence electrons. However, η 1 complexes (η 3 complexes) are also known in which the Cp ligand contributes two or four electrons.
safety instructions
Since cyclopentadiene is only traded and sold as dicyclopentadiene due to its low resistance, there are no hazardous substance labeling and transport regulations for cyclopentadiene. Nevertheless, caution is advised when handling the substance.
Cyclopentadiene is highly flammable and very volatile . Its vapors are heavier than air and form explosive mixtures with it. It also reacts with the oxygen in the air and forms explosive peroxides . Cyclopentadiene also reacts with a number of other chemicals, in which there is strong heat development and in some cases there is a risk of ignition or explosion.
Cyclopentadiene has an irritant effect on the eyes , respiratory tract and skin and leads to disorders of the central nervous system ; in higher concentrations it has a narcotic effect . Due to the distinct odor, accidental exposure to dangerous concentrations of the gaseous substance is not to be expected.
Individual evidence
- ↑ a b c d e f g h i Entry on 1,3-cyclopentadiene in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
- ↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Physical Constants of Organic Compounds, pp. 3-130.
- ↑ Swiss Accident Insurance Fund (Suva): Limit values - current MAK and BAT values (search for 542-92-7 or cyclopentadiene ), accessed on November 2, 2015.
- ↑ Entry on dicyclopentadiene. In: Römpp Online . Georg Thieme Verlag, accessed on May 25, 2014.
- ^ Carey, Sundberg: Organic Chemistry. VCH 1995.
- ↑ Christoph Elschenbroich: Organometallchemie . BG Teubner Verlag, 2008, ISBN 978-3-8351-0167-8 ( table valence electrons in the Google book search).