Polyenes
Polyenes are olefins , i.e. organic compounds that contain two or more carbon-carbon double bonds ( C = C double bonds for short ). They belong to the group of alkenes . Depending on the number of double bonds in the molecule, a distinction is made between dienes (with two), trienes (with three), tetraenes (with four), pentaenes (with five double bonds), etc.
The polyenes include, for example, the carotenes and polyethine .
Classification and nomenclature
| Isomeric pentadienes | |
|---|---|
.svg) conjugated diene 1,3-pentadiene |
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.svg) isolated diene 1,4-pentadiene |
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.svg) cumulative diene 1,2-pentadiene |
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Polyenes differ in the arrangement of the double bonds, i.e. the constitution of the molecule:
- If the double and single bonds are in direct alternation in a carbon chain, one speaks of conjugated double bonds (Latin conjugare : connect to a pair). Here the carbon double bonds are separated by exactly one carbon single bond, such as B. at 1,3-pentadiene . Interactions (conjugation) can take place between the double bonds, through which the single bonds in between can acquire a “partial double bond character”. However, it must be taken into account that the length of the = CC = bond, even without “conjugation”, is shorter than with alkanes (sp 2 -sp 2 bond). Such compounds are sometimes referred to as conjuens .
- In the case of isolated double bonds , the double bonds are separated from one another by at least two CC single bonds , e.g. B. at 1,4-pentadiene . As a rule, the double bonds do not interact with one another.
- In the case of cumulative double bonds (Latin cumulatus : heaped, cumulus : heap), the series of carbon double bonds is not interrupted by single carbon bonds, e.g. B. with 1,2-pentadiene . The carbon atoms are arranged linearly along the cumulative double bonds, since the central carbon atoms are in sp hybridization . Compounds with two cumulative double bonds are called allenes , compounds with three or more cumulative double bonds are called cumulenes .
In analogy to the polyenes and polyynes (with more triple bonds) compounds are referred to with several C-C double and - triple bonds than polyenynes .
The name of a polyene, like that of an alkene, is derived from that of the corresponding alkane , with the ending -an being replaced by the Greek numeral representing the number of double bonds ( di , tri , tetra , etc.) and the ending -en , which characterizes the alkenes . The positions of the double bonds are indicated by prefixed Arabic numbers separated by commas. The numbering of the bonds is done in such a way that the smallest possible numbers can be used in the name of the alkene. The numbers are linked to the name with a hyphen. Example: butane (alkane) → butene (alkene, one double bond) → 1,2-butadiene (alkene / diene, two double bonds at position 1 and 2 in the carbon chain)
Occurrence
Polyenes are widespread in nature. Mostly there are colored connections. Among the polyenes, for example, is one of the carotenoids , Annulenes and cyclooctatetraene , cyanine - and polymethine - dyes , Fecapentaene , Gamone and leukotrienes . Fatty acids with several double bonds (polyene) as linoleic acid and arachidonic acid are also part of the polyenes as various so-called macrolides , including macrolide - antibiotics and antifungal agents such as Nystatin , Natamycin , Amphotericin B and candicidin .
properties
structure
The bond length of the single and double bonds in polyenes differs from those in alkanes or alkenes such as ethene . CC single bonds are usually 0.154 nm in length . CC double bonds are 0.134 nm long. However, CC single bonds that follow a CC double bond are shorter than 0.1526 nm.
This is due to the fact that, due to the sp² hybridization of the carbon atom that is involved in the single and double bond (in the case of 1,3-butadiene in the right figure, the carbon atoms 2 and 3), the binding Molecular orbitals do not have such a spatial extent as in sp3 hybridization, for example, in the case of alkanes.
Conformation
Polyenes have CC single bonds around which parts of the molecule can rotate. This enables different, energetically different states ( conformations ) of the molecule.
The rotation around the single bonds results in conformations in which two double bonds are on the same side as seen from the single bond or on opposite sides. One then speaks of cisoid or transoid double bonds.
configuration
Due to the CC double bonds occurring in the molecule, the phenomenon of cis-trans isomerism occurs in polyenes . Two different spatial orientations ( configurations ) of the carbon chain are possible at each double bond . Since these configurations can only be converted into one another by breaking and reestablishing bonds , in this case the double bond, this results in at least physically different connections. With an increasing number of double bonds in the polyene molecule, the possibility of different combinations of cis and trans double bonds and thus the number of isomers increases .
Mesomerism
In the case of the conjugated polyenes, the mesomerism also has an influence on the bond lengths and relationships, similar to benzene and other aromatics .
1. CC double bonds between carbon atoms 1 and 2 or 3 and 4;
2. CC double bond between carbon atoms 2 and 3;
3. Resonance hybrid orbital from states 1. and 2.
There are two mesomeric boundary structures of 1,3-butadiene, which are characterized by different electron distributions ( see figure above). All carbon atoms are sp² hybridized . Namely, each carbon atom of a perpendicular to the molecular plane standing p. Z - atomic orbital has obtained by overlapping with a p z a orbital of an adjacent carbon atom double bond (more specifically, a can bond) form. But this is also possible with more distant carbon atoms, but less likely.
Hence, one can formulate two structures for 1,3-butadiene:
- Overlapping of the p z orbitals of carbon atoms 1 and 2 or 3 and 4 and formation of the corresponding double bonds.
- A central double bond is formed by overlapping the p z orbitals of carbon atoms 2 and 3. The p z orbitals of carbon atoms 1 and 4 can also interact with one another. However, from a spatial point of view, this is very unlikely, so that 1,3-butadiene is only present in small proportions in this mesomeric structure.
Neither structure 1 nor 2 can, on their own, represent the actual bonding relationships. By superimposing these two structures, a so-called resonance hybrid for 1,3-butadiene is obtained, a hybrid of the two mesomeric limiting forms 1 and 2. This shows a ( delocalized ) electron density distributed over the entire molecule . With larger polyenes, the number of possibilities for overlapping and thus the expansion of the electron system increases.
Based on these considerations, it can be summarized that the double bonds in 1,3-butadiene and other conjugated polyenes have the properties of single bonds to a certain extent and, conversely, single bonds also have a slight double bond character.
stability
The so-called heat of hydrogenation is a relative measure of the stability of hydrocarbons with multiple bonds such as polyenes . This is the energy that is released when hydrogen to the double bonds is added is ( hydrogenation ). It is observed that less energy is released in the hydrogenation of conjugated polyenes than in the case of non-conjugated polyenes.
use
There are polyenes that are used in the form of antifungal agents to treat fungal infections . These are so-called macrocycles , larger ring-shaped polyenes with frequently hydroxylated areas. This gives these compounds an amphiphilic character. Natamycin , Filipin , Nystatin and Amphotericin B may be mentioned as examples .
Individual evidence
- ^ Siegfried Hauptmann : Organic Chemistry, 2nd Edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 245, ISBN 3-342-00280-8 .
- ↑ a b Joachim Buddrus: Fundamentals of organic chemistry . Walter de Gruyter, 2011, ISBN 3-11-024894-8 , p. 368 ( limited preview in Google Book search).