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Common name for trans-1,2-diphenylethylene, aka, stilbene, or more specifically, (E)-stilbene, is the alkene, ethene with two phenyl groups on either carbon of the parent chain. The name was derived from the Greek word stilbos, which means shining. It should also be noted, there is also a (Z)-stilbene which is sterically hindered and less stable because of it. Also notice that (Z)-stilbene has a melting point of 5°C to 6°C, while the melting point of (E)-stilbene is in the 125°C area, this illustrates the significant differences between the two. Because (E)-stilbene is so much more common than (Z)-stilbene, this page will focus only on (E)-stilbene.
Its risk and safety phrases are:
Uses of Stilbene
Halogenation of Stilbene
Simple hydrocarbons are relatively unreactive. In order to form more complex molecules, it is generally necessary to introduce more reactive functional groups. Alkenes (olefins) containing the carbon-carbon double bond may be halogenated to form alkyl halides, which are more capable of undergoing further chemical reactions.
An example of halogenation is the bromination of (E)-stilbene. Bromine is somewhat of a special case due in no small part, to its relatively enormous size compared to carbon. Since elemental bromine (Br2) is volatile and highly corrosive, pyridinium tribromide is commonly used to generate Br2 in situ.
In either case the halogenation of stilbene is as follows; as seen in the first reaction, the Br-Br bond attracts the attention of the π-bonding electrons (π-bonds being e- dense and Br being very electronegative), this leads to a weakening and eventual break (heat, plays a crucial role in driving the break forward) of the π-bond. Once the π-bond has been broken, its electrons are transferred to Br2, causing the Br-Br bond to be severed when the bonding electrons are transferred to the other bromine. At this stage there is the positively charged intermediate and the loose bromine ion (Br-), coming from the opposite direction as the Br2, the Br- loosens one of the C-Br Bonds, leaving the final brominated product.
It doesn't matter which stilbene is used for this reaction, (E)-stilbene and (Z)-stilbene will both produce 1,2-dibromo-1,2-diphenylethane. However, bromination of the (Z) isomer results in a racemic mixture of dl stilbene dibromide, while the bromination of an (E) isomer results in a majority meso-stilbene dibromide along with minor amounts of the dl enantiomers. The minor dl product in the bromination of (E)-stilbene is a result of resonance in the bromonium bridge intermediate, where a positive charge is located on one of the two bridge carbons, thus allowing for a syn-addition of a bromine anion.
trans-stilbene isomerizes to cis-stilbene under the influence of light. The reverse path can be induced by heat or light.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Stilbene". A list of authors is available in Wikipedia.|