Stilbene

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:

• R22 — harmful if swallowed
• R37 — irritating to respiratory system
• R38 — irritating to skin
• R62 — possible risk of impaired fertility
• S22 — do not breathe dust
• S26A — in case of contact with eyes, rinse immediately with water and seek medical advice
• S36 — wear suitable protective clothing
• S37 — wear suitable gloves
• S45 — in case of accident or if you feel unwell seek medical advice immediately, and show the label if possible

Additional recommended knowledge

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Uses of Stilbene

Stilbene is used in manufacture of dyes and optical brighteners, and also as a phosphor and a scintillator.

Stilbene is one of the gain mediums used in dye lasers.

Many stilbene derivates (stilbenoids) are present naturally in plants. An example is resveratrol and its cousin, pterostilbene.

Stilbene Reactions

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.

Green halogenation

To green this reaction, the pyridinium tribromide in situ generation of Br₂ is replaced with hydrobromic acid, which is oxidized by hydrogen peroxide(H₂O₂).

2HBr + H₂O₂ → Br₂ + 2H₂O

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.

Photochemical reaction

Main article: Diarylethene

trans-stilbene isomerizes to cis-stilbene under the influence of light. The reverse path can be induced by heat or light.

More Information

Spectra

• IR Spectra
• Mass Spectra
• HNMR Spectra
• CNMR Spectra

References

• Spectral Database for Organic Compounds, SDBS
• (E)-Stilbene on ChemExper
• Green Organic Chemistry, Strategies, tools, and Laboratory Experiments. Kenneth M, Doxsee & James E. Hutchison.
• Organic Chemistry, Eighth Edition. T.W. Graham Solomonds & Craig B. Fryhle.