THIOPHENE Priyanka S, group 15. Scientific adviser is Kozub Svetlana

THIOPHENE

Priyanka S, group 15. Scientific adviser is Kozub Svetlana.

Thiophene was discovered as a contaminant in benzene. It was observed that isatin forms a blue dye if it is mixed with sulfuric acid and crude benzene. The formation of the blue indophenin was long believed to be a reaction with benzene. Victor Meyer was able to isolate the substance responsible for this reaction from benzene. This new heterocyclic compound was thiophene.

Thiophene and its derivatives occur in petroleum, sometimes in concentrations up to 1–3%. The thiophenic content of oil and coal is removed via the hydrodesulfurization (HDS) process. In HDS, the liquid or gaseous feed is passed over a form of molybdenum disulfide catalyst under a pressure of H2. Thiophenes undergo hydrogenolysis to form hydrocarbons and hydrogen sulfide. Thus, thiophene itself is converted to butane and H2S. More prevalent and more problematic in petroleum are benzothiophene and dibenzothiophene.

At room temperature, thiophene is a colorless liquid with a mildly pleasant odor reminiscent of benzene, with which thiophene shares some similarities. The high reactivity of thiophene toward sulfonation is the basis for the separation of thiophene from benzene, which are difficult to separate by distillation due to their similar boiling points (4 °C difference at ambient pressure). Like benzene, thiophene forms an azeotrope with ethanol

Thiophene is considered aromatic, although theoretical calculations suggest that the degree of aromaticity is less than that of benzene. The "electron pairs" on sulfur are significantly delocalized in the pi electron system. As a consequence of its aromaticity, thiophene does not exhibit the properties seen for conventional thioethers. For example the sulfur atom resists alkylation and oxidation. However, oxidation of a thiophene ring is thought to play a crucial role in the metabolic activation of various thiophene-containing drugs, such as tienilic acid and the investigational anticancer drug OSI-930. In these cases oxidation can occur both at sulfur, giving a thiophene S-oxide, as well as at the 2,3-double bond, giving the thiophene 2,3-epoxide, followed by subsequent NIH shift rearrangement.

PYROLLE

Sahil Soodan, group 15. Scientific adviser is Kozub Svetlana.

Properties: Pyrrole has very low basicity compared to conventional amines and some other aromatic compounds like pyridine. This decreased basicity is attributed to the delocalization of the lone pair of electrons of the nitrogen atom in the aromatic ring. Pyrrole is a very weak base with a pK_aH of about –1 to –2. Protonation results in loss of aromaticity, and is, therefore, unfavorable.Like many amines, pyrrole slowly decomposes on exposure to air and light. It turns brown over time due to accumulation of impurities such as polypyrrole and various amine oxides. It is usually purified by distillation immediately before use.

Synthesis: Pyrrole is prepared industrially by treatment of furan with ammonia in the presence of solid acid catalysts.

One synthetic route to pyrrole involves the decarboxylation of ammonium mucate, the ammonium salt of mucic acid. The salt is typically heated in a distillation setup with glycerol as a solvent.

Reactivity: The NH proton in pyrroles is moderately acidic with a pK_a of 16.5. Pyrrole can be deprotonated with strong bases such as butyllithium and sodium hydride. The resulting alkali pyrrolide is nucleophilic. Treating this conjugate base with an electrophile such as methyl iodide gives N-methylpyrrole. The resonance contributors of pyrrole provide insight to the reactivity of the compound. Like furan and thiophene, pyrrole is more reactive than benzene towards electrophilic gnificant for producing functionalized pyrroles are the aromatic substitution because it is able to stabilize the positive charge of the intermediate carbocation..Pyrrole undergoes electrophilic aromatic substitution predominantly at the 2 and 5 positions. Two such reactions that are especially similar Mannich reaction and the Vilsmeier-Haack reaction , both of which are compatible with a variety of pyrrole substrates. Pyrroles react with aldehydes to form porphyrins. For example, benzaldehyde condenses with pyrrole to give tetraphenylporphyrin. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as under Lewis acid catalysis, heating, or high pressure.Pyrrole polymerizes in light. An oxidizing agent, such as ammonium persulfate, can also be used, typically at 0 °C and in darkness to control the polymerization.

Comercial Use: Pyrrole is essential to the production of many different chemicals. N-methylpyrrole is a precursor to N-methylpyrrolecarboxylic acid, a building-block in pharmaceutical chemistry.Although there is a claim that pyrrole is used as an additive to cigarettes, it is typically listed as a constituent of tobacco smoke and not as an ingredient.