Pyridine as a catalyst and reagent

 Since pyridine is abundant and cheap and has an extremely rich chemistry, it is not surprising that it has many applications. One of the simplest ways to brominate benzenes is not to bother with the Lewis acid catalysts recommended but just to add liquid bromine to the aromatic compound in the presence of a small amount of pyridine. Only about one mole percent is needed and even then the reaction has to be cooled to stop it getting out of hand. As we have seen, pyridine attacks electrophiles through its nitrogen atom. This produces the reactive species, the N-bromo-pyridinium ion, which is attacked by the benzene. Pyridine is a better nucleophile than benzene and a better leaving group than bromide. This is another example of nucleophilic catalysis.

Another way to use pyridine in brominations is to make a stable crystalline compound to replace the dangerous liquid bromine. This compound, known by names such as pyridinium tribromide, is simply a salt of pyridine with the anion Br3 −. It can be used to brominate reactive compounds such as alkenes.

Both of these methods depend on the lack of reactivity of pyridine’s π system towards electrophiles such as bromine. Notice that, in the first case, both benzene and pyridine are present together. The pyridine attacks bromine only through nitrogen (and reversibly at that) and never through carbon. Oxidation of alcohols is normally carried out with Cr(VI) reagents but these, like the Jones’ reagent (Na₂Cr₂O₇ in sulfuric acid), are usually acidic. Some pyridine complexes of Cr(VI) compounds solve this problem by having the pyridinium ion (pKa 5) as the only acid. The two most famous are PDC (pyridinium dichromate) and PCC (pyridinium chloro chromate). Pyridine forms a complex with CrO₃ but this is liable to burst into flames. Treatment with HCl gives PCC, which is much less dangerous. PCC is particularly useful in the oxidation of primary alcohols to aldehydes as over-oxidation is avoided in the only slightly acidic conditions .

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المزيد

الآخبار الصحية

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المزيد

الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

Furan and thiophene are oxygen and sulfur analogues of pyrrole

Five-membered aromatic heterocycles are good at electrophilic substitution

Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution

Pyridones are good substrates for nucleophilic substitution

Nucleophilic substitution is easy with pyridines

Pyridine is bad at electrophilic aromatic substitution

Pyridine is a very unreactive aromatic imine

Aromaticity survives when parts of benzene’s ring are replaced by nitrogen atoms

The Claisen ester disconnection: a 1,3-diO relationship needing two carbonyl groups

Aldol-style disconnections with N and O in a 1,3-relationship: I

Look out for concealed functional group relationships

Donor and acceptor synthons