Nitrogen anions can be easily made from pyrrole
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص740
2025-07-14
425
Nitrogen anions can be easily made from pyrrole
Pyrrole is much more acidic than comparable saturated amines. The pKa of pyrrolidine is about 35, but pyrrole has a pKa of 16.5, making it some 1023 times more acidic! Pyrrole is about as acidic as a typical alcohol so bases stronger than alkoxides will convert it to its anion. We should not be too surprised at this as the corresponding hydrocarbon, cyclopenta diene, is also extremely acidic, with a pKa of 15. The reason is that the anions are aromatic with six delocalized π electrons. The effect is much greater for cyclopentadiene because the hydrocarbon is not aromatic and much less for pyrrole because it is already aromatic and has less to gain. In all of the reactions of pyrrole that we have so far seen, new groups have added to the carbon atoms of the ring. The anion of pyrrole is useful because it reacts at nitrogen. The nitrogen atom has two lone pairs of electrons in the anion: one is delocalized around the ring but the other is localized in an sp2 orbital on nitrogen. This high-energy pair is the new HOMO and this is where the molecule reacts. N-acylated derivatives in general can be made in this way. A commonly used base is sodium hydride (NaH) but weaker bases produce enough anion for reaction to occur.
●Anions of pyrroles react with electrophiles at the nitrogen atom.
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