McMurry (Kap 18,8) - heterocyclic amines Flashcards
A heterocycle is a cyclic compound that contains atoms of two or more elements in its ring.
What is a heterocyclic amine?
A heterocyclic amine is a type of heterocyclic compound that contains an amine functional group (-NH2, -NHR, or -NR2) within a ring structure.
Have many important biological properties.
Does heterocycles have the same chemistry as their open chain counterparts?
Yes, for example lactams and acyclic amides behave similarly.
BUT, in certain cases, like when the ring is unsaturated, heterocyckles have unique and interisting properties.
How is the heterocyclic compound pyrrole produced?
Obtained commercially by treatment of furan with ammonia over an alumina catalyst at 400grader.
Furan (the oxygen analog of pyrrol is obtained by an acid catalyzed dehydration of the five carbon sugars found in ek.s.corncobs.
Although pyrrole appears to be an amine, and a conjugated diene - its chemichal properties are not consistent with that.
Pyrrole is not basic like most other amines - pyrrolinium ion pKa is 0,4. (its supposed to be around 10).
Unlike most conjugated dienes, pyrrole undergoes electrophillic substitution reaction - rather then addition.
What are the reasons for these properties?
Pyrrole is aromatic. Because nitrogen is part of the atomaticity, protonation on it would destroy the aromacitity (hence why it does not act as a base),
The carbon atoms of pyrrole are more electron rich and more nucleophillic than typical c-c double bonds, the pyrrole ring is therefore more reactive towards the electrophiles.
The N in pyrrole is electron poor, while the pyrrole carbons are more electron rich.
How is the chemistry of pyrrole?
The same as that of an activated benzene ring.
heterocycles are more reactive towards electrophiles then benzenes are, and low temparatures are necessary to control the reactions.
- Halogenation
- Nitration
- Sulfonation
- Fredel - Crafts acylation
can be accomplished.
How does the electrophillic substitutions occur with these heterocycles?
Occurs at C2 - the position next to the nitrogen. (see page 667 for examples).
Other common five membered heterocyclic amines include imidazole and thiazole.
SEE PAGE 667.
What is pyrridine?
Nitrogen containing heterocyclic analog of benzene.
Flat, aromatic molecule, bond angles 120, c-c bond lenghts of 139 pm.
Pyridine has a pka of 5,25 - stronger base then pyrrole, but a weaker base then alkylamine. (the lone pair electrons on pyridine are in sp2 orbital, while on an alkylamine nitrogen are in sp3.
Because the s otbitals have their maximum e density at the nucleus, but p orbitals have a node at their nucleus - an orbital with a more characteristic from an s orbital - held more closely to the positively charged nuclei and are less available for bonding.
As result the sp2 hybridized N atom is less basic then the sp3 - because it is held more closely to the nuclei and are less available for bonding.
(undergoes electrophillic aromatic substitution reaction - but with difficulty unlike benzene) - see page 668-669 for example.
