Aromatic, Heterolytic Chemistry Flashcards
What are heterocyclic compounds?
Heterocyclic compounds are cyclic organic compounds where one or more carbon atoms in the molecule’s backbone have been replaced by an atom other than carbon (e.g., O, N, S).
What are the two main types of heterocyclic compounds based on saturation?
Saturated – e.g., tetrahydrofuran (THF), piperidine
Unsaturated – e.g., pyran, dihydrothiophene
What defines an aromatic heterocyclic compound?
Aromatic heterocycles are compounds that:
Are cyclic
Are planar
Have (4n + 2) π-electrons (Hückel’s rule)
Have delocalization of π-electrons
Contain at least one heteroatom
Give examples of monocyclic aromatic heterocycles.
Pyridine, pyrrole, furan, thiophene
Give examples of polycyclic aromatic heterocycles.
Quinoline, isoquinoline, indole, purine
Which orbital does the lone pair on the nitrogen in pyridine occupy?
The lone pair on nitrogen occupies an sp² hybridized orbital and is not part of the aromatic π system.
How is the aromatic sextet formed in pyridine?
The aromatic sextet is formed from the 5 π electrons in the ring + 1 from nitrogen’s unhybridized 2p orbital, making 6 π electrons total.
Does the lone pair on nitrogen in pyridine participate in resonance?
No. The lone pair is in an sp² orbital and does not participate in delocalization or resonance of the π system.
What kind of resonance structures are formed in pyridine?
Structures showing alternating single and double bonds with the nitrogen maintaining its lone pair outside of the delocalized system.
In what cases do lone pairs on nitrogen participate in electron delocalization?
Pyridine: No
Pyrrole: Yes (lone pair in p orbital, part of sextet)
Is the lone pair on nitrogen in pyrrole part of the aromatic sextet?
Yes. It occupies a p orbital and is involved in delocalization, contributing to the 6 π electrons needed for aromaticity.
What is the hybridization of nitrogen in pyrrole?
sp² hybridized – 3 sp² orbitals form sigma bonds, and the lone pair is in the p orbital.
How does nitrogen in pyrrole contribute to resonance?
The nitrogen donates its lone pair into the π system, allowing resonance structures that maintain aromaticity.
Why is pyrrole aromatic?
It has 6 π electrons (4 from double bonds + 2 from nitrogen’s lone pair), and the system is planar and cyclic, satisfying aromaticity conditions.
Which heterocycles involve the heteroatom in π-delocalization?
Compound Heteroatom in π-delocalization?
Pyridine No
Pyrrole Yes
Furan Yes
Thiophene Yes
Quinoline No
Isoquinoline No
Indole Yes
Purine Yes (specifically N at position 9)
Which nitrogen in purine is involved in π-delocalization?
Nitrogen at position 9 is involved in π-delocalization in purine.
