Types of Heterocycles

Uploaded by lamechivanes on 09.02.2010


In this webcast, we’re going to take a survey
of different types of aromatic heterocycles
that we have yet to discuss.
First, we’re going to talk about fused ring systems,
move on to two ah atom heterocyclic systems
and multi-atom heterocyclic systems.
First, let’s took a look at ah furan and thiophene shown here.
So furan and thiophene are related in structure
because sulfur lies directly below oxygen
on the periodic table
and, thus, has a similar electron configuration
to oxygen.
If we look at the, the two pictures of sulfur,
of the sulfur and the oxygen,
we see that each are sp2 hybridized,
each have two lone pairs.
Ah, in each case, one, one lone pair occ-occupies the p orbital
that contributes to the π system
and the aromaticity of the molecule,
and the other lone pair occupies an sp2 hybrid orbital, um,
perpendicular and away from the ring.

If we move on and look at the resonan-
resonance stabilization energies,
ah, of these different, ah, compounds.
If we look at thiophene, pyridine, pyrrole, and furan
and compare them to the resonance stabilization energy
of benzene,
we notice that these heterocyclic systems
don’t have quite as much
resonance stabilization energy as benzene.
Although you don’t need to memorize the data in this table,
it’s just worth noting, ah, qualitatively
that these heterocyclic systems are not quite as stable
relative to the benzene system.
As we all- and as we all know,
benzene is quite inert to many reactions, ah,
therefore, the heterocyclic systems
may not be quite as inert, ah, to, to certain reactions.

If we move on and take a look at fused ring systems,
ah, first, we’ll take a look at benzofuran here.
Benzofuran is basically benzene fused to fur-ah, a furan ring.

If we move on and take a look at this, ah, indole structure,
this indole structure is the most important
out of all five of these fused ring systems
because it makes up the amin, ah,
the side chain of the amino acid tryptophan.
And it’s basically, its substructure is benzene,
ah, fused to a pyrrole system.

Now, it’s worth noting- because
indole here is the side chain of an amino acid,
it’s worth noting why we’re studying these heterocyclos-
heterocyclic compounds to begin with.
Because they show up so frequently in
bioorganic reactions and biomolecules,
and we need a good grasp of these
before we begin to study these, ah, reactions,
ah, of bioorganic reactions
and biomolecules later in the semester.
So if we move on to these fused ring systems,
we see that, we see benzothiophene here
which is basically benzene fused to a thiophene ring.
And move on and see quinoline and isoquinoline here
which only differ in the position of the nitrogens.

Here we have two atom aromatic heterocyclic systems,
and we have two different classes shown here.
The first class is the azoles (āzoles) or azoles (ăzoles).
[clears throat]
Of these three structures,
we want to key in on the imidazole structure.
The imidazole structure
is the most important out of these three
because it makes up the side chain
of the amino acid histidine.

If we move on and look at the diazene structures,
again, we want to key in on the middle structure, pyrimidine.
Now pyrimidine is a substructure of,
ah, nucleic acid base pairs,
and these, as we all know, are critical, ah,
to the functioning of life at the marl-molecular level.

If we move on and take a look at, ah,
here a multi-atom aromatic heterocycle,
and we have the structure of purine.

Ah, purine is rapidly in equilibrium
between these two structures here
– shown, shown right here.
And, ah, both, both, ah, structures are in fact aromatic
and we could make a quick count of π electrons
if we needed to to, ah, confirm that.
So in this slide, we’re given-
this is an old test problem, ah, an exam problem.
We’re given the structure of guanine.

Again, we see a multi-atom fused,
ah, heterocyclic system.
So the first part of this problem says
to draw all the lone pairs in the heterocycle.
So if we remember that, ah, nitrogens want three bonds
and one lone pair,
and oxygens want two bonds and two lone pairs,
we can click-quickly fill in all the lone pairs
on the guanine structure, as shown here.

The second part of the question
asks us to con- consider all the π electrons in both rings,
and how many π electrons are there in all?
And note we’re not just concerned with the π electrons
on the, one the ring directly.
We need to consider the π electrons
of the atoms appended to the ring.
So if we know, if we know from our connectivity diagram
shown from, ah, previous webcasts
that a two connected nitrogen only contributes one,
ah, electron to the aromatic system
because the lone pair is in an sp2 hybrid orbital and not,
ah, conjugated to the ring,
and that three connected nitrogens ah
do contribute both of their lone pair-
both electrons in the lone pair to the aromatic system,
we can make a quick count of all the π electrons.
If we start here at this, at this nitrogen
and make a count of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11.
11 π electrons for the core of this structure.
And, again, we need to consider the atoms appended off
this molecule.
So the oxygen is double bonded so it contributes one,
ah, electron – one π electron also.
So that would be our 12th electron.
And the three connected nitrogen, ah,
bonded down here is also –
also has its lone pair in a p orbital and would,
ah, account for our 13th and 14th, ah, π electron
count here.
And the third part of this question says
circle all the electron lone pairs
that are involved in π bonding.
So if we remember, again, that, ah,
two connected ah nitrogens, their lone pairs are not,
ah, involved in π bonding
because they’re in sp2 hybrid orbital-orbitals,
and the three connected nitrogens,
their lone pairs are involved in π bonding
because they’re in p orbitals,
we can quickly circle all the three connected nitrogens
because they’re involved in π bonding. trogens, their lone pairs are not,
ah, involved in π bonding
because they’re in sp2 h