Name: 五元異構體作為C-親核體的作用 (Five-membered Heteroaromatics as C-Nucleophiles)
Uploaded: 2021-01-14T05:20:48.000Z
Duration: 6 min 5 s
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I want to compare and contrast the behavior

in which the π system serves as the nucleophile.

The electrophilic aromatic substitution reactions

like we were con- considering in the previous webcast,

but now with the 5-membered heteroaromatics.

We'll take as our 5-membered ring our prototypical reaction,

the reaction of thiophene, that system there,

Acetyl nitrate is just a good source of an electrophilic

nitronium cation, but it's going to react

in a slightly different mode of reactivity.

the reaction takes place at a very low temperature,

compared to the previous case of the 6-membered ring.

And the second thing to note is that the mode, the, of

substitution takes place in this position,

is going to be displaced so this is carbon-2 of thiophene,

that undergoes substitution with the nitryl group.

There's a couple of ways we could understand why

the substitution takes place at C2 rather than C3.

One thing that we might do is to do a SHMO calculation

and look at the highest occupied molecular orbital

and we'd find that the grayest- greatest coefficient

of the highest occupied molecular orbital

Another way we might anticipate this reactivity

is to compare the two different types of intermediates

that form by attack at C2 and by attack at C3.

Let me outline that for you and what I would encourage you to do

and compare the reaction pathway for C2 attack

C2 attack involves three resonance contributors,

between nitrogen and oxygen in acetyl nitrate

and we'll end up making a new carbon nitrogen bond,

and this intermediate is a carbocation intermediate

that is delocalized onto those positions in the ring

resonance interation gives us three resonance contributors

one of which, the best one, is the last one

because it has an octet of electrons on every atom.

Attack at C3 ult- provides us with a less stable intermediate.

one of which is again, having a positive charge on sulfur,

but you can see that we have less d- delocalization

The intermediate that results from attack at C2

is more stable, the reaction pathway proceeds

through a transition state to the C2 attack

that’s lower in energy than the C3 attack.

That pathway, the C2 pathway, is faster than C3.

And that has to do with the highest occupied

molecular orbital energy level being elevated.

the more nucleophilic that pair of electrons.

So there’s really two reasons why benzene,

towards electrophilic aromatic substitution.

So the first thing that I’ve already mentioned

where the π electrons are raised up in energy,

they’re more nucleophilic than the benzene ring

you would in fact see those electrons in the HOMO

But the other thing to note is that the transition state,

and the intermediate that results is more stable

with a complete octet on every single atom.

Here’s one resonance contributor and that’s the carbocation,

we have this nonbonding pair of electrons

that can do an end to A or an end to π*-type donation

a resonance contributor of that intermediate

that has an octet of electrons on every atom.