Nucleophile

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chiddler

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In a diels alder rxn, the nucleophile is:
A. The diene
B. The dienophile

3jQvH.png


it looks like both the diene and dienophile have some component of nucleophilic activity, doesn't it?

EK says that it's A, "The diene attacks the dienophile".

?
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I've never thought about Diels-Alder like that, but maybe it's because the diene has more electrons than the dienophile, so it contributes more to the new bonds being formed and thus is "attacking?"

That's my best guess, though I don't particularly like that explanation.

Edit: And EK is correct, apparently. http://masterorganicchemistry.com/2010/04/01/nucleophile-attacks-electrophile/

Though they are each attacking each other: http://books.google.com/books?id=9Dm6n4dDdU0C&lpg=PA230&ots=fWxgrpyHuP&dq=diene%20attacks%20dienophile&pg=PA230#v=onepage&q=diene%20attacks%20dienophile&f=false

Conclusion: 😕
 
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huh. interesting.

maybe it's not really a concerted mechanism but it's written that way out of convention/convenience
 
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If memory serves correctly, diels-alder reactions occur more readily when the dienophile has EWG as substituents. Based on that, it's easy to remember that the diene acts as the nucleophile.
 
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that's a good point. and even not knowing that, the conjugated system of the diene is much more likely to form a carbanion for attack, too.
 
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chiddler said:
that's a good point. and even not knowing that, the conjugated system of the diene is much more likely to form a carbanion for attack, too.
Click to expand...

Why is it more likely to form a carbanion? I thought smaller alkyl groups had more stable carbanions (Me- vs But-)?

Is it because there is greater resonance in a diene than an alkene?
 
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MedPR said:
Why is it more likely to form a carbanion? I thought smaller alkyl groups had more stable carbanions (Me- vs But-)?

Is it because there is greater resonance in a diene than an alkene?
Click to expand...

i figured that the resonance allows greater partial charged character on the carbons. not sure anymore, though because you're right about this too.
 
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chiddler said:
i figured that the resonance allows greater partial charged character on the carbons. not sure anymore, though because you're right about this too.
Click to expand...

Orgo has by far and away the least content on the MCAT, but some of the topics are so dense it's ridiculous.
 
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Greater resonance = more stability (delocalization of charge). The diene has fewer resonance forms and is thus less stable/more nucleophilic.

This is a pretty minute detail, so don't get caught up in the confusion. I think it is more important you understand that the Diels-Alder reaction is a concerted mechanism (meaning only one transition state exists).
 
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gettheleadout said:
Simply because instead of being localized on one carbon, the carbanionic lone pair is spread out over the two end carbons.
Click to expand...

i don't understand that. it has partial negative and partial positive. yes they are spread out, but that both charges exist in resonance says that the lone pair of one carbon is spread out to the other. (assuming they have similar contributions to resonance structure)

there are two carbons that are capable of sending their lone pair off, so the spreading is cancelled out because they give each other their lone pairs.

like two quarterbacks throwing the football to the other. as opposed to throwing it to a regular player who throws it back.
 
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chiddler said:
i don't understand that. it has partial negative and partial positive. yes they are spread out, but that both charges exist in resonance says that the lone pair of one carbon is spread out to the other. (assuming they have similar contributions to resonance structure)

there are two carbons that are capable of sending their lone pair off, so the spreading is cancelled out because they give each other their lone pairs.

like two quarterbacks throwing the football to the other. as opposed to throwing it to a regular player who throws it back.
Click to expand...

I think you can think of it as one carbon being able to share the negative with another carbon rather than having to hold on to it all the time.

It's kind of like why Iodide is more stable (and less reactive) than Bromide. If an electrophile were to try and react with Iodide, it would need to search through Iodide's huge cloud to find the electron. Bromide, however, has the same valence electron but a much smaller cloud, so the electrophile can find it much easier. If you have a carbanion with no resonance, the elctrophile only has to look in one place to find the electron. If there is resonance, however, the electron isn't necessarily going to always be in the same place. Electrophile can't find it = can't react = more stable.
 
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yes that's true. they do have to traverse the length of the diene, too, to reach their respective carbons.
 
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