DAT Destroyer Organic #180

[florida21]

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I am confused on this question because the answer key says that the LDA would come in and deprotonate the most acidic hydrogen in the multi-cyclic compound and there would be an SN2 reaction. However, I thought that LDA was too big of a base and would form the less substituted alkene of an E2 reaction.

 

[510586]

LDA deprotonates the acidic hydrogen that is least sterically hindered. I just assumed you couldn't deprotinate sp2 carbons with double bonds so it left all the single carbons left.and the least hindered sp3 carbon was the one opposite the oxygen. Not 100% sure why it is acidic since I'm used to doing LDA with alpha hydrogens. Hopefully someone can come clarify this

 

[NavyDDS1990]

yea you can't deprotonate sp2 carbons and all thats left are sp3 carbons. But I think the reason why LDA deprotonates the carbon opposite of oxygen is because due to conjugation and how Oxygen is pulling its electron density towards itself which makes that carbon more vulnerable to get deprotonated.
please correct me if my reasoning is wrong.

 

[510586]

yea you can't deprotonate sp2 carbons and all thats left are sp3 carbons. But I think the reason why LDA deprotonates the carbon opposite of oxygen is because due to conjugation and how Oxygen is pulling its electron density towards itself which makes that carbon more vulnerable to get deprotonated.
please correct me if my reasoning is wrong.

I see the oxygen pulling but the conjugation is not as clear to me. I can't see where it resonates to after it deprotonates.

 

[NavyDDS1990]

I see the oxygen pulling but the conjugation is not as clear to me. I can't see where it resonates to after it deprotonates.

you're right haha no conj. most likely due to oxygen pulling way the electron making that sp3 carbon vulnerable

 

[florida21]

ok thank you!

 

[DAT Destroyer]

I am confused on this question because the answer key says that the LDA would come in and deprotonate the most acidic hydrogen in the multi-cyclic compound and there would be an SN2 reaction. However, I thought that LDA was too big of a base and would form the less substituted alkene of an E2 reaction.

LDA is a strong base but rather poor nucleophile. In this reaction,no SN2 is possible since it is an Aryl Halide. The only option is to remove the most acidic proton. Indeed it is acidic, since it can be stabilized from 2 adjacent benzene rings. Once the anion is formed, we can finish the job and do the SN2.

Hope this helps..

Dr. Romano

 

[510586]

LDA is a strong base but rather poor nucleophile. In this reaction,no SN2 is possible since it is an Aryl Halide. The only option is to remove the most acidic proton. Indeed it is acidic, since it can be stabilized from 2 adjacent benzene rings. Once the anion is formed, we can finish the job and do the SN2.

Hope this helps..

Dr. Romano

So stabilization using the benzene rings involves conjugation? I'm assuming a resonance of it is having a lone pair somewhere in one of the benzenes when resonating? I didn't see it earlier

 

[DAT Destroyer]

So stabilization using the benzene rings involves conjugation? I'm assuming a resonance of it is having a lone pair somewhere in one of the benzenes when resonating? I didn't see it earlier

Either benzene ring can resonance-stabilized this anion. Recall, the more stable the anion..... the more ACIDIC will that hydrogen be.....hence easier to come off. If for example, LDA reacted with an alcohol....what would you get ? Hopefully you see that it would act as a base and yield the alkoxide anion......plus LDA with a hydrogen on it ...namely diisopropyl amine. Note that resonance stabilization is not involved here,,,,,,but LDA is a very strong base.

Dr. Romano

 

[510586]

Either benzene ring can resonance-stabilized this anion. Recall, the more stable the anion..... the more ACIDIC will that hydrogen be.....hence easier to come off. If for example, LDA reacted with an alcohol....what would you get ? Hopefully you see that it would act as a base and yield the alkoxide anion......plus LDA with a hydrogen on it ...namely diisopropyl amine. Note that resonance stabilization is not involved here,,,,,,but LDA is a very strong base.

Dr. Romano

Either benzene ring can resonance-stabilized this anion. Recall, the more stable the anion..... the more ACIDIC will that hydrogen be.....hence easier to come off. If for example, LDA reacted with an alcohol....what would you get ? Hopefully you see that it would act as a base and yield the alkoxide anion......plus LDA with a hydrogen on it ...namely diisopropyl amine. Note that resonance stabilization is not involved here,,,,,,but LDA is a very strong base.

Dr. Romano

Is this correct?

 

[DAT Destroyer]

Is this correct?

Yes! Good solid work,,,,🙂

Dr. Romano