Orgo Question

[flyingforever]

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I got the first part of this reaction. I don't understand why Br is substituted for NH2. Isn't Br better leaving group than Cl?

 

[mx41]

Br is a better leaving group in a SN reaction, yes. But this isn't an SN reaction, its a Nucleophilic Aromatic Substitution reaction
Therefore, the most electronegative atom attracts electrons greater than other atoms down the column.. and leaves first
F > Cl > Br > I

 

[flyingforever]

Br is a better leaving group in a SN reaction, yes. But this isn't an SN reaction, its a Nucleophilic Aromatic Substitution reaction
Therefore, the most electronegative atom attracts electrons greater than other atoms down the column.. and leaves first
F > Cl > Br > I

thanks!!!

 

[RSD2014]

]

I got the first part of this reaction. I don't understand why Br is substituted for NH2. Isn't Br better leaving group than Cl?

In Nucleophilic Aromatic substitution
EWG (NO2) = Ortho/para director
EDG (including Halogen) = Meta director

NH2 is the nucleophile that will substitute the Leaving Group (Br).
NO2 activates the ring at ortho/para. Br is in the para position so it is substituted.
NO2 wouldn't do anything with Cl. Cl is in Meta position.

 

[PreDental88]

In Nucleophilic Aromatic substitution
EWG (NO2) = Ortho/para director
EDG (including Halogen) = Meta director

NH2 is the nucleophile that will substitute the Leaving Group (Br).
NO2 activates the ring at ortho/para. Br is in the para position so it is substituted.
NO2 wouldn't do anything with Cl. Cl is in Meta position.

I think there are specific details that are incorrect in this logic.

NO2 = EWG (deactivating, not activating) = meta director

 

[RSD2014]

I think there are specific details that are incorrect in this logic.

NO2 = EWG (deactivating, not activating) = meta director

In Electrophilic Aromatic Substitution <--------
EDG = Ortho/para director
EWG (NO2) Meta director <------

In Nucleophilic Aromatic substitution <--------
EWG (NO2) = Ortho/para director <--------
EDG (including Halogen) = Meta director

 

[Ibraiz]

Can someone please show the mechanism of the second step in this reaction. All I know about the NaNH2 is that its a strong base and from reviewing the Organic Chemistry section of DAT Destroyer, I know that it there are two halides on an alkane, NaNH2 will take the halides and form an alkyne.

I am on the right track?

 

[RSD2014]

I think there are specific details that are incorrect in this logic.

NO2 = EWG (deactivating, not activating) = meta director

Part 2 is Nucleophilic Aromatic substitution not Electrophilic Aromatic Substitution.

In Nucleophilic Aromatic substitution <--------
EWG (NO2) = Ortho/para director <--------
EDG (including Halogen) = Meta director

Electrophilic Aromatic Substitution <--------
EDG = Ortho/para director
EWG (NO2) Meta director <------

 

[RSD2014]

I think there are specific details that are incorrect in this logic.

NO2 = EWG (deactivating, not activating) = meta director

]

According to L.G. Wade Organic Chemistry Textbook:
Electrophilic Aromatic Substitution <--------
EDG = Ortho/para director
EWG (NO2) Meta director <------

In Nucleophilic Aromatic substitution <--------
EWG (NO2) = Ortho/para director <--------
EDG (including Halogen) = Meta director


Part 1 is Electrophilic Aromatic Substitution, Br (Halogen) is a deactivating ortho/para director that directs the substitution of H with Cl.

Part 2 is Nucleophilic Aromatic substitution not Electrophilic Aromatic Substitution.
NH2 is the nucleophile that will substitute the Leaving Group (Br).
NO2 activates the ring at ortho/para. Br is in the para position so it is substituted.
NO2 wouldn't do anything with Cl. Cl is in Meta position.

 

[PreDental88]

Part 2 is Nucleophilic Aromatic substitution not Electrophilic Aromatic Substitution.

In Nucleophilic Aromatic substitution <--------
EWG (NO2) = Ortho/para director <--------
EDG (including Halogen) = Meta director

Electrophilic Aromatic Substitution <--------
EDG = Ortho/para director
EWG (NO2) Meta director <------

Oh that's right! I didn't notice this was nucleophilic rather than electrophilic. My apologies!

 

[RSD2014]

Can someone please show the mechanism of the second step in this reaction. All I know about the NaNH2 is that its a strong base and from reviewing the Organic Chemistry section of DAT Destroyer, I know that it there are two halides on an alkane, NaNH2 will take the halides and form an alkyne.

I am on the right track?

Ibraiz, a Strong base deprotonate a H next to LG, LG leaves and a benzyne is formed. a nucleophile (could be the same strong base) attacks one end of the benzyne to form 1 product or it can attack the other end of benzyne to form a 2nd product.

But most importantly, for Nucleophilic aromatic substitution we need these conditions:


Nucleophilic Aromatic Substition
EWG (NO2) = Ortho/para director
EDG (including Halogen) = Meta director

In Addition-Elimination (1 major product)
Requires a Strong EWG (NO2) on aromatic ring. LG must be in Ortho or para.
If LG at Meta Position, NO RXN! It won't go Addition-Elimination but we can force it to go Elimination-Addition if we use HEAT.

In Elimination-Addition (3 possible products)
Without EWG on aromatic ring to activate benzene we must use
STRONG BASE + HEAT.

We can get Meta & Para mixture product. It is also possible to get ORTHO, META & PARA product, depending on the molecule you start with.

 

[Ibraiz]

So if there is no other nucleophile beside the strong base it self in the Nucleophilic Aromatic Substitution reaction, then strong base always acts as a nucleophile (replacing the leaving group)?

Does Nucleophilic Aromatic Substitution every stop at benzyne and not do the substitution? I guess it won't be called substitution then it would be elimination but is it possible to have an elimination product rather than a substitution product?

 

[RSD2014]

So if there is no other nucleophile beside the strong base it self in the Nucleophilic Aromatic Substitution reaction, then strong base always acts as a nucleophile (replacing the leaving group)?

Does Nucleophilic Aromatic Substitution every stop at benzyne and not do the substitution? I guess it won't be called substitution then it would be elimination but is it possible to have an elimination product rather than a substitution product?

There's a mechanism drawn with explanation here:
Nucleophilic Aromatic Substitution

 

[Ibraiz]

Thanks RSD2014, I think I have it down now. Has anyone taken the DAT came across intense reactions like this? I don't think I came across this mechanism in KBB or DAT Destroyer.

 

[Baylor2011]

I can't even find nucleophilic aromatic substitution in my two college organic books that I have haha, this is odd..

 

[RSD2014]

Thanks RSD2014, I think I have it down now. Has anyone taken the DAT came across intense reactions like this? I don't think I came across this mechanism in KBB or DAT Destroyer.

anytime. Nucleophilic aromatic substitution is not that important. It's a topic that was only briefly covered as the last topic of my ochem class.

I think electrophilic aromatic substitution is more important. concentrate on this instead. 🙂