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Pls help w/ nucleophilic substitution

I've been searching online everywhere to find the answer to my Q but I can't find anything :frown: so basically, I'm looking over the nucleophilic substitution mechanism with ammonia being the nucleophile. I understand so far that the ammonia attacks the carbon atom, causing the carbon-bromine bond to break, leaving a carbocation with the ammonia in place of the bromine atom and a positive charge on the nitrogen. But I don't understand the next step, where another ammonia molecule comes along and attacks the hydrogen atom instead of the positive nitrogen?!?!?!?! Why does it attack the hydrogen??? :frown:(( Please help !!!!!
Reply 1
Avatar for alow
alow
19
The process where another other ammonia takes a proton to neutralise the alkylmmonium ion to a neutral alkylamine is reversible and exists in a equilibrium.

If you think about how the +ve charge on the N will effect the electron density around it, and the polarity of the H bonds, the reasoning for attacking an H should become clear.
Reply 2
Avatar for slothgenius
slothgenius
OP
1
:bump::bump:
Reply 3
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slothgenius
OP
1
Original post by alow
The process where another other ammonia takes a proton to neutralise the alkylmmonium ion to a neutral alkylamine is reversible and exists in a equilibrium.

If you think about how the +ve charge on the N will effect the electron density around it, and the polarity of the H bonds, the reasoning for attacking an H should become clear.


so the positive charge on the N means that it will be trying to attract electrons? So electron density is greater around the N than the H, meaning there is a small positive charge on the H? Is that wrong?
Reply 4
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alow
19
Original post by slothgenius
so the positive charge on the N means that it will be trying to attract electrons? So electron density is greater around the N than the H, meaning there is a small positive charge on the H? Is that wrong?


That's how I'd rationalise it. You should probably also think that the N already has 8 electrons around it so will be quite a poor electrophile. With reactions where a nucleophile attacks a Nitrogen, one of the other bonds to the nitrogen pretty much always breaks (e.g. in an oxaziridine the N-O bond breaks) to allow the new group to attach. In your reaction, you would have to release a carbanion or hydride which is really unfavourable.
Reply 5
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slothgenius
OP
1
Original post by alow
That's how I'd rationalise it. You should probably also think that the N already has 8 electrons around it so will be quite a poor electrophile. With reactions where a nucleophile attacks a Nitrogen, one of the other bonds to the nitrogen pretty much always breaks (e.g. in an oxaziridine the N-O bond breaks) to allow the new group to attach. In your reaction, you would have to release a carbanion or hydride which is really unfavourable.


Thank you so much for the help!
Reply 6
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alow
19
Original post by slothgenius
Thank you so much for the help!


No problem :smile:
Original post by slothgenius
I've been searching online everywhere to find the answer to my Q but I can't find anything :frown: so basically, I'm looking over the nucleophilic substitution mechanism with ammonia being the nucleophile. I understand so far that the ammonia attacks the carbon atom, causing the carbon-bromine bond to break, leaving a carbocation with the ammonia in place of the bromine atom and a positive charge on the nitrogen. But I don't understand the next step, where another ammonia molecule comes along and attacks the hydrogen atom instead of the positive nitrogen?!?!?!?! Why does it attack the hydrogen??? :frown:(( Please help !!!!!


hey- you PM'd me about uni advice but I can't reply because I think you've disabled private messages, just wondering where you'd like me to reply?

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