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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Monoamine on March 15, 2021, 01:50:32 AM

Title: Alkylation of Amines via an "ether synthesis" why doesn't it stop!
Post by: Monoamine on March 15, 2021, 01:50:32 AM

So it is well known that you can alkylate amines with Bromoalkanes. My textbook says that this is generally not a good approach though, since this will create a quaternary ammonium cation.
Now, I'm having trouble understanding why this should be, since the liberated Br- ion should form a salt with the amine and so all the lone pairs in the amine should be used up and shouldn't be available for further bonding. (See attached picture).

Incidentally, if you do end up with a quaternion ammonium cation, how can you get rid of one of the extra alkyl constitutes, and is there a way of selecting which one you want to remove?

Any insight is greatly appreciated!  :D

Title: Re: Alkylation of Amines via an "ether synthesis" why doesn't it stop!
Post by: mjc123 on March 15, 2021, 06:58:35 AM

RR₂NH is neutral, not a cation.
Initial attack gives RR₂NH₂⁺Br^-, from which HBr is eliminated. When you get to the quaternary ammonium, you can't eliminate HBr.

Title: Re: Alkylation of Amines via an "ether synthesis" why doesn't it stop!
Post by: Monoamine on March 15, 2021, 07:54:32 PM

Thanks for he clarification.