Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: TheSupremeOne on 07/12/2004 11:22:33

Title: What is the mechanism of this amine-based chemical reaction?
Post by: TheSupremeOne on 07/12/2004 11:22:33

I have this answer to a question and I don't understand the answer:

Name the reactants and write equations to show the formation of the seconday amine you have drawn in a)? (4 marks)

To begin with my answer is the same as the book:

the secondary amine is CH3CH2NHCH3
reactants bromoethane and methylamine

the book says the equation is :
CH3CH2Br + 2CH3NH2 => CH3CH2NHCH3 + CH3NH3+Br-

I don't understand where the product CH3NH3+Br- came from I have read through and drawn out of the mechanism for ages surely the primary structure is CH3CH2NH2?

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: Ylide on 12/12/2004 04:03:48

This appears to be a nucleophilic attack by methylamine on bromoethan.  The free electron pair on the nitrogen of methylamine attacks the formally positive charged carbon that bromine is attached to.  The causes an sn2 replacement of bromine by methyl amine.  The leaving group Br- will abscond a hydrogen from the now positively charged  nitrogen of the newly formed isopropyl amine:

CH3CH2Br + CH3NH2 => CH3CH2NH2+CH3 + Br- => CH3CH2NHCH3 + HBr

Since there is a second methyl amine in the reaction and amines are basic, the HBr reacts with it in a standard acid/base reaction forming a salt (methylammonium bromide):

CH3NH2 + HBr => CH3NH3+Br-

This is really a side reaction, I'm assuming the methyl amine is in excess so the HBr reacts with it.  The first reaction is really the important one, the nucelophilic substitution of methyl amine on ethyl bromide.



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Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: TheSupremeOne on 12/12/2004 10:59:42

Thanks [:D]

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: mvw on 13/11/2011 03:45:05

So it's been 7 years and...

I have the same problem, but exactly in reverse. In the text is not drawn with 2 arrows to indicate equilibrium.
The problem is:
CH3CH2NHCH3 + HBr =>  My suggestion is CH3CH2Br + CH3NH2

But is this a reversible reaction? Why does he not show it as reversible?

TIA
mvw

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: Bored chemist on 13/11/2011 09:38:43

Actually you have the same problem in the same direction, but with a different amine.
Look at Ylide's equation.
CH3NH2 + HBr => CH3NH3+Br-

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: mvw on 13/11/2011 17:41:37

First off, thanks so much for replying to an old post. 
I think we're not on the same page yet.
That was methylamine.  I understand the ionic part.
I have ethyl methylamine, a secondary amine. If it works out the same as your suggestion the positive charge is on the Hydrogen, but it's in the middle of the structure.
I'm thinking this:  CH3CH2NHCH3 + HBr → CH3CH2NH2⁺CH3 Br^-

Anyway, thanks again.
mvw

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: damocles on 13/11/2011 21:07:46

Quote from: mvw on 13/11/2011 17:41:37

First off, thanks so much for replying to an old post. 
I think we're not on the same page yet.
That was methylamine.  I understand the ionic part.
I have ethyl methylamine, a secondary amine. If it works out the same as your suggestion the positive charge is on the Hydrogen, but it's in the middle of the structure.
I'm thinking this:  CH3CH2NHCH3 + HBr → CH3CH2NH2⁺CH3 Br^-

Anyway, thanks again.
mvw

The in-line representation of a chemical formula does not represent a molecular geometry at all accurately. It is one-dimensional. Even a structural diagram is 2-dimensional.

Your thought is exactly right. There is no problem with the positive charge being "in the middle" -- it is not.
If it helps, you could write the in-line formula as


Br^- NH2⁺(CH3)CH2CH3

Title: Re: What is the mechanism of this amine-based chemical reaction?
Post by: mvw on 13/11/2011 21:38:16

Thanks so much. I hope to hang around here more often