The Naked Scientists

The Naked Scientists Forum

Author Topic: What is the mechanism of this amine-based chemical reaction?  (Read 17398 times)

Offline TheSupremeOne

  • First timers
  • *
  • Posts: 2
    • View Profile
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?
« Last Edit: 13/11/2011 22:40:14 by chris »


 

Offline Ylide

  • Moderator
  • Hero Member
  • *****
  • Posts: 905
    • View Profile
    • http://clem.mscd.edu/~mogavero
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.



This message brought to you by The Council of People Who Are Sick of Seeing More People
 

Offline TheSupremeOne

  • First timers
  • *
  • Posts: 2
    • View Profile
Thanks :D
 

Offline mvw

  • First timers
  • *
  • Posts: 3
  • If it's not broken, I can't fix it.
    • View Profile
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
 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8648
  • Thanked: 42 times
    • View Profile
Actually you have the same problem in the same direction, but with a different amine.
Look at Ylide's equation.
CH3NH2 + HBr => CH3NH3+Br-
 

Offline mvw

  • First timers
  • *
  • Posts: 3
  • If it's not broken, I can't fix it.
    • View Profile
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
 

Offline damocles

  • Hero Member
  • *****
  • Posts: 756
  • Thanked: 1 times
    • View Profile
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
 

Offline mvw

  • First timers
  • *
  • Posts: 3
  • If it's not broken, I can't fix it.
    • View Profile
Thanks so much. I hope to hang around here more often
 

The Naked Scientists Forum


 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums