How does excess ammonia prevent further substitution reactions?

Watch
sportyegg
Badges: 16
Rep:
?
#1
Report Thread starter 3 years ago
#1
I understand (or rather, just memorised) the fact that excess ammonia in nucleophilic substitution reactions in the preparation of amides prevents further substitution reactions....but how? Would anyone mind explaining?
0
reply
Tasnim:)
Badges: 9
Rep:
?
#2
Report 3 years ago
#2
the likelihood of ammonia being substituted further decreases because there will be more of a chance that the other reactant will be reacting with the ammonia molecules not just a few of the ammonia molecules.......idk did that make any sense at all?
0
reply
sportyegg
Badges: 16
Rep:
?
#3
Report Thread starter 3 years ago
#3
Hmm kind of but the ending lost me 😂 thanks tho
1
reply
BTAnonymous
Badges: 22
Rep:
?
#4
Report 3 years ago
#4
(Original post by sportyegg)
I understand (or rather, just memorised) the fact that excess ammonia in nucleophilic substitution reactions in the preparation of amides prevents further substitution reactions....but how? Would anyone mind explaining?
Say you have a 50:50 ratio of ammonia and the haloalkane. They will react to form primary, secondary, tertiary and quartnerary amines pretty easily.

Now create a mixture so the ratio of ammonia to haloalkane is now 75:25. The haloalkanes are now more likely to react with the ammonia to form the primary amines rather than reacting with the primary, secondary or tertiary amines. Of course, you will still get a mixture containing these different types of amines but the concentration of primary amines will be much greater than that of secondary, tertiary and quartnerary amines compared to the 50:50 solution.

This also seemed non-intuitive when I was first taught this topic and it really confused me for a while until it clicked.
3
reply
sportyegg
Badges: 16
Rep:
?
#5
Report Thread starter 3 years ago
#5
(Original post by BTAnonymous)
Say you have a 50:50 ratio of ammonia and the haloalkane. They will react to form primary, secondary, tertiary and quartnerary amines pretty easily.

Now create a mixture so the ratio of ammonia to haloalkane is now 75:25. The haloalkanes are now more likely to react with the ammonia to form the primary amines rather than reacting with the primary, secondary or tertiary amines. Of course, you will still get a mixture containing these different types of amines but the concentration of primary amines will be much greater than that of secondary, tertiary and quartnerary amines compared to the 50:50 solution.

This also seemed non-intuitive when I was first taught this topic and it really confused me for a while until it clicked.

Ah yes, gotcha! Thanks so much!
0
reply
yoyomynameis...
Badges: 10
Rep:
?
#6
Report 3 years ago
#6
(Original post by sportyegg)
Hmm kind of but the ending lost me 😂 thanks tho
You use excess ammonia and ethanol. By using ethanol you prevent nucleophilic substituion by water. When you use ammonia you get a primary amine, however, it still has a lone pair, which can act as a nucleophile itself and can react with the haloalkane to form a secondary amine. You don't want this, so you add excess ammonia. Ammonia is a better nucleophile and will react with the haloalkane, so the primary amine doesn't react to give a secondary amine, increasing yield of the primary amine. It's a bad synthetic technique so we use catalysts or we can take the haloalkane, hydrolyse it to form an alcohol, oxidise this to form an aldehyde and then reduce the aldehyde using HCN, to form a hydrocynitrile, which you can reduce by adding H2 and nickel to get the primary amine. Much better but more steps.
0
reply
sportyegg
Badges: 16
Rep:
?
#7
Report Thread starter 3 years ago
#7
(Original post by yoyomynameis...)
You use excess ammonia and ethanol. By using ethanol you prevent nucleophilic substituion by water. When you use ammonia you get a primary amine, however, it still has a lone pair, which can act as a nucleophile itself and can react with the haloalkane to form a secondary amine. You don't want this, so you add excess ammonia. Ammonia is a better nucleophile and will react with the haloalkane, so the primary amine doesn't react to give a secondary amine, increasing yield of the primary amine. It's a bad synthetic technique so we use catalysts or we can take the haloalkane, hydrolyse it to form an alcohol, oxidise this to form an aldehyde and then reduce the aldehyde using HCN, to form a hydrocynitrile, which you can reduce by adding H2 and nickel to get the primary amine. Much better but more steps.
Thank you!
0
reply
supdudes
Badges: 5
Rep:
?
#8
Report 3 years ago
#8
just because it annoys me nucleophilic substitution forms an amine not an amide thats addition-elimination!
0
reply
sportyegg
Badges: 16
Rep:
?
#9
Report Thread starter 3 years ago
#9
(Original post by supdudes)
just because it annoys me nucleophilic substitution forms an amine not an amide thats addition-elimination!
Yeah I know sorry! My phone wasn’t letting me edit it was annoying me too ahh
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Should the school day be extended to help students catch up?

Yes (77)
29.96%
No (180)
70.04%

Watched Threads

View All