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why is this?

why can CH3CH2NH2 not be formed from when CH3I is heated in excess in a sealed tube with alcoholic ammonia?

Thanks :smile:
Reply 1
Avatar for upthegunners
upthegunners
OP
10
this is an exam question on edexcel AS
Reply 2
Avatar for shengoc
shengoc
12
Original post by upthegunners
this is an exam question on edexcel AS


OK, lets see,

MeI + alcoholic NH3 ---> Me-NH2

compare the ability of NH3 vs. MeNH2 as a nucleophile. Which one is a better nucleophile?

A nucleophile attacks an electrophilic species, i.e. richer in electron density due to the inductive ability of Me (or any generic alkyl groups) will increase electron density around N of the amine - hence in terms of increasing nucleophilicity,

NH3 < MeNH2 < Me2NH < Me3N

Hence, once MeNH2 is produced, you have more MeI in your reacting system,

MeNH2 + MeI ---> Me2NH

Me2NH + MeI ---> Me3N

etc. can also form quarternary ion; cf ammonium like ion, but
NMe4+ I- species

Hope that is all clear now.
Reply 3
Avatar for upthegunners
upthegunners
OP
10
Original post by shengoc
OK, lets see,

MeI + alcoholic NH3 ---> Me-NH2

compare the ability of NH3 vs. MeNH2 as a nucleophile. Which one is a better nucleophile?

A nucleophile attacks an electrophilic species, i.e. richer in electron density due to the inductive ability of Me (or any generic alkyl groups) will increase electron density around N of the amine - hence in terms of increasing nucleophilicity,

NH3 < MeNH2 < Me2NH < Me3N

Hence, once MeNH2 is produced, you have more MeI in your reacting system,

MeNH2 + MeI ---> Me2NH

Me2NH + MeI ---> Me3N

etc. can also form quarternary ion; cf ammonium like ion, but
NMe4+ I- species

Hope that is all clear now.


Wow, that's great! Thanks :smile:

Seems quite advanced for ASlevel, though..:redface:
Reply 4
Avatar for shengoc
shengoc
12
Original post by upthegunners
Wow, that's great! Thanks :smile:

Seems quite advanced for ASlevel, though..:redface:


it is not really; it is about what you understand about nucleophilicity of amine group as how good is the electron donating ability of the lone pair of nitrogen.

also, inductive effect of methyl groups also explain why tertiary carbocation is more stable than secondary than primary (you'd come across this in a level at some point if you haven't)

also, then, it is about understanding reaction dynamics as a whole and not just as discrete as what we usually write on paper (i.e. representing MeI + NH3 forming MeNH2 does not mean all the MeI is used up in that one go; what happens is say, you make one MeNH2 but there are more MeI lying around, etc, you get me...)

:confused: don't be! :wink:

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