Amine reactions

[DeathandTaxes]

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I am confused as to how 3 ethyl groups ( as opposed to just 1 ) could form. So the NH3 can SN2 the primary carbon. Once it does that, there are no other leaving groups. Is my thinking wrong here? I would think the answer should be B.

 

[premed1001]

i also thought the answer would be B.

the only thing i can think of is that the ethyl group is more electronegative then a hydrogen atom, so it can easily displace another hydrogen on the nitrogen. this is only a guess

 

[Czarcasm]

View attachment 181673

I am confused as to how 3 ethyl groups ( as opposed to just 1 ) could form. So the NH3 can SN2 the primary carbon. Once it does that, there are no other leaving groups. Is my thinking wrong here? I would think the answer should be B.

You're forgetting that an ammonium cation is formed which is mildly acidic and will easily rid one of it's protons, reforming it's lone pair to continue attacking electrophiles. In each case, the e-donating contribution of alkyl substituents increase the electron density of the lone pair (it's nucleophilicity); however, for the same reason, it becomes more difficult to remove a proton (less acidic due to increasing instability/reactivity). It'll continue attacking electrophiles until no more protons remain, which is why D is the correct answer. Technically, it could even go one further step and create an alkyl ammonium ion.

 

[premed1001]

You're forgetting that an ammonium cation is formed which is mildly acidic and will easily rid one of it's protons, reforming it's lone pair to continue attacking electrophiles. It'll continue until no more protons remain, which is why D is the correct answer. Technically, it could even go one further step and create an alkyl ammonium ion.

but why does it trade the hydrogen for an ethyl group? is it because carbon is more electrophilic/electronegative?

also using the same logic, why doesnt the lone pairs on an alcohol reacted with an alkyl halide, take it to an ether?

i found this, maybe it can help:
http://research.cm.utexas.edu/nbauld/CHAPTER 21_files/image045.gif

 

[Czarcasm]

but why does it trade the hydrogen for an ethyl group? is it because carbon is more electronegative?

also using the same logic, why doesnt the lone pairs on an alcohol reacted with an alkyl halide, take it to an ether?

It doesn't trade it for a hydrogen. The NH3 has a free lone pair to attack the electrophile and displace the leaving group. Once the alkyl ammonium is formed, it'll rid one of it's protons via deprotonation because it's mildly acidic. This "free's up" the lone pair again to repeat the process.

 

[premed1001]

nevermind

 

[Czarcasm]

ethyl amine is fairly stable is it not?

Compared to a neutral amine, it is not. Remember alkyl substituents donate some electron density inductively, which in turn makes it's free lone pair more negative and therefore, more reactive. The key part of this question is what would happen if excess electrophile was added. Sure, after one reaction it'd stop but provided there's so much electrophile floating around, it'll keep reacting as much as possible.

 

[premed1001]

Compared to a neutral amine, it is not. Remember alkyl substituents donate some electron density inductively, which in turn makes it's free lone pair more negative and therefore, more reactive. The key part of this question is what would happen if excess electrophile was added. Sure, after one reaction it'd stop but provided there's so much electrophile floating around, it'll keep reacting as much as possible.

makes sense thanks!