Ochem/SN2/acidity?

[vintik]

hey could someone answer the following question: mix dopamine, ethanol, dry hcl, what do u get? obviously amino group gets protonated. but the phenol groups shouldnt get protonated because they're more acidic than water(the congugate base of the hydronium). so SN1 reaction cannot happen. on the other hand ethanol may get protonated. and then both phenol groups or only 1 of them might attack ethanol in SN2 reaction? and perhaps even the carboxylic acid group might attack the protonated ethanol in sn2 rxn?

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[ThePandaFactor]

Dopamine doesn't have any carbonyl groups, and amines and acids don't really have a reaction, so you probably just get the protonated amine.

 

[vintik]

Dopamine doesn't have any carbonyl groups, and amines and acids don't really have a reaction, so you probably just get the protonated amine.

but ethanol reacts with hcl and it then becomes very receptive to nucleophilic attack. and phenol oxygen can definitely act as a nucleophile(and it has 2 phenol groups). carboxylic acid oxygen would be a much much weaker nucleophile but still who knows.

 

[ThePandaFactor]

I guess you're right, maybe it can form the ethyl ethers, since there is a large excess of alcohol.

 

[vintik]

but then again, nonprotonated alcohol is a better nucleophile than a phenol. so it might just keep switch protons between the alcohol molecules... or perhaps in rare instances the phenol will attack the protonated alcohol and even the carboxylic acid will attack the alcohol, forming both ethers and an ester.

 

[161927]

Phenol is not a good nucleophile because its lone pairs are in conjugation with the benzene ring. I would not predict phenol to react with an ethanol. Phenol is also not likely to hold on to an extra acidic hydrogen.

The amine will be protonated.

Now getting into more unusual product formation: Under acidic conditions, ethanol may be protonated and lose a molecule of water, to form an alkene (or a carbocation intermediate). The triply-activated benzene ring of dopamine may react with this carbocation intermediate through an electrophilic aromatic substitution. (Just a guess)

 

[vintik]

Phenol is not a good nucleophile because its lone pairs are in conjugation with the benzene ring. I would not predict phenol to react with an ethanol. Phenol is also not likely to hold on to an extra acidic hydrogen.

The amine will be protonated.

Now getting into more unusual product formation: Under acidic conditions, ethanol may be protonated and lose a molecule of water, to form an alkene (or a carbocation intermediate). The triply-activated benzene ring of dopamine may react with this carbocation intermediate through an electrophilic aromatic substitution. (Just a guess)

ah your explanation actually made it a bit more clear. it means that alcohol after losing h2o may now react in SN1 rxn, as opposed to sn2. so it would react with the more substituted phenol group. but there are 2 phenol groups, so what stops it from being added to the other group as well? and by the same sn1 mechanism it might also add to the carboxylic acid in esterification?
so in that case if the answer choices are: ch3ch2 not added at all; added to the more substituted phenol, added to both phenol groups and the acid group, under what conditions would 1 choice be better than another?

 

[ThePandaFactor]

EtOH is a primary alcohol so the carbocation will be very unstable. I don't think it will be activated enough to react in an electrophilic aromatic substitution.

 

[Kaustikos]

Phenol is not a good nucleophile because its lone pairs are in conjugation with the benzene ring. I would not predict phenol to react with an ethanol. Phenol is also not likely to hold on to an extra acidic hydrogen.

The amine will be protonated.

I agree. The reaction conditions for a phenol, or any substituent on a benzene ring, are just more stringent than regular alcohols. Since we're dealing with the question of which gets protonated, I would say the phenol would not become protonated under these conditions.

Amines always react with HCl - it's one of the methods to remove an amine compound from other compounds in filtration. But protonation of an alcohol is also a great way to undergo Sn1 reactions (actually, protonation is probably the only method you would have to know for the mcat as a way to remove an alcohol other than toluene or tosylate). I think the question comes down to which would get protonated more easily/frequently - the amine or the alcohol. Then there's the question of which is the better nucleophile/leaving group. The amine group would, in my opinion, get protonated more likely than the ethanol. This would lead to a strong reaction site at the carbon bonded to the amine.

But under your conditions mentioned, I would say not added at all. The conditions just don't meet the requirements for a phenol to undergo any reaction. If anything, the ethanol would become an ethene.

 

[vintik]

hey thanks you guys!