Can anyone help me with this basicity questions on Amines?

[americanpierg]

Why is the salt (CH3)4N+ CL- less basic than the alkyl amine (CH3CH2)2NH?

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[americanpierg]

Why is the salt (CH3)4N+ CL- less basic than the alkyl amine (CH3CH2)2NH?

NVRMND, I'm ******ed. But can anyone tell me why the trimethyl amine (CH3)3N is less basic than the dimethyl amine (CH3)2NH?

 

[LittleRocker]

NVRMND, I'm ******ed. But can anyone tell me why the trimethyl amine (CH3)3N is less basic than the dimethyl amine (CH3)2NH?

Normally it would be more basic but its less because of steric hindrance

 

[sleepy425]

ok, so it turns out that this has to do with the solvation of the cation formed. so let's think about it:
a neutral amine: you've got a polar region around the nitrogen and a nonpolar region around the alkyl groups.

a protonated amine: you've got a charged region (which means it's effectively super polar) around the nitrogen and a nonpolar region around the alkyl groups.

in both cases, water wants to solvate the molecule at the polar/charged end. however, this "need" to solvate is much much more important when the amine is protonated because unsolvated cations are rather unstable.

ok, so now we have to look at what having more alkyl groups (trimethylamine) will do to this: if you have much more steric bulk around the molecule, it is more difficult for the water molecules to solvate the cation. thus, the trimethylamine cation is less stable so neutral trimethyl amine is more stable, which lowers the pKa (less basic). Dimethylamine has less steric bulk so the protonated form can be solvated more easily. However, it still is rather bulky, and if you'll notice, the pKa of methylamine is 10.66 while the pKa of dimethylamine is 10.73. They are effectively the same basicity, even though the dimethylamine has an extra donating methyl group, so this is another example of the solvation ability influencing basicity.

Excellent question, but for the record, this will never be on the MCAT because it's not a well known effect. Hope this helps... Lemme know if you have any questions

 

[LittleRocker]

ok, so it turns out that this has to do with the solvation of the cation formed. so let's think about it:
a neutral amine: you've got a polar region around the nitrogen and a nonpolar region around the alkyl groups.

a protonated amine: you've got a charged region (which means it's effectively super polar) around the nitrogen and a nonpolar region around the alkyl groups.

in both cases, water wants to solvate the molecule at the polar/charged end. however, this "need" to solvate is much much more important when the amine is protonated because unsolvated cations are rather unstable.

ok, so now we have to look at what having more alkyl groups (trimethylamine) will do to this: if you have much more steric bulk around the molecule, it is more difficult for the water molecules to solvate the cation. thus, the trimethylamine cation is less stable so neutral trimethyl amine is more stable, which lowers the pKa (less basic). Dimethylamine has less steric bulk so the protonated form can be solvated more easily. However, it still is rather bulky, and if you'll notice, the pKa of methylamine is 10.66 while the pKa of dimethylamine is 10.73. They are effectively the same basicity, even though the dimethylamine has an extra donating methyl group, so this is another example of the solvation ability influencing basicity.

Excellent question, but for the record, this will never be on the MCAT because it's not a well known effect. Hope this helps... Lemme know if you have any questions

oh you just had to 1 up me haha

 

[sleepy425]

oh you just had to 1 up me haha

LOL! sorry, i was writing the post when you submitted yours lol

 

[Kaustikos]

So can one not just use the knowledge of electron donating/electron withdrawing groups to figure out which is mroe acidic/less acidic? It works in the case of trimethyl vs dimethyl amines.

 

[sleepy425]

So can one not just use the knowledge of electron donating/electron withdrawing groups to figure out which is mroe acidic/less acidic? It works in the case of trimethyl vs dimethyl amines.

no, it doesn't. trimethyl amine, by that reasoning, should be more basic (more electron donating methyl groups) than dimethyl amine, but it's almost 1 pKa unit lower (less basic) than dimethylamine. steric effects on solvation ability are important for these molecules.

 

[Kaustikos]

no, it doesn't. trimethyl amine, by that reasoning, should be more basic (more electron donating methyl groups) than dimethyl amine, but it's almost 1 pKa unit lower (less basic) than dimethylamine. steric effects on solvation ability are important for these molecules.

nevermind. I realized my mistake. electron donating decreases acidity. Other way around

 

[cleopetra]

So can one not just use the knowledge of electron donating/electron withdrawing groups to figure out which is mroe acidic/less acidic? It works in the case of trimethyl vs dimethyl amines.

Yes , you can !!! actually trimethyl amine should be more basic than dimethyl amine because it can donate electron pair more easily but its Nitrogen has bulky group around it and steric hinderance plays an important role here. Bulky groups do not allow electron pair acceptors to come near it and hence it becomes less basic than dimethyl amine.

 

[cleopetra]

So can one not just use the knowledge of electron donating/electron withdrawing groups to figure out which is mroe acidic/less acidic? It works in the case of trimethyl vs dimethyl amines.

Oh,, one important thing... the answer i gave above is based on the fact that electron pair donar are considered as base and electron pair acceptors are acid. I believe you memorized that electron pair donating gps. decreases acidity,, well I would say it is better to understand why that happens.
Acid when gives proton then it remains in the anion form and electron donating grps. increases negative charge making it less stable and it reattaches itself to proton ( acidity decreases). opposite happens with electron pair withdrawing gps.
Sleepy425 is right. And the answer I gave on the basis of Electron pair donar tendency is also right.

 

[IkerUnzalu]

The more stable the conjugate base the more acidic. And correct me if I'm wrong but I think the stabilty of a conjugate base is based on its electronegativity.

 

[IkerUnzalu]

So an acyl chloride will be more acidic than a carboxilic acid because according to periodic trends the cloride ion is more electronegative than the oxygen conjugate of the OH on the carboxilic acid. I'm just trying to remember this stuff so idk if it's 100% correct, so feel free to tweek my recollection.... I won't take offense.

 

[bluemonkey]

So an acyl chloride will be more acidic than a carboxilic acid because according to periodic trends the cloride ion is more electronegative than the oxygen conjugate of the OH on the carboxilic acid. I'm just trying to remember this stuff so idk if it's 100% correct, so feel free to tweek my recollection.... I won't take offense.

What proton is an acid chloride going to donate? The alpha hydrogen?

 

[IkerUnzalu]

Uh..... no..... I think what happends is that acyl chlorides are extremely reactive b/c the carbonyl oxygen and the chloride ion have very srtong electron withdrawing tendencies so that the carbonyl carbon is left with a large positive charge leaving it susceptible to nucleophilic attack even by weak nucleophiles. So..... if it was in aqueous solution then water would become the nucleophile yielding a carboxilic acid that is in equilibrium through resonance between the conjugate oxygen and the carbonyl oxygen and then the rxn will also yield HCL which is very acidic ph of 2. So I think that to say acyl chloride is more acidic was a mistake on my behalf I should've restated my original statement and said that they are more reactive. But for acidity it's always about stability. The more stable the conjugate base the more acidic. So like an alchol is less likely to give up its hydrogen because it's conjugate base would not be able to stabilize itself through resonance like a carboxilic acid but water is actually more acid because it acts as a bronsted base and acid so its conjugate base the OH can easily stabilze itself by picking up a proton and even another to yield a hydronium ion which is more unstable thus it will easily lose its proton to cotribute to acidity. These exchanges of hydrogens are happening very quickly so stability and acidity should be analyzed relative to the enviroment (the solution) that these rxn are taking place, but the general gist is always assumed to be under aqueous solutions. Again I'm not 100% sure but for the most part this is correct.

 

[Kaustikos]

Oh,, one important thing... the answer i gave above is based on the fact that electron pair donar are considered as base and electron pair acceptors are acid. I believe you memorized that electron pair donating gps. decreases acidity,, well I would say it is better to understand why that happens.
Acid when gives proton then it remains in the anion form and electron donating grps. increases negative charge making it less stable and it reattaches itself to proton ( acidity decreases). opposite happens with electron pair withdrawing gps.
Sleepy425 is right. And the answer I gave on the basis of Electron pair donar tendency is also right.

Yeah, I just think the idea is that I cannot just use this "trick" all the time, as tehre are exceptions to that rule. But MCAT does not test the exceptions, from what I hear.

 

[IkerUnzalu]

I don't wana infrige on
MCAT regulations, but I actually had a question that involved basicity of two molecules. I can't be specific but the MCAT does test on these questions.

 

[Kaustikos]

I don't wana infrige on
MCAT regulations, but I actually had a question that involved basicity of two molecules. I can't be specific but the MCAT does test on these questions.

No, that wasn't the point I meant. What I meant was that the mcat will test that, but it tends not to test you on exceptions to the rules for biochem/organic chem.

 

[IkerUnzalu]

Oh I know but someone on an earlier post said that questions regarding basicity are not likely to be on the test and I'm telling whoever wrote that that I had a question regarding the very same thing we are all discussing.