Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: mrlucky0 on February 28, 2008, 11:43:02 PM
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Can someone explain to me why a methylamine, CH3NH2 (Pka = 10.64) is more basic then a trimethylamine, (CH3)3-N (Pka = 9.87)? From what I know, I thought less electronegative substituents, like alkyl groups increase the basicity. Hence, shouldn't trimethylamine be more basic because it's got 3 methyl groups?
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I believe basicity is determined by the availability of the lone pare on the nitrogen.
Your right about the methyl substituent are less electronegative, but do draw in electrons inductively (-I effect) from the nitrogen.
So if you think about it:
1 methyl substituent draws let say 0.1% of electrons (methylamine),
3 methyl substituent must draw 0.1% x 3 = 0.3% of electrons (trimethylamine).
This should explain why trimethylamine is less basic then methylamine.
Hope this helps.
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Hey mrlucky0 and refid
I must say, refid, that i do not agree with you - methylgroups in amines, and ammonium-ions for that matter, are electrondonating and not electronwithdrawing.
The basicity trends of amines are most easily explained by looking at the acidity of ammonium-ions, which (in solution) follows the trend given below:
NH4+ > (CH3)3NH+ > CH3NH3+ > (CH3)2NH2+
That is NH4+-ions are the most acidic (least basic) ones, whereas (CH3)2NH2+-ions are the least acidic (most basic) ones.
This trend can be explained by two conflicting effects - polarizability and solvation.
If you look at gas phase acidities the trend is as follows:
NH4+ > CH3NH3+ > (CH3)2NH2+ > (CH3)3NH+
Which is what we would normally expect since more methyl-groups donates more electrons spreading out the positive charge and therefore stabilizing the ammonium-ion (making it less acidic, and the conjugate base more basic).
On the other hand solvation effects leads to the following trend:
(CH3)3NH+ > (CH3)2NH2+ > CH3NH3+ > NH4+
This trend can be explained by the size of the ammonium-ion - the larger, the least well solvated, therefore least stable and hence more acidic (less basic).
A combination of these two trends gives the acidity (and therefore basicity) trend of ammonium-ions (amines) in solution.
I hope this answers your question!
Rico
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Excellent explanation, Rico! I agree completely!
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Can someone explain to me why a methylamine, CH3NH2 (Pka = 10.64) is more basic then a trimethylamine, (CH3)3-N (Pka = 9.87)? From what I know, I thought less electronegative substituents, like alkyl groups increase the basicity. Hence, shouldn't trimethylamine be more basic because it's got 3 methyl groups?
Trimethylamine is more basic then methylamine. Your data are pKas that say directly about acidities of ammonium cations
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Opps..Thanks rico
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yes AWK, the pka values are the pka's of their conjugated acids,
-the highest pka the stronger base? it is? methylamine should be more basic than trimethyl amine.
the pka's value of alkyl amines are around 30????
thanks!
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http://www.chemicalforums.com/index.php?page=pkavalues