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Basicity in amines

  1. Sep 15, 2012 #1
    This is no homework question, i came across this when i was checking out the pKb values for some amines. The pKb value of diethylamine is less that of dimethylamine in aqueous phase according to my textbook. This indicates that the former is more basic. My question is why diethylamine is a stronger base than dimethylamine?
    What i think is when the latter is protonated, it is easily protonated because of less steric hindrance. Also, the protonated dimethylamine is more solvated than the protonated diethylamine due to its smaller size. These two factors support that dimethlylamine is a stronger base.
    Next comes the inductive effect, the +I effect of -C2H5 is greater than that -CH3 due to which protonated diethylamine is more stable than the protonated dimethylamine. This supports that diethylamine is a stronger base.
    How should i determine the basicity order here when two factors support that dimethylamine is a stronger base than diethylamine but one factor supports that diethylamine is stronger base than dimethylamine?

    Any help is appreciated. :smile:
     
  2. jcsd
  3. Sep 15, 2012 #2
    Anyone?
     
  4. Sep 16, 2012 #3
    Is there no one who could help me with my simple question? :(
     
  5. Sep 16, 2012 #4
    This is not something that can be quantitatively analyzed just from knowledge of the above two factors. Which effect dominates can only be found out experimentally, and in this case stabilisation due a more electron-donating group is more important than cation solvation (I assume all this is in an aqeous medium).
     
  6. Sep 16, 2012 #5
    Finally someone replied! Thanks Shubham! :smile:
    Yes, everything is in aqueous medium, i already mentioned that.

    But what if a question appears on these two amines? Am i supposed to memorize the pkb values? For example, if i wanted to determine which is more basic from, say, triethylamine and trimethylamine in aqueous medium, how should i go on answering it?
     
  7. Sep 16, 2012 #6

    AGNuke

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    Gold Member

    You can compare between Ethylamine, diethylamine or triethylamine, but the problem is, you can't readily compare between trimethylamine or triethylamine. But, since you know the comparison between dimethylamine and diethylamine, you can solve this question.

    You may probably know, trimethylamine as well as triethylamine, both are weaker than their "di" counterparts. (Look into NCERT). Now triethylamine do not take the hit as bad as trimethylamine, because steric hindrance can't go worse (affecting hydration), so the stabilization of cation by +I groups matters, so TEA > TMA.
     
  8. Sep 16, 2012 #7
    Thanks AGNuke for the explanation, i get it now! :smile:

    Btw, this question is from NCERT, i mean i was checking out the pkb values from the chapter "Amines" and stuck on these.
     
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