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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: NewtoAtoms on March 22, 2009, 03:28:20 AM

Title: Which amine is the weakest base?
Post by: NewtoAtoms on March 22, 2009, 03:28:20 AM

Hello Organic Chemists,

Would someone please shed some light on this question for me? I have done most of the leg work but I am stuck between two choices

Which amine is the weakest base?

1.  CH3CH2NH2                       5.6 x 10^-4           pKb = 3.2       STRONG BASE
2.  (CH3)2NH                          5.1 x 10^-4            pKb = 3.3       STRONG BASE
3.  C6H5CH2NH2    (benzyl amine)                        pKb = 4.67      STRONG BASE
4.  C6H5NH2         (Aniline)                                 pKb = 9.42      WEAK BASE
5.  C6H5CH2NHCH3  (N-methyl benzyl amine)     ???

Now I know that arylamines are weaker bases than alkylamines, therefore 1,2 are eliminated.
My issue is between 4,5.  I am leaning toward Aniline being the weakest bast, however I have researched N-methyl benzyl amine extensively and I can't find out about it. 

Can anyone shed some light on this for me.

I would be so grateful and thank you for your time.

Newtoatoms

Title: Re: Which amine is the weakest base?
Post by: macman104 on March 22, 2009, 04:23:22 AM

You should not need pkb information for this (in fact it's a crutch).  You should be able to evaluate these compounds qualitatively.  I imagine that's actually what this problem is expecting as well.

What is the difference between aniline and N-methyl benzyl amine?  How does this group (think about sterics, resonance and induction) affect the resulting cation that forms when the nitrogen takes a proton?

Title: Re: Which amine is the weakest base?
Post by: NewtoAtoms on March 22, 2009, 12:12:50 PM

Hello macman,

Steric hinderance decreases nucleophile strength and increases base strength.

Therefore if I were to compare the structures and their substitutents, without comparing Pkb values I would conclude:

         H
         l
C6H5-N:             Aniline
         l               less sterically hindered
         H

          H
          l
C6H5 - N            N-methyl benzyl amine   
          l              more sterically hindered
       HCH
         H

Because N-methyl benzyl amine is more sterically hindered it is the stronger base, because "Steric hinderance decreases nucleophile strength and increases base strength"

I find this very confusing, and I have tried to understand it here, am I at all correct?

Title: Re: Which amine is the weakest base?
Post by: macman104 on March 22, 2009, 04:15:18 PM

N-methyl benzyl amine should be more basic, but not because of steric hindrance.  Think about inductive effects.  Which way is the dipole in the C-N bond of the methyl group?  Does that increase or decrease electron density, and how does that affect the stability of a positive charge on the nitrogen (when it acts as a base).