Which is the stronger Lewis Base? NH3 or NCL3?

Hello,

As was pointed out in the comments by mathtalk, a Lewis base is a
compound which can "donate" an electron pair.

In the molecules which you have mentioned, a central nitrogen is
connected to 3 molecules of either hydrogen or chlorine.  Organic
molecules have tetrahedral symmetry in their bonding orbitals.  All of
the orbitals are full when they have two electrons each.  Nitrogen has
5 valence electrons which are arranged into the four orbitals - thus
three of these orbitals must form bonds with another molecule and the
remaining orbital has a full pair of electrons.  It is this pair of
electrons that acts as a Lewis base.

Now, consider a difference between hydrogen and chlorine - chlorine is
more electrophillic than hydrogen (it pulls electrons more closely
towards it's nucleus).  When you have three very electrophillic atoms
connected to the central nitrogen you will have a very strong "pull"
of negative charge (the electrons) from the nitrogen towards the
chlorine atoms - this is known as an inductive effect.  This "pulling"
of the electron density away from the nitrogen decreasing the net
negative charge near those two lone-pair electrons, rendering them
less negative in NCl3 than they are in NH3.  Thus, NCl3 will be a
worse Lewis base than NH3 due to the decrease in the net negative
charge upon the lone-pair electrons.

A good reference for this effect in organic chemistry can be found
here:
http://www.chemguide.co.uk/basicorg/acidbase/bases.html#top 
http://www.cm.utexas.edu/bauld/teach/acidsbases.html#str  (See section
on inductive effects)

Please let me know if you need further explanation.

synarchy

Hello 031987,

"the PH of NH3 (ammonia) is 11.4"
SOURCE: http://www-plb.ucdavis.edu/Courses/f02/bis102/Lec2_solutions.pdf
"The pH of ammonia is approximately 12"
SOURCE: ://www.google.ca/search?q=cache:KlLWSQYIDFgJ:www.cst.cmich.edu/users/Benja1dw/BIO101/tools/quiz/inorganic_chemistry.htm+%22ph+of+ammonia%22&hl=en&ie=UTF-8

If you can find the pH of NCL3, you should be able to solve this
question.

tisme-ga


Search Strategy:

"ph of ammonia"

Hi, tisme-ga:

A "Lewis base" is defined with reference to the ability to donate an
electron pair.  So strictly speaking the pH of these compounds would
not settle this question.

regards, mathtalk-ga