Lewis Acid-Base questions

[Zellmer, Robert]

I know this is right before the exam so hopefully you have a chance to read it.
I got a couple of questions today about Lewis acids and bases.  I went over some
of the following during lecture and the review so you can watch it.  The following
came about from a question sent to me concerning whether CO2, CO and SnCl4
were Lewis acids or bases.  The following was my response.

CO2 acts as a Lewis ACID in water.  The C atom is somewhat electron deficient
due to the double bonds between it and the O atoms.  It accepts a pair of electrons
from the O in the H2O and forms an intermediate first and then carbonic acid.  I
went over this reaction in lecture, showing how CO2 reacts with H2O to form
carbonic acid, H2CO3.

For CO, the C atom has a lone pair of electrons on it and a partial negative charge.
It's a strange molecule because C doesn't like to have a non-bonded pair of
electrons on it, it has a formal charge of -1 on it and the dipole moment in CO
points toward the C atom, with the C atom being the negative end of the dipole.
The C atom really wants to share the pair of electrons and thus it acts as a Lewis
base.  This is what happens in carbon monoxide poisoning.  The CO acts as a
Lewis base and binds with the Fe ion in your hemoglobin.  It binds more strongly
than O2 so it doesn't take a lot to kill you.

SnCl4 can act as a Lewis acid because Sn can expand its valence and bond to
more than 4 atoms.  It can violate the octet rule.  This occurs for row three atoms
and below.  So technically any atom that can expand its valence shell beyond an
octet could act as a Lewis acid.

Remember, all Arrhenius acids/bases are Bronsted-Lowry acids/bases and all
BL acids/bases are Lewis acids/bases.  However, it doesn't always go back the
other way.  BF3 is a Lewis acid but not a BL or Arrhenius acid.

Dr. Zellmer
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