Which is Lewis acid and base?

1.) Determine
a)CN- (is it Lewis acid or base)
b)MgBr2 (is it Lewis acid or base)

Write acid-base reaction for both CN- and MgBr2 and show how electrons shifted.

It would be a great help if you could just tell me if I'm right assuming that CN- is a base and MgBr2 an acid, I'm not sure how to write the reactions...

1 Answer
Jan 2, 2017

A "Lewis acid" is an electron pair acceptor, and a.........

Explanation:

.............a "Lewis base" is an electron-pair donor.

If we look at the standard reaction for a Bronsted acid, H^+, with a "Bronsted base", ""^(-)OH it is easy to see which is the "Lewis acid", and which is the "Lewis base".

H^(+) + ""^(-)OH rarr H_2O

So which species has donated the electrons, and which species has accepted the electron to form the H-O bond?

Now you have given us "cyanide anion", ""^(-)C-=N, which is moderately basic (and also of course highly poisonous):

H_2O(l) + ""^(-)C-=N rightleftharpoons H-C-=N(aq) + ""^(-)OH

Given what I have said, clearly cyanide anion is the electron pair donor, and therefore is the Lewis base.

The designation of MgBr_2 is much less clear cut. In aqueous solution, magnesium bromide speciates to Mg^(2+) and Br^- species, as the aquated ions:

MgBr_2(s) rarr Mg^(2+) +2Br^-

The magnesium ion is solvated by several water molecules, so that we could reasonably represent this as [Mg(OH_2)_6]^(2+). In other words, the magnesium cation has accepted electron density, i.e. donation, from the water ligands, and here has acted as a Lewis acid. Which is the Lewis base in this scenario?

Are you happy with this treatment?