Question

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...

Answer 1

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?