# 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...

Jan 2, 2017

A $\text{Lewis acid}$ is an electron pair acceptor, and a.........

#### Explanation:

.............a $\text{Lewis base}$ is an electron-pair donor.

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

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 $\text{cyanide anion}$, ""^(-)C-=N, which is moderately basic (and also of course highly poisonous):

${H}_{2} O \left(l\right) + {\text{^(-)C-=N rightleftharpoons H-C-=N(aq) + }}^{-} O H$

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

The designation of $M g B {r}_{2}$ is much less clear cut. In aqueous solution, magnesium bromide speciates to $M {g}^{2 +}$ and $B {r}^{-}$ species, as the aquated ions:

$M g B {r}_{2} \left(s\right) \rightarrow M {g}^{2 +} + 2 B {r}^{-}$

The magnesium ion is solvated by several water molecules, so that we could reasonably represent this as ${\left[M g {\left(O {H}_{2}\right)}_{6}\right]}^{2 +}$. 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?