[bigmo89]

Hello, I'm looking for some Lewis acid/base reaction where I would need to predict the products. I'm having a little trouble determining which reactant would be the acid and base. Any problems or explanation on how I can figure out which is the acid and which is the base would be appreciate.

[Honclbrif]

Bases are usually pretty easy to find because 99 times out of 100 they have a lone pair. Acids can be a little trickier because you're looking for the absence of electron density, but a positive charge or vacant orbitals are usually good signs.

[bigmo89]

How would I know what bond to break and form though. Also thanks for letting know how to identify which is the acid or base, really makes a lot of sense now.

[Honclbrif]

"How would I know what bond to break and form though?"

Executive summary: Most of these reactions involve turning lone pairs into bonds when a base attacks an acid, and bonds into lone pairs when valance would be exceeded by formation of a new bond.

Consider the reaction of ammonia and borane:

:NH₃ + BH₃  H₃N-BH₃

To draw the mechanism, you would draw a curved arrow starting from the the lone pair on nitrogen (Lewis base) which attacks the vacant p-orbital on boron (Lewis acid), turning that lone pair into a bond. No need to break bonds in this case, just to form them.

Consider the reaction of ethylene and hydrobromic acid:

H₂C=CH₂ + H-Br  H₃C-CH₂⁺ + Br^-

The pi-cloud in ethylene (Lewis base) attacks the H in H-Br (Lewis acid) with a curved arrow. The immediate result of this would be carbon with 5 bonds and hydrogen with 2 bonds, exceeding their valance. Therefore, you "break" the pi-bond so carbon does not exceed its valance. To avoid exceeding H's valance, you break the H-Br bond by pushing the electron pair to Br, forming a new lone pair on Br^-.