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Pushing Electrons and Curly Arrows

Lesson 2

This is a simple acid / base reaction showing the formation of the hydronium ion produced when hydrogen chloride is dissolved in water.  In order to develop an understanding of curly arrows, it is useful to analyse the bond changes that are occuring.
Water is functioning as a BASE accepting a PROTON and hydrogen chloride as an ACID donating a PROTON.  Let's consider the reaction in discrete steps.  Formation of the proton by the acid requires that we break the H-Cl bond, we do this by taking electrons out of the bond :

breaking a sigma bond between H and Cl
Do the charges balance ? Yes, we have a neutral molecule going to  +ve and -ve whose charges cancel.

Next the reaction of the base with the proton requires that we make a new O-H bond.  This requires that we put electrons BETWEEN the atoms that are to be BONDED:

making a sigma bond between O and H

Do the charges balance ? Yes, we have a neutral molecule with a +ve ion giving a +ve ion. 

So the curly arrows  have accounted for all the bonding changes that occur in this process. We started with H2O and HCl we now have H3O+ and Cl-

However, we should consider this reaction as a single process where the BASE abstracts the PROTON from the ACID :

This is just the two separate steps rolled into one.  Start from the lone pair on oxygen, make the O-H bond, need to break a bond (since H can only have 1 pair of electrons around it) so break the H-Cl bond giving the electrons to the electronegative Cl.


Can you draw the curly arrows  for the reverse process ?