Having identified basic sites as areas of high electron density and acidic protons as hydrogen atoms of low electron density, we can now establish how curved arrows are used to indicate the movement of electrons in acid-base reactions. The rules are basically the same as for resonance structures.
(a) Full headed arrows indicate electron pair movement.
(b) The arrow always originates at the basic site, i.e. the source of electrons (typically nonbonding electrons, but can also be a p-bond).
(c) The arrow indicates a newly formed σ-bond between the basic site of one molecule and the acidic proton of another molecule. As this new bond is being formed, the atom (or group of atoms) to which the acidic proton is attached leaves as the conjugate base. Sometimes this atom or group of atoms is referred to as a leaving group.
SINCE STRONG ACIDS HAVE WEAK CONJUGATE BASES, THE BEST LEAVING GROUPS ARE WEAK BASES. IN OTHER WORDS, EQUILIBRIUM FAVORS DISPLACEMENT OF THE WEAKER BASE BY THE STRONGER BASE.
The red arrow originates at the electron source and moves towards the acidic proton. It indicates a new bond that forms between the oxygen and the acidic proton in HCl.
As oxygen bonds to the acidic proton, chlorine leaves with the electrons as chloride ion, the conjugate base of HCl. It can therefore be referred to as the leaving group.
In this case, water acts as the leaving group.