The reaction mechanism we see here is called a nucleophilic substitution, and is abbreviated S N2. The C-Cl bond breaks as the new C-O bond forms, and the chlorine leaves along with its two electrons. However, in order for a new bond to form between the hydroxide oxygen and the carbon, one of the bonds already on the carbon must break – otherwise, there will be five bonds to carbon and the octet rule will be violated. The hydroxide ion – specifically, the electronegative oxygen atom in the hydroxide ion – has high electron density due to the polarity of the hydrogen-oxygen bond. One very important key to understanding just about any reaction mechanism is the concept of electron density, and how it is connected to the electron movement (bond-breaking and bond-forming) that occurs in a reaction. Despite its simplicity (and despite the fact that the reactants and products are inorganic rather than organic), this reaction allows us to consider for the first time many of the fundamental ideas of organic chemistry that we will be exploring in various contexts throughout this text. You have undoubtedly seen this reaction before in general chemistry. The product is water (the conjugate acid of hydroxide) and chloride ion (the conjugate base of HCl). This is an acid-base reaction: a proton is transferred from HCl, the acid, to hydroxide, the base. For our first example of chemical reactivity, let’s look at a very simple reaction that occurs between hydroxide ion and hydrochloric acid:

Hydroxide oxygen charge