CoachThe reaction mechanism of a reaction is the series of steps by which the reactants become the products. Each elementary step is a single event in which a few pairs of electrons move at once. A species formed in one step and used up in a later one is a reaction intermediate: it does not appear in the overall equation.
Examples
Example 40.8 (A carbocation intermediate)
When hydrogen bromide adds to ethene, the reaction takes two steps. In the first, the double bond of ethene gives a pair to the hydrogen of , and the pair goes to bromine, which leaves as ; the carbon that lost its share of the double bond is left with only six electrons and a positive charge: a carbocation, . In the second step, a lone pair of forms a bond with that carbon. The carbocation is an intermediate.