Bases usually contain a lone pair of electrons which could form a bond to a proton. Most, but not all, bases include a negative charge. We certainly have noticed an example of a reaction of bases with halogenoalkanes to give alkenes within the last chapter, and others are going to follow in this particular section. Bases may also react to form bonds to other elements, especially carbon, and when reacting in this way they are termed nucleophiles. We have already come across nucleophilic reactions in the SNl and SN2