I have a quick question about E1 reactions. In my book (Wade), when it lists examples, the base that transfers electrons to a H in order to deprotonate the molecule is always the base listed in the chemical equation, but in the solutions to some of the end-of-chapter problems, it sometimes uses a Br- base to deprotonate the molecule instead. For example:
H2C-CH-CH3 ---> Br- + H2C-CH-CH3 ---> H2C=CH-CH3
In the solutions manual, the Br- that is a product of the first, slow reaction is used to deprotonate the molecule; the solvent is water, however. How do I know that Br- will be used as the base and not the solvents, such as HOH or HOCH3-? The book always just uses the solvent for deprotonation.