Overall, in an elimination reaction, a pi bond is formed by eliminating, or removing, one group from each carbon of the pi bond.
In this section, the elimination reactions occur through loss of a leaving group and a proton transfer. There are two types of elimination reactions, E2 and E1.
An E2 reaction occurs when a strong base is used. The mechanism is concerted meaning the proton transfer (which is the source the electrons for the pi bond) and loss of a leaving group on a neighboring carbon occurs simultaneously.
If only a weak base is present, the reaction needs to be heated in order for an E1 reaction to occur. In an E1 reaction, first, the leaving group is lost to form a carbocation. As always with carbocations, be on the lookout for a rearrangement! Then in a separate step, a proton transfer from a neighboring carbon results in formation of a pi bond.
Watch the videos below for more details. Then see if you can decide yourself which pathway is favored by playing the recommended practice puzzles.