Final answer:
E2 elimination requires antiperiplanar geometry but can sometimes adjust under certain conditions. Elimination can also occur through E1 or E1cB mechanisms that do not require antiperiplanar geometry if a stable carbocation or carbanion intermediate is formed, respectively.
Step-by-step explanation:
When usual antiperiplanar geometry for an E2 elimination is not available due to the trans arrangement of halogens, such as with bromines, other elimination mechanisms may occur. The E1 mechanism is one possibility, where the rate-determining step is the formation of a carbocation by loss of a leaving group, followed by deprotonation to form the double bond. Alternatively, the E1cB mechanism can occur if the substrate has a hydrogen that can form a stabilized carbanion upon deprotonation, facilitating the loss of the leaving group.
In E1 reactions, the formation of a carbocation is the first step, and this intermediate can then lead to either substitution (SN1) or elimination (E1), depending on the reaction conditions and the presence of a base. On the other hand, in the E1cB mechanism, deprotonation occurs first leading to a carbanion, which then ejects the leaving group. Both E1 and E1cB do not require the antiperiplanar geometry that E2 eliminations do.
E2 reactions, although generally requiring antiperiplanar geometry, can also sometimes occur with less ideal geometries under particular conditions, such as the presence of a strong base and when a secondary or tertiary alkyl halide substrate is involved. E2 is considered a concerted mechanism, and in some instances, rotations around σ-bonds can allow for the necessary alignment for elimination, even in trans-halogenated substrates.