Final answer:
To predict the major alkene product in an E1 reaction, consider the stability of the carbocation that will form. The most substituted and stable alkene is often the major product.
Step-by-step explanation:
To predict the major alkene product of an E1 reaction, consider the stability of the carbocation intermediate. The most stable alkene will be formed from the most substitution, often resulting in a more substituted and thus more stable alkene product. For a typical E1 elimination reaction, a good leaving group such as a halide ion leaves first, forming a carbocation.
The next step involves the removal of a proton (H+) from an adjacent carbon atom, which leads to the formation of the alkene. In the case of secondary substrates, whether an E1 reaction occurs can be substrate-dependent. However, for tertiary substrates, the E1 mechanism is favored.
Compounds with secondary and tertiary carbons tend to undergo E1 more readily due to the increased stability of the resultant carbocation.
Considering an E1cB mechanism, it is important when a carbanion intermediate is stabilized, typically through resonance. This mechanism is relevant in biological systems, especially in reactions involving carbonyl compounds.
The reaction's specifics, including the structure of the substrate and the conditions applied, will ultimately determine which alkene is the major product. However, as a general rule, the most substituted, stable product is expected in E1 reactions.