Final answer:
The major intermediate carbocation is the one with the greatest degree of alkyl substitution, typically a tertiary carbocation. Factors such as inductive effect and potential for carbanion stabilization by resonance also influence the stability and formation of reaction intermediates in E1 and E1cB mechanisms.
Step-by-step explanation:
To determine which of the carbocations formed is the major intermediate, we must consider the stability of the potential carbocations. The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon because these groups can donate electron density through the inductive effect, thereby stabilizing the positive charge. Thus, a tertiary carbocation (R3C+) is more stable than a secondary (R2CH+) or a primary (RCH2+) carbocation.
Considering these stability rules, the carbocation that is formed predominantly will be the one that has the greatest degree of alkyl substitution. In the case of reactions involving tertiary alkyl halides or allylic phosphates, the formation of a tertiary carbocation would be favored over secondary or primary carbocations.
Furthermore, according to the provided information, in the presence of enzymes, the formation of enantiomers is restricted due to the chiral nature of the active sites. This directs the attack of the nucleophile to only one side, leading to a stereospecific outcome. Nevertheless, when dealing with non-enzymatic reactions, the planarity of the carbocation intermediate allows for attack from either side, potentially creating a mixture of enantiomers if the product is chiral.
Key Mechanisms
The E1 mechanism and the E1cB mechanism both involve the formation of carbocations. The E1 mechanism leads to the formation of a carbocation through the departure of a leaving group. In contrast, the E1cB mechanism first involves the formation of a carbanion through the removal of a proton, which then promotes the elimination of the leaving group. This suggests that we must also consider the potential for carbanion stabilization by resonance when predicting which reaction pathway will be preferred.