In: Chemistry
1. Mechanistically, what is the major difference between an E1 and E2 elimination process?
2. Why is phosphoric acid or sulfuric acid used in the E1 elimination reactin rather than HCl or HBr?
3. If the elimination reaction can result in the formation of to different alkenes which one with predominate in the product?
For E1 elimination reaction
For E2 elimination reaction:
2. Why is phosphoric acid or sulfuric acid used in the E1 elimination reaction rather than HCl or HBr?
If H2SO4 or H3PO4, were used as acid in the reaction with alcohol then elimination reaction proceeds where as the use of HCl or HBr results in substitution reaction. This is because the HSO4– anion is a very poor nucleophile , being quite stabilized by resonance. The negative charge is delocalized over three different oxygens.
The similar is the case with phosphoric acid too
The treating of an alcohol with a strong hydrohalic acid – think HCl, HBr, or HI – resulted in the formation of alkyl halides. With a tertiary alcohol this proceeds through an SN1 mechanism. [Protonation of alcohol, then loss of H2O to form a carbocation, then attack of nucleophile on carbocation].
3. If the elimination reaction can result in the formation of two different alkenes which one with predominate in the product?
If an alcohol is treated with a strong acid (like sulfuric acid, H2SO4) and heat, you obtain one major product (an alkene) and a minor product (also an alkene).
The major product of an elimination reaction is the more substituted alkene. This is because the transition state leading to the more substituted alkene is lower in energy and therefore will proceed at a higher rate.
This is based on the zaitsev’s rule. It states that in a regioselective E1 or E2 reaction the major product is the more stable alkene, (i.e., the alkene with the more highly substituted double bond). For example: E1 reaction always follow Zaitsev's rule; with E2 reactions, there are exceptions