Question

For the following dehydrohalogenation (E2) reactions, draw all Zaitsev products, showing the stereochemistry clearly. Show the product stereochemistry clearly. If there is more than one organic product, both products may be drawn in the same box.

Answer 1

Concepts and reason

Organic reactions are classified into four types; addition, substitution, elimination and rearrangement reactions.. Based on the structure of reactant, the mechanism of elimination reaction goes through ${{\rm{E}}_{\rm{1}}}$ or ${{\rm{E}}_{\rm{2}}}$ pathway.

${{\rm{E}}_{\rm{2}}}$ Elimination reaction is bi-molecular reaction. In ${{\rm{E}}_{\rm{2}}}$ elimination reaction, the product formed in a single-step via transition state. Rate of the reaction depends upon the concentration of both base and the substrate.

Examples of elimination reactions are:

i)Dehydration

ii)Dehydrohalogenation

iii)Dehydrogenation

Fundamentals

In an ${{\rm{E}}_{\rm{2}}}$ elimination reaction, the structure of alkene product formed, depends on the nature of base (bulkier or lighter) used in the reaction to attract the acidic proton in the substrate. If bulky base is used, Hoffman product is formed. If the base is not sterically hindered Zaitsev’s product is formed.

• Hoffman product: Due to steric hindrance of the bulky base, least substituted alkene is preferred.

• Zaitsev’s product: Most substituted double bond in alkene is more stable, if the used base is not steric hindered.

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