Question

Predict the alkene formed by dehydrohalogenation of 1-bromo-1-cyclohexylethane.

Hint:

Elimination reactions usually follow Saytzeff rule → more substituted alkene major.

Answer 1

Dehydrohalogenation is a reaction in which a hydrogen atom and a halogen atom (such as bromine or chlorine) are eliminated from a molecule, forming an alkene. According to Saytzeff’s rule, in this elimination reaction, the more substituted alkene is generally the preferred product. This rule states that when there is more than one possible alkene product, the one with the greater number of alkyl groups attached to the double bond will form predominantly due to its increased stability.
Step 1: Understanding Saytzeff’s rule.
Saytzeff’s rule predicts that the most stable, and hence preferred, product of dehydrohalogenation will be the alkene where the double bond is attached to the carbon with the greatest number of alkyl groups. This is because the more substituted alkenes are more stable due to hyperconjugation and inductive effects.
Step 2: Elimination of HBr.
When HBr is eliminated from the molecule, the hydrogen atom and the bromine atom are removed from adjacent carbon atoms. In this case, there is more than one possible site for the elimination, but according to Saytzeff’s rule, the elimination will favor the formation of the more substituted alkene.
Step 3: Product formed.
For this specific example, the product formed is: \[ \text{1-ethylcyclohexene (more substituted alkene)} \] This product is the more stable alkene because it has a higher number of alkyl groups attached to the double bond.
Final Answer:
The formation of 1-ethylcyclohexene is the result of dehydrohalogenation following Saytzeff’s rule, where the more substituted alkene is favored due to its greater stability.