Question

Identify total number of '$\beta$' elimination reaction: (i) (ii) (iii) (iv) (v)

Answer 1

4

Answer 2

A $\beta$-elimination reaction (also known as 1,2-elimination) involves the removal of two groups from adjacent carbon atoms, leading to the formation of a double bond. Typically, one group is a leaving group (e.g., halide) from the $\alpha$-carbon and the other is a hydrogen from the $\beta$-carbon.

Let's analyze each reaction:

(i) Reaction (i): The substrate is 2-chloro-2-methylpropane (tert-butyl chloride). The reagent is alcoholic KOH and heat. This is a classic dehydrohalogenation reaction. A hydrogen atom from a $\beta$-carbon and the chlorine atom from the $\alpha$-carbon are removed to form 2-methylpropene. This is a $\beta$-elimination reaction (E2 mechanism).

(ii) Reaction (ii): The substrate is 2,3-dichlorobutane (a vicinal dihalide). The reagent is zinc dust and heat. This is a dehalogenation reaction where two halogen atoms are removed from adjacent carbons, forming a double bond (2-butene). This is a type of 1,2-elimination, hence a $\beta$-elimination reaction.

(iii) Reaction (iii): The substrate is 1-bromo-1,2-dimethylcyclohexane. The bromine at C1 is in an axial position. The $\beta$-carbon C6 has an axial hydrogen which is anti-periplanar to the axial bromine at C1. The reagent, sodium methoxide ($\text{CH}_3\text{O}^-\text{Na}^+$), is a strong base that promotes E2 elimination. Elimination of the axial hydrogen from C6 and the axial bromine from C1 will occur, forming 1,2-dimethylcyclohexene. This is a $\beta$-elimination reaction.

(iv) Reaction (iv): The substrate is 1-bromo-1,2-dimethylcyclohexane. The bromine at C1 is in an equatorial position. For an E2 elimination to occur, the $\beta$-hydrogen must be anti-periplanar to the leaving group.

• At C2, the hydrogen is axial, which is not anti-periplanar to the equatorial bromine.
• At C6, neither the axial nor the equatorial hydrogen is anti-periplanar to the equatorial bromine.

Since the required anti-periplanar geometry cannot be achieved in this conformation, E2 elimination is not possible. Therefore, this is not a $\beta$-elimination reaction under typical E2 conditions.

(v) Reaction (v): The substrate is 1-fluoro-2-methylpropane (isobutyl fluoride). The reagent is hydroxide ion ($\text{OH}^-$) and heat. Although fluorine is a poor leaving group, with a strong base and high temperature, dehydrohalogenation (a $\beta$-elimination) can occur, forming 2-methylpropene. This is a $\beta$-elimination reaction.

Conclusion: Reactions (i), (ii), (iii), and (v) are $\beta$-elimination reactions. Reaction (iv) is not. Total number of $\beta$-elimination reactions = 4.