Question

Elimination of bromine from 2 -bromobutane results in the formation of (1) equi-molar mixture of 1 -and 2 -butene (2) predominantly-2-butene (3) predominantly-I-butene (4) predominantly-2-butyne

Short answer

Predominantly 2-butene (Option 2)

Step-by-step solution

Identify the Type of Reaction

Determine the chemical reaction involved in the elimination process. This involves understanding the elimination (E2) reaction mechanism, where a halide is removed from an alkane to form an alkene.

Understand the Structure

Draw the structure of 2-bromobutane. It has a butane chain (4 carbon atoms) with a bromine atom attached to the second carbon.

Analyze Possible Products

Identify the possible products from the elimination reaction. Removing the bromine and a hydrogen atom from the adjacent carbon atoms (following the Zaitsev's rule) can form 1-butene and 2-butene.

Apply Zaitsev's Rule

According to Zaitsev's rule, the more substituted alkene (the one with more alkyl groups attached to the carbon-carbon double bond) is favored. Hence, 2-butene will be the major product.

Conclusion

Based on the analysis and Zaitsev's rule, the elimination of bromine from 2-bromobutane predominantly results in the formation of 2-butene.

Key concepts

E2 Reaction Mechanism

An E2 (bimolecular elimination) reaction is a single-step reaction where a hydrogen atom and a leaving group (like a halide) are removed from adjacent carbon atoms. In this reaction, both the elimination of the leaving group and the deprotonation by a base occur simultaneously. The transition state involves the breaking of the C-H and C-Halide bonds and the formation of the alkene double bond.
The E2 reaction typically requires a strong base to abstract the proton, and it's highly dependent on the steric hindrance of the substrate. Prioritize the formation of the most stable alkenes.

Zaitsev's Rule

Zaitsev's rule is a guideline used to predict the outcomes of elimination reactions. It states that the most substituted (and thus more stable) alkene product is favored. In practical terms, the rule implies that the hydrogen is removed from the β-carbon (the carbon atom next to the carbon with the leaving group) that has the fewest hydrogen atoms attached.
This tendency ensures that the resulting alkene (double bond) is the most stable one. Multiple factors influence this stability, including hyperconjugation and the inductive effect from alkyl groups attached to the double-bonded carbons.

2-Bromobutane

2-bromobutane is an organic compound with the formula C₄H₉Br. In its structure, a bromine atom is attached to the second carbon of a butane chain. This positioning makes it an ideal candidate for E2 elimination reactions.
When you look at 2-bromobutane, you will see four carbon atoms in a chain with a bromine atom attached to the second carbon. This setup offers two β-hydrogens to choose from for the elimination reaction, leading to the creation of either 1-butene or 2-butene.

Alkene Formation

During an elimination reaction, the removal of a halide and a hydrogen from a molecule results in the formation of an alkene. Alkenes are hydrocarbons containing at least one carbon-carbon double bond. The stability of the produced alkene depends significantly on Zaitsev's rule, which favors the formation of the most substituted double bond.
When 2-bromobutane undergoes an E2 reaction, it mainly forms 2-butene due to its higher substitution level compared to 1-butene. The reaction prefers to form the more stable 2-butene, which is predicted and explained by Zaitsev's rule.