Question

Elimination of bromine from 2 -bromobutane results in the formation of (a) equimolar mixture of 1 and 2 -butene (b) predominantly 2 -butene (c) predominantly 1-butene (d) predominantly 2-butyne

Short answer

The elimination of bromine from 2-bromobutane predominantly results in 2-butene.

Step-by-step solution

Understanding the Reaction: Dehydrohalogenation

The elimination of bromine from 2-bromobutane involves the removal of hydrogen bromide (HBr) to form a double bond. This is a dehydrohalogenation reaction, often facilitated by a strong base such as KOH or NaOH, with a preference for forming alkenes.

Applying Zaitsev's Rule

Zaitsev's rule states that the more substituted alkene will be the major product in an elimination reaction. In 2-bromobutane, elimination leads to the formation of both 1-butene and 2-butene as potential products. However, 2-butene is more substituted compared to 1-butene.

Predicting the Major Product

Based on Zaitsev's rule, the elimination reaction will predominantly produce the more substituted alkene. In this case, that is 2-butene because the double bond is located between carbons 2 and 3, where it has more alkyl groups attached, making it more stable.

Conclusion on Product Formation

The elimination reaction of bromine from 2-bromobutane will predominantly yield 2-butene, following the principle that more substituted, stable alkenes are favored during such reactions.

Key concepts

Dehydrohalogenation

Dehydrohalogenation is an elimination reaction where a hydrogen halide, such as hydrochloric acid (HCl) or hydrobromic acid (HBr), is removed from an alkyl halide to form an alkene. In this process, we typically require a strong base like potassium hydroxide (KOH) or sodium hydroxide (NaOH) to induce the reaction. The base abstracts a hydrogen atom attached to a carbon adjacent to the halogen-bearing carbon. This leads to the formation of a double bond between the two carbon atoms that lose their respective hydrogen and halogen. Consequently, an alkene is produced, and a halide ion and water are typically byproducts.

This reaction is particularly important in organic synthesis because it serves as one of the fundamental methods to produce alkenes from simpler precursor molecules. The choice of the base and the reaction conditions, such as temperature and solvent, can influence the reaction pathway and the resulting alkene product.

Zaitsev's Rule

Zaitsev's Rule is a guiding principle in organic chemistry that helps us predict the major alkene product in an elimination reaction. According to this rule, when faced with multiple possibilities for product formation, the more substituted alkene is preferentially formed. This preference results from the greater stability conferred by having more alkyl groups attached to the carbons of the double bond.

In the context of organic reactions, a more substituted alkene means that the sp2 hybridized carbons of the double bond are bonded to a greater number of carbon atoms instead of hydrogen atoms. This enhanced substitution generally stabilizes the alkene due to hyperconjugation and disperses any potential electron density concentrations, making the molecule more stable energetically.

In the specific example of 2-bromobutane undergoing dehydrohalogenation, Zaitsev's Rule helps us understand why 2-butene, which has more alkyl groups surrounding its double bond than 1-butene, is the predominant product.

Alkenes Formation

In the world of chemistry, forming alkenes through elimination reactions is an intriguing process. During an elimination reaction, a small molecule like water or a hydrogen halide is removed from the reactant, leading to the formation of an alkene. Alkenes are hydrocarbons that contain carbon-carbon double bonds in their structure, and these double bonds define their unique reactivity.

The formation of alkenes is accompanied by a few strategic steps depending on the starting material and conditions. For instance, in the just-discussed dehydrohalogenation of 2-bromobutane, the elimination of HBr results in a double bond forming between carbon atoms. The presence of a base is crucial here, as it facilitates the removal of a proton, thereby promoting the formation of the double bond.

Moreover, the importance of alkenes in synthesis cannot be overstated; they lead to various possibilities in synthetic pathways, such as polymerization and the creation of myriad other chemical compounds.