Question

An alkene (A) with molecular formula \(\mathrm{C}_{7} \mathrm{H}_{12}\) adds HBr to give a single alkyl halide (B) \(\left(\mathrm{C}, \mathrm{H}_{13} \mathrm{Br}\right) . \mathrm{A}\) on catalytic hydrogenation gives 1,1 -dimethyl cyclopentane. The alkene (A) is (A) (B) (C) (D)

Short answer

The structure of alkene A is: \( \\ \text{ Br } - \text{ H } \\ \, \, | \, \, \, \, \, \, \, \textbackslash \, \\ \text{CH}_3 - \text{CH} = \text{CH} - \text{CH}_2 - \text{CH} = \text{CH} - \text{CH}_3 \\ \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \textbackslash \\ \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, \, CH_3 \\ \)

Step-by-step solution

The addition of HBr to alkene A

First, we need to understand the addition of HBr to alkene A, which occurs according to Markovnikov's rule. Essentially, it states that when HX (in our case, HBr) is added to an alkene, the hydrogen (H) will attach to the carbon with fewer alkyl substituents, and the halogen (Br) will be added to the more substituted carbon. The fact that there is only one alkyl halide product (B) indicates that alkene A has only one reactive site.

Analyze the hydrogenation product

After hydrogenation, alkene A forms 1,1-dimethylcyclopentane. Hydrogenation saturates the alkene, creating a structure with no double bonds. Since the product is a 1,1-dimethylcyclopentane, the original alkene must contain a 5-membered ring (cyclopentene) with methyl groups at position 1. So the alkene should have a molecular formula of C5H8 (cyclopentene) plus two additional carbons.

Determine alkene A's structure

Considering the information obtained from Steps 1 and 2, the structure of the alkene A should be a 1-methylcyclopentene with a substituent methyl group at the more substituted carbon site to comply with Markovnikov's rule. The structure of alkene A is: (A) Br - H | \ CH3 - CH = CH - CH2 - CH = CH - CH3 \ CH3 We can verify that this structure has a molecular formula of C7H12, and when HBr gets added, it forms an alkyl halide with a molecular formula of C7H13Br. Finally, upon hydrogenation, it forms 1,1-dimethyl cyclopentane which confirms our proposed structure for alkene A.

Key concepts

Cycloalkene Structure

Cycloalkene structures are characterized by the presence of a carbon-carbon double bond nestled within a carbon ring. Understanding these structures helps us predict how they react with other chemicals. For example, in a 5-membered ring like cyclopentene, the double bond forms part of the ring, facilitating unique chemical properties.

• A cycloalkene typically has fewer hydrogens than a comparable linear alkene due to the ring structure.
• The spatial arrangement of atoms can influence the reactivity and stability of the compound.

In our exercise, the structure for compound A must include a cyclopentene ring with an additional methyl group, which affects the location of the carbon-carbon double bond. The outcome of any reactions, such as with HBr or hydrogenation, depends on the positioning and number of these bonds.

Molecular Formula

A molecular formula, such as \(\mathrm{C}_{7}\mathrm{H}_{12}\), tells us not just about the number of carbon and hydrogen atoms, but their potential arrangement. For cycloalkene compounds, missing hydrogens due to rings and double bonds lead to a lower hydrogen count compared to linear alkanes.

• The given exercise hints that compound A has seven carbons and 12 hydrogens, indicating one or more rings and double bonds.
• Adding HBr changes the hydrogen count by creating a saturated alkyl halide with a formula of \(\mathrm{C}_{7} \mathrm{H}_{13} \mathrm{Br}\).

This formula transformation helps us pinpoint structural changes and confirms connections with the reaction products like the hydrogenated form. The analysis of molecular formulas aids in reconstructing the possible structure of an alkene, especially in test problems.

Catalytic Hydrogenation

Catalytic hydrogenation involves the addition of hydrogen (H₂) across double bonds in the presence of a catalyst, typically a metal like palladium or platinum. This process transforms unsaturated compounds into saturated ones by removing double bonds.

• Hydrogenation of an alkene like 'A' results in the complete saturation of the double bonds.
• The resulting compound, such as 1,1-dimethylcyclopentane, reflects the original ring’s size and placement of substituents.

This reaction was pivotal in the exercise, as it confirmed the presence of a cyclopentene ring with a particular substitution. In essence, the transformation achieved through catalytic hydrogenation indicates not only the structure but also offers insights into chemical stability and transformation pathways of cycloalkenes.