Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

The products formed when bromocylohexane and sodium propynide are heated together are (a) cyclohexene (b) 2 - cyclohexyl propyne (c) propyne (d) 1 - cyclohexyl propene

Step-by-Step Solution

Answer

The main product is cyclohexene.

Step 1: Understand the Reactants

Bromocyclohexane is an alkyl halide, specifically having a bromine atom attached to a cyclohexane ring. Sodium propynide is the sodium salt of propyne (HC≡CNa), which serves as a nucleophile in this reaction.

Step 2: Consider the Reaction Type

The reaction of an alkyl halide with a nucleophile like sodium propynide typically leads to a substitution reaction or an elimination reaction. In this context, sodium propynide acts as a strong base, promoting an elimination reaction (E2 reaction for secondary and tertiary alkyl halides).

Step 3: Identify the Elimination Product

In an E2 reaction, proton abstraction occurs by the base, resulting in the elimination of HBr from bromocyclohexane, forming a double bond. The major product is then cyclohexene, formed by the elimination of the bromine atom and a hydrogen atom from the cyclohexane ring.

Step 4: Analyze the Possibility of Substitution

Though substitution could occur, in the presence of a strong base like sodium propynide, elimination to form an alkene (cyclohexene) is favored over substitution reactions, such as forming 2-cyclohexyl propyne.

Step 5: Conclusion

After evaluating both possible pathways—substitution and elimination—the main product expected from heating bromocyclohexane with sodium propynide is cyclohexene due to the favored E2 elimination reaction.

Key Concepts

Understanding Alkyl Halides

Alkyl halides are a class of organic compounds that consist of an alkyl group attached to a halogen atom such as chlorine, bromine, or iodine. These compounds are significant in organic chemistry due to their reactivity and role in various chemical reactions. In the case of bromocyclohexane, the compound is an example of a secondary alkyl halide, where the bromine atom is attached to a cyclohexane ring. This attachment creates a site of potential reactivity because the carbon-bromine bond is polar, with the bromine being more electronegative.

• Alkyl group: A group of carbon and hydrogen atoms arranged in a chain.
• Halogen: An atom that can act as a substituent in organic chemistry, often possessing high electronegativity.

The reactivity of alkyl halides in reactions like nucleophilic substitution or elimination largely depends on the nature of the halogen and the structure of the alkyl chain. The bond between carbon and bromine in bromocyclohexane is susceptible to breaking, making it a good candidate for reactions where the halogen can be replaced or eliminated.

Nucleophilic Substitution and Reaction Pathways

In organic chemistry, nucleophilic substitution reactions involve the replacement of a leaving group, usually a halogen in alkyl halides, by a nucleophile. The term nucleophile refers to an atom or molecule that donates an electron pair to form a new chemical bond. Sodium propynide, which acts as a nucleophile, consists of a negatively charged carbon atom due to the presence of a lone pair of electrons.

• Nucleophile: A species that can donate an electron pair to form a covalent bond.
• Leaving group: A group that can readily depart with an electron pair during a reaction to form a stable product.

In the reaction between bromocyclohexane and sodium propynide, the potential for nucleophilic substitution exists but is often overshadowed by the tendency for an elimination pathway. This is because sodium propynide can also act as a strong base, favoring elimination over substitution, especially in secondary and tertiary alkyl halides. The distinction lies in the nature of the base and the structure of the alkyl halide, which influence whether substitution or elimination will dominate.

Cyclohexene Formation via E2 Elimination Reaction

Cyclohexene formation from bromocyclohexane and sodium propynide is a classic example of an E2 (bimolecular elimination) reaction. An E2 elimination reaction involves the removal of a proton and a leaving group simultaneously, leading to the formation of a double bond. In the case of bromocyclohexane, sodium propynide acts as a strong base, abstracting a hydrogen atom from the cyclohexane ring while displacing the bromine atom.

• E2 reaction: A concerted process where both the proton and leaving group are removed in one single step.
• Double bond formation: Results in the formation of cyclohexene, an unsaturated hydrocarbon.

This process creates cyclohexene, where the double bond indicates the formation of an alkene. The tendency for E2 elimination is enhanced by the presence of a strong base like sodium propynide, which ensures the reaction proceeds towards the formation of the alkene over potential substitution products. The E2 mechanism advantages the elimination path due to its high efficiency in forming stable carbon-carbon double bonds.