This discussion focuses on the synthetic process of converting a double bond into a triple bond, which is a crucial transformation in organic chemistry. The initial step involves halogenation, where a double bond reacts with diatomic iodine (I₂). This reaction results in the formation of vicinal dihalides, where two halogen atoms are added across the double bond. The mechanism involves the formation of a cyclic halonium ion intermediate, followed by nucleophilic attack by iodide ions, leading to the final vicinal dihalide product.

To convert the vicinal dihalide into a triple bond, a double elimination reaction is performed using excess base, typically represented as H^-. This process requires two equivalents of base to eliminate the two halogen atoms, resulting in the formation of a triple bond. A third equivalent of base is then used to deprotonate the terminal hydrogen, yielding an alkynide ion. The term "excess" indicates that sufficient base should be used to ensure complete reaction, which in this case means three equivalents in total.

Next, the alkynide ion can react with a primary alkyl halide, such as phenyl CH₂I, through an SN2 reaction. This step is facilitated by the strong nucleophilicity of the alkynide, allowing it to effectively displace the leaving group and form a new carbon-carbon bond. The product of this reaction is a terminal alkyne with a phenyl group attached.

The final step involves catalytic hydrogenation, where the alkyne is treated with hydrogen gas (H₂) in the presence of a nickel catalyst. This reaction reduces the triple bond to a single bond, resulting in a saturated hydrocarbon. The final product is a five-carbon chain attached to a benzene ring, demonstrating the successful transformation through this synthetic pathway.

Understanding this sequence of reactions is essential, as it highlights the importance of halogenation and elimination reactions in organic synthesis. Mastery of these concepts will be beneficial for exams, particularly in recognizing how to manipulate functional groups to achieve desired molecular structures.