The reaction of the unusual hydrocarbon spiropentane with chlorine and light is one of the best ways of preparing chlorospiropentane. (a) Explain why chlorination is such a useful preparative method in this case, (b) Write the reaction mechanism.

Spiropentane has a unique bicyclic structure, consisting of two three-membered rings sharing a single carbon atom. This causes a significant strain on the molecule since the carbon bond angles deviate from the ideal tetrahedral angle (109.5 degrees). The three-membered rings have angles closer to 60 degrees, which makes them much more reactive than other hydrocarbons. Chlorination of spiropentane is essential because it selectively reacts with the strained carbon-carbon bonds in the three-membered rings. During the reaction, the chlorine atoms can form new bonds with the carbon atoms, relieving some of the strain in the molecule. This leads to an increase in the stability of the product, chlorospiropentane, which makes it easier to isolate and work with. Therefore, chlorination serves as an efficient preparative method for converting spiropentane to chlorospiropentane with great selectivity and stability.

The reaction between spiropentane and chlorine in the presence of light is a radical reaction, where the radicals are generated by homolytic cleavage of the bonds. The reaction mechanism is as follows: Step 1: Initiation In this step, chlorine molecules absorb light and undergo homolytic cleavage to form two chlorine radicals: \[ Cl_2 + \text{light} \longrightarrow 2 \cdot Cl \] Step 2: Propagation Here, the chlorine radical reacts with the spiropentane molecule, breaking a carbon-hydrogen bond in the three-membered ring and forming a carbon-chlorine bond. This step generates a new spiropentane radical: \[ C_5H_8 + \cdot Cl \longrightarrow C_5H_7Cl + \cdot H \] Now, the spiropentane radical reacts with a chlorine molecule to form chlorospiropentane and release a new chlorine radical: \[ C_5H_7Cl + Cl_2 \longrightarrow C_5H_7Cl_2 + \cdot Cl \] Step 3: Termination The termination occurs when two radicals combine, ending the reaction: \[ \cdot H + \cdot Cl \longrightarrow HCl \] \[ C_5H_7Cl + \cdot Cl \longrightarrow C_5H_7Cl_2 \] These are the steps that constitute the reaction mechanism of spiropentane with chlorine and light to form chlorospiropentane.