Question

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.

Short answer

Chlorination is an efficient preparative method for spiropentane because it selectively reacts with the strained carbon-carbon bonds in the three-membered rings, leading to greater stability in the resulting chlorospiropentane product. The reaction mechanism involves a radical reaction process with three steps: initiation (formation of chlorine radicals), propagation (formation of chlorospiropentane), and termination (combination of radicals to end the reaction).

Step-by-step solution

Part (a): Understanding the usefulness of chlorination

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.

Part (b): Writing the reaction mechanism

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.

Key concepts

Radical Reactions

In organic chemistry, radical reactions are pivotal in forming new bonds and creating complex molecules. These reactions involve species known as radicals, which are atoms or molecules containing an unpaired electron. Due to this unpaired electron, radicals are highly reactive. This makes them crucial players in many chemical reactions.

A typical radical reaction proceeds through a series of steps: initiation, propagation, and termination. During initiation, radicals are created, typically through the breaking of a bond, a process called homolytic cleavage. Propagation involves these radicals reacting with stable molecules, thus creating new radicals and perpetuating the reaction. Finally, termination happens when radicals combine to form stable products, effectively ending the reaction chain.

In the context of the chlorination of spiropentane, radicals play a critical role. The chlorine radicals generated initiate the reaction, converting the strained spiropentane into chlorospiropentane.

Chlorination

Chlorination is a widely used method in organic chemistry for introducing chlorine atoms into a compound. This process is particularly useful because it can increase the reactivity and alter the properties of the original molecule.

Chlorine's high reactivity allows it to interact with various organic substrates. This makes it invaluable in the synthesis of new organic molecules. Chlorination often utilizes light or heat to initiate reactions, as seen in the case of spiropentane. It's a highly selective process that can relieve strain in cyclic compounds like spiropentane's three-membered rings.

Through radical mechanisms, chlorination effectively transforms spiropentane by breaking specific carbon-hydrogen bonds and replacing them with carbon-chlorine bonds. This results in the formation of chlorospiropentane, a more stable compound, showcasing the utility of chlorination in organic synthesis.

Reaction Mechanism

A reaction mechanism provides a detailed step-by-step description of the transformation of reactants into products. In the chlorination reaction of spiropentane, understanding the mechanism is essential to comprehend how the reaction proceeds.

The mechanism begins with initiation, where the energy from light causes chlorine molecules to undergo homolytic cleavage, forming chlorine radicals. This initiation is crucial, as it sets the stage for the reaction.

During propagation, these radicals react with spiropentane, initially breaking a carbon-hydrogen bond to form a carbon-chlorine bond and a hydrogen radical. This newly formed radical continues to react with another chlorine molecule, creating chlorospiropentane and regenerating a chlorine radical, thereby sustaining the reaction cycle. Finally, termination occurs when two radicals combine and cease the radical chain reaction, forming stable molecules like HCl or fully substituted hydrocarbons.

Understanding each step helps to predict the outcome of reactions and adjust conditions for optimal product yield.

Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of hydrogen and carbon atoms. They form the foundation for numerous complex organic structures. There are several types of hydrocarbons based on structure: alkanes, alkenes, alkynes, and aromatic hydrocarbons.

Spiropentane is a unique hydrocarbon with a bicyclic structure. It consists of two three-membered carbon rings sharing a single carbon atom. This unusual architecture results in highly strained carbon bonds due to the deviation from ideal bond angles.

The high reactivity of hydrocarbons like spiropentane makes them prime candidates for chemical modifications, such as chlorination. Introducing a chlorine atom to a hydrocarbon can significantly alter its chemical properties, paving the way for the creation of various derivatives with enhanced stability or reactivity. Understanding the behavior of hydrocarbons under different conditions is crucial for manipulating and optimizing their chemical transformations.