Question

(a) When cyclopropane is treated with HI, 1-iodopropane is formed. A similar type of reaction does not occur with cyclopentane or cyclohexane. How do you account for the reactivity of cyclopropane? (b) Suggest a method of preparing ethylbenzene, starting with benzene and ethylene as the only organic reagents.

Short answer

(a) The reactivity of cyclopropane is due to the angle strain and weakened C-C bonds in its three-membered ring, which makes it more reactive towards reagents like HI. In contrast, cyclopentane and cyclohexane are less reactive as they have more stable ring structures with less angle strain. (b) Ethylbenzene can be prepared from benzene and ethylene by initially converting ethylene to ethyl chloride via chlorination with Cl2, and then performing a Friedel-Crafts alkylation with benzene in the presence of an AlCl3 catalyst.

Step-by-step solution

(a) Understand the given reaction

Cyclopropane reacts with HI to form 1-iodopropane. The reaction mechanism involves the breaking of the C-C bond in cyclopropane and the formation of a new C-I bond. However, this reaction does not occur in the case of cyclopentane or cyclohexane, which suggests that the chemical properties of cyclopropane are different.

(a) Analyze the structure of cyclopropane, cyclopentane, and cyclohexane

Cyclopropane has a three-membered ring, whereas cyclopentane and cyclohexane have five- and six-membered rings, respectively. The three-membered ring of cyclopropane is characterized by its small size and angle strain. This strain results in weakened C-C bonds and higher reactivity.

(a) Explain the reactivity of cyclopropane

The strain and weakened C-C bonds in cyclopropane make it more reactive towards reagents like HI, which can break the C-C bond and form a more stable product, 1-iodopropane. In contrast, cyclopentane and cyclohexane have more stable ring structures with less angle strain, making them less reactive towards similar reactions.

(b) Identify the reaction needed to form ethylbenzene

Preparing ethylbenzene from benzene and ethylene requires a reaction that adds an ethyl group (C2H5) to the benzene ring. This can be achieved through Friedel-Crafts alkylation.

(b) Friedel-Crafts alkylation reaction

Friedel-Crafts alkylation involves the electrophilic substitution of an alkyl group on an aromatic ring using an alkyl halide. In our case, we can use benzene and ethylene, but we need to transform the ethylene to ethyl chloride (C2H5Cl) first.

(b) Prepare ethyl chloride

To prepare ethyl chloride from ethylene, chlorination can be performed, which involves treating the ethylene with chlorine gas (Cl2) and an appropriate catalyst like FeCl3. The reaction proceeds as follows: \(C_2H_4 + Cl_2 \rightarrow C_2H_5Cl\)

(b) Perform Friedel-Crafts alkylation to form ethylbenzene

With ethyl chloride (C2H5Cl) prepared, the Friedel-Crafts alkylation can be carried out with benzene. The reaction requires aluminum chloride (AlCl3) as a catalyst. The reaction proceeds as follows: \(C_6H_6 + C_2H_5Cl + AlCl_3 \rightarrow C_6H_5C_2H_5 + HCl + AlCl_3\) Overall, ethylbenzene can be prepared from benzene and ethylene by first reacting ethylene with chlorine gas to form ethyl chloride and then reacting it with benzene in the presence of an AlCl3 catalyst.

Key concepts

Cyclopropane Reactivity

Cyclopropane, a simple cyclic alkane with a three-membered carbon ring, is notable for its unique chemical reactivity. Unlike its larger counterparts, cyclopentane and cyclohexane, cyclopropane's bonds are associated with significant angle strain due to the deviation from the ideal tetrahedral angle of 109.5 degrees. This strain leads to a higher potential energy state, making the C-C bonds of cyclopropane much more reactive.

The angle strain results from the three carbon atoms of cyclopropane being held together in a triangle, which causes the C-C-C bond angles to be 60 degrees, far less than the ideal bond angles for sp3 hybridized carbons. This strained ring system is less stable and more reactive, and the bonds can be more easily broken during chemical reactions, such as when cyclopropane reacts with hydrogen iodide (HI) to form 1-iodopropane. During this reaction, the C-C bond is cleaved and a new C-I bond is formed, which relieves some of the ring strain. The increased reactivity can lead to a variety of reactions that are not typically seen in larger ring compounds that are less strained.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a type of electrophilic aromatic substitution reaction where an alkyl group is introduced to an aromatic ring, such as benzene. This reaction is facilitated by the use of a strong Lewis acid catalyst, such as aluminum chloride (AlCl3).

The reaction mechanism involves the generation of a highly reactive carbocation intermediate from the alkyl halide, which is then attracted to the delocalized π electrons of the benzene ring. This interaction results in the formation of a new carbon-carbon bond, with the substitution taking place at one of the hydrogen atoms on the aromatic ring. However, as a potential drawback, Friedel-Crafts alkylation can lead to carbocation rearrangement, which might complicate the product distribution. To avoid such complications and to achieve specific product formation, careful consideration of reaction conditions and starting materials is necessary.

Chemical Reaction Mechanisms

The chemical reaction mechanism is a detailed step-by-step description of the process through which reactants are converted into products. It explains not only which bonds break and which bonds form, but also the order in which these events occur, including the formation and fate of any intermediate species. Understanding these mechanisms is essential for predicting the behavior of chemical reactions under various conditions and for the synthesis of complex molecules.

Each step within a mechanism can be categorized as uni-molecular or bi-molecular, depending upon whether it involves one or two reactive species, respectively. Rate-determining steps, transition states, intermediates, and catalyst involvement are also integral aspects of reaction mechanisms. Knowledge of these mechanisms allows chemists to control reactions to favor the formation of desired products while minimizing side products.

Synthesis of Ethylbenzene

Ethylbenzene is commonly synthesized through the alkylation of benzene with ethylene, often by employing the Friedel-Crafts reaction. The first step in synthesizing ethylbenzene involves the formation of ethyl chloride from ethylene by reaction with chlorine in the presence of a suitable catalyst. Ethyl chloride acts as an alkylating agent.

Once ethyl chloride is prepared, it is reacted with benzene using aluminum chloride (AlCl3) as a Lewis acid catalyst to facilitate the Friedel-Crafts alkylation. The AlCl3 catalyst aids in generating the highly reactive carbocation intermediate from ethyl chloride, which then attacks the benzene ring to form ethylbenzene. The simplicity of using benzene and ethylene as starting materials for the production of ethylbenzene demonstrates the principle of using less complex molecules to build more complex structures through the understanding of reaction mechanisms including this electrophilic aromatic substitution.