Question

(a) One test for the presence of an alkene is to add a small amount of bromine, which is a red-brown liquid, and look for the disappearance of the red-brown color. This test does not work for detecting the presence of an aromatic hydrocarbon. Explain. (b) Write a series of reactions leading to para-bromoethylbenzene, beginning with benzene and using other reagents as needed. What isomeric side products might also be formed?

Short answer

The bromine test for alkenes doesn't work for aromatic hydrocarbons because it would disrupt their aromaticity, which is thermodynamically unfavorable. Synthesis of para-bromoethylbenzene can be achieved through Friedel-Crafts alkylation, nitration, reduction, diazotization, and Sandmeyer reaction, starting with benzene. Isomeric side products may form due to reactions at ortho and meta positions.

Step-by-step solution

Part (a) Explanation

Bromine reacts with alkenes via an electrophilic addition reaction, which quickly forms a dibromoalkane. The disappearance of the red-brown color is due to the formation of a colorless dibromoalkane product. However, in the case of aromatic hydrocarbons, such as benzene, the bromine cannot undergo electrophilic addition because it would disrupt the aromaticity of the compound. The loss of aromaticity is thermodynamically unfavorable due to the high resonance energy of the aromatic compound. Thus, the bromine test does not work for aromatic hydrocarbons because they do not readily undergo electrophilic addition reactions with bromine.

Part (b) Synthesis of para-bromoethylbenzene

We will synthesize para-bromoethylbenzene starting from benzene in a series of reactions and using other reagents as needed. Step 1: Friedel-Crafts alkylation React benzene with ethyl chloride (CH2CH3Cl) in the presence of aluminum chloride (AlCl3) catalyst to produce ethylbenzene, where the ethyl group is bonded directly to the benzene ring: \(C_6H_6 + CH_2CH_3Cl \xrightarrow{AlCl_3} C_6H_5CH_2CH_3 + HCl\) Step 2: Nitration React ethylbenzene with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) to introduce the nitro group (NO2) at the para position of ethylbenzene: \(C_6H_5CH_2CH_3 + HNO_3 \xrightarrow{H_2SO_4} p-C_6H_4(NO_2)CH_2CH_3 + H_2O\) Step 3: Reduction Reduce the nitro group in para-nitroethylbenzene to an amine group (NH2) using a reducing agent such as iron (Fe) and hydrochloric acid (HCl): \(p-C_6H_4(NO_2)CH_2CH_3 + 3Fe + 6HCl \rightarrow p-C_6H_4(NH_2)CH_2CH_3 + 3FeCl_2 + 2H_2O\) Step 4: Diazotization React the amine group with sodium nitrite (NaNO2) and hydrochloric acid (HCl) to form a diazonium salt : \(p-C_6H_4(NH_2)CH_2CH_3 + NaNO_2 + HCl \rightarrow p-C_6H_4(N_2^+Cl^-)CH_2CH_3 + NaCl + 2H_2O\) Step 5: Sandmeyer reaction Replace the diazonium group with a bromine atom using copper bromide (CuBr) catalyst: \(p-C_6H_4(N_2^+Cl^-)CH_2CH_3 + CuBr \rightarrow p-C_6H_4(Br)CH_2CH_3 + CuCl + N_2\) The final product is para-bromoethylbenzene. During some of the reaction steps (e.g., nitration and hydrocarbon alkylation), the substrates have the potential to react at the ortho and meta positions as well as at the para position, leading to the formation of corresponding isomeric side products.

Key concepts

Electrophilic Addition Reaction

Electrophilic addition reactions are a key type of reaction in organic chemistry, especially when dealing with alkenes. In this process, a molecule with a high electron density, like an alkene double bond, reacts with an electrophile. An electrophile is a species attracted to electrons due to its electron-deficient nature.

In the presence of bromine, alkenes undergo an electrophilic addition reaction. The pi bonds in the alkene are electron-rich, which means they can easily interact with the electron-poor bromine, leading to the formation of a dibromoalkane. This reaction results in the disappearance of the red-brown color of bromine, a key indicator of this reaction occurring.

However, aromatic hydrocarbons like benzene do not undergo electrophilic addition with bromine in the same manner. This is because any addition would disturb the stable aromaticity of benzene, making the reaction thermodynamically unfavorable due to the loss of resonance energy. Instead, aromatic hydrocarbons are more likely to undergo substitution reactions, where the aromatic character is preserved.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a classic method to introduce alkyl groups into an aromatic compound. The process requires an aryl halide and a strong Lewis acid catalyst, commonly aluminum chloride (AlCl₃). This catalyst helps in the formation of a highly reactive carbocation intermediate that facilitates the alkylation.

During the reaction, an alkyl group is transferred to the benzene ring, yielding a substituted aromatic compound. In the example of reacting benzene with ethyl chloride, the ethyl group attaches directly to the benzene, creating ethylbenzene. It is important to note that the catalyst is not consumed in the reaction and can be reused.

Friedel-Crafts alkylation is selective, but it can also lead to the formation of isomeric side products. These are caused by rearrangements of the carbocation intermediate or by subsequent reactions at different positions on the aromatic ring.

Diazonium Salt

Diazonium salts are an important class of compounds in organic chemistry utilized for various substitution reactions. They are typically formed when an aromatic amine, like aniline, reacts with sodium nitrite (NaNO₂) and hydrochloric acid under cold conditions. This process is known as diazotization.

The diazonium salt is extremely versatile and reactive due to its ability to act as an intermediate in further reactions. It possesses a diazonium group (N₂⁺), which makes it prone to substitution since N₂ is a very stable leaving group. This reaction is useful because it allows for the introduction of various functional groups into an aromatic ring.

While diazonium salts open avenues to numerous synthetic transformations, they must be handled with care due to their instability, especially under heat. These salts are typically used immediately after their preparation to maximize efficiency and safety.

Sandmeyer Reaction

The Sandmeyer reaction is a powerful tool in organic synthesis for the introduction of halogen atoms into aromatic rings via diazonium salts. This transformation is characterized by the replacement of the diazonium group with a halogen like bromine, chlorine, or iodine.

In a typical Sandmeyer reaction, a diazonium salt reacts with a copper(I) halide, such as copper(I) bromide (CuBr), under mild conditions. The copper salt aids in the conversion by facilitating the exchange of the diazonium group with a bromine atom, forming the aryl bromide desired product.

This reaction is highly valuable in organic synthesis because it allows for the selective and efficient introduction of various functional groups that are otherwise difficult to add directly. Its applications extend to pharmaceuticals and dye manufacturing, showcasing its versatility in creating complex molecules.