Question

Write the structures of the products that form when ethylene reacts with each of the following substances by an addition reaction. (Assume that needed catalysts or other conditions are provided.) (a) \(\mathrm{H}_{2}\) (c) \(\mathrm{Br}_{2}\) (e) \(\operatorname{HBr}(g)\) (b) \(\mathrm{Cl}_{2}\) (d) \(\mathrm{HCl}(g)\) (f) \(\mathrm{H}_{2} \mathrm{O}\) (in acid)

Short answer

The products are: (a) Ethane ((C_2H_6)), (b) 1,2-dichloroethane ((C_2H_4Cl_2)), (c) 1,2-dibromoethane ((C_2H_4Br_2)), (d) Chloroethane ((C_2H_5Cl)), (e) Bromoethane ((C_2H_5Br)), (f) Ethanol ((C_2H_5OH)).

Step-by-step solution

Identify the Type of Reaction

The reactions described are all addition reactions. In an addition reaction, atoms or groups of atoms are added to a double bond in an alkene, such as ethylene ((C_2H_4)).

Reaction with (H_2)

When ethylene reacts with hydrogen ((H_2)) in the presence of a catalyst (typically palladium, platinum, or nickel), an alkane is formed by adding a hydrogen to each carbon atom of the double bond. This is called hydrogenation. The structure will be ethane ((C_2H_6)).

Reaction with (Br_2) and (Cl_2)

When ethylene reacts with bromine ((Br_2)) or chlorine ((Cl_2)), a dihalogenated alkane is formed. Each halogen atom adds across the double bond. With (Br_2), the product is 1,2-dibromoethane ((C_2H_4Br_2)), and with (Cl_2), the product is 1,2-dichloroethane ((C_2H_4Cl_2)).

Reaction with (HBr) and (HCl)

Ethylene reacts with hydrogen halides like hydrogen bromide ((HBr)) and hydrogen chloride ((HCl)) to form haloalkanes. The hydrogen atom adds to one carbon, and the halogen atom adds to the other. The products are bromoethane ((C_2H_5Br)) for the reaction with (HBr) and chloroethane ((C_2H_5Cl)) for the reaction with (HCl).

Reaction with Water ((H_2O))

The reaction of ethylene with water, in the presence of an acid catalyst, leads to the formation of ethanol. This is a hydration reaction where (H_2O) adds across the double bond. The product is ethanol ((C_2H_5OH)).

Key concepts

Hydrogenation of Alkenes

Hydrogenation is a type of chemical reaction in which hydrogen molecules are added to the double bond of an alkene, converting it into an alkane. In the context of the presented exercise, ethylene \(C_2H_4\) reacts with hydrogen gas \(H_2\) to form ethane \(C_2H_6\), which is an alkane. This transformation is facilitated by the use of a metal catalyst such as palladium, platinum, or nickel.

The hydrogenation process is not only pivotal in the field of organic chemistry for creating saturated compounds, but it also plays a significant role in the food industry, for instance, in converting vegetable oils to margarine. Being aware of the catalysts' function and the reaction conditions allows for a controlled alteration of organic molecules, a vital skill for synthesizing a broad range of chemical products.

Halogenation of Alkenes

Halogenation denotes the addition of halogens, such as bromine \(Br_2\) and chlorine \(Cl_2\), across the double bond of alkenes to yield dihalogenated alkanes. According to our exercise, ethylene \(C_2H_4\) reacts with \(Br_2\) to form 1,2-dibromoethane \(C_2H_4Br_2\) and with \(Cl_2\) to produce 1,2-dichloroethane \(C_2H_4Cl_2\).

The reactions are characteristic tests for unsaturation in hydrocarbons because they proceed with the decolorization of the halogen reagents. In practical applications, halogenation is essential for the creation of many industrial chemicals and pharmaceuticals, and understanding the mechanism is crucial for predicting the outcome in complex organic syntheses.

Hydrohalogenation of Alkenes

In the context of our exercise, hydrohalogenation is the process where hydrogen halides such as hydrogen bromide \(HBr\) and hydrogen chloride \(HCl\) add to the double bond of an alkene, resulting in a haloalkane. Specifically, ethylene \(C_2H_4\) reacts with \(HBr\) to give bromoethane \(C_2H_5Br\) and with \(HCl\) to yield chloroethane \(C_2H_5Cl\).

Hydrohalogenation follows the Markovnikov rule, which states that the hydrogen atom from the hydrogen halide will add to the carbon of the double bond with the greatest number of hydrogen atoms. This rule helps students predict the major product of hydrohalogenation reactions. These reactions are widely used in the chemical industry for the synthesis of alcohols and other intermediates.

Hydration of Alkenes

Finally, in a hydration reaction, water \(H_2O\) adds to an alkene to form an alcohol. For our ethylene example, when reacted with water in acidic conditions, ethanol \(C_2H_5OH\) is produced. This reaction also follows Markovnikov's rule where the hydrogen from the water molecule attaches to the carbon with more hydrogen atoms, and the hydroxyl group \(OH\) attaches to the other carbon.

Understanding hydration is crucial for students as it is a fundamental method for producing alcohols, which are valuable in both the pharmaceutical and manufacturing industries. Students must recognize that the acid catalyst is required to protonate the alkene, making it more susceptible to the nucleophilic attack by water. Knowledge of this mechanism assists in grasping more complex organic reactions and processes.