Question

Carbon compounds containing double bonds (such compounds are called alkenes) react readily with many other reagents. In each of the following reactions, identify which atoms are oxidized and which are reduced, and specify the oxidizing and reducing agents. a. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{Cl}_{2}(g) \rightarrow \mathrm{ClCH}_{2}-\mathrm{CH}_{2} \mathrm{Cl}(l)\) b. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{Br}_{2}(g) \rightarrow \mathrm{BrCH}_{2}-\mathrm{CH}_{2} \operatorname{Br}(l)\) c. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{HBr}(g) \rightarrow \mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{Br}(l)\) d. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{H}_{2}(g) \rightarrow \mathrm{CH}_{3}-\mathrm{CH}_{3}(g)\)

Short answer

In summary: a. Oxidized atom: C in CH2=CH2; Reduced atom: Cl in Cl2; Oxidizing agent: Cl2; Reducing agent: CH2=CH2 b. Oxidized atom: C in CH2=CH2; Reduced atom: Br in Br2; Oxidizing agent: Br2; Reducing agent: CH2=CH2 c. No oxidation or reduction occurs in this reaction. d. No oxidation or reduction occurs in this reaction.

Step-by-step solution

Reaction a analysis

For the given reaction: \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{Cl}_{2}(g) \rightarrow \mathrm{ClCH}_{2}-\mathrm{CH}_{2} \mathrm{Cl}(l)\) We first need to determine the oxidation states of each atom in the reactants and the products. Oxidation states: C: -2 (in both reactants and products) H: +1 (in both reactants and products) Cl: 0 (in reactants) and -1 (in products) Since Cl goes from an oxidation state of 0 to -1, it is reduced. Therefore, Cl is the oxidizing agent. The atom being oxidized is C (in the alkene) because it loses one bond; thus, CH2=CH2 is the reducing agent.

Reaction b analysis

For the given reaction: \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{Br}_{2}(g) \rightarrow \mathrm{BrCH}_{2}-\mathrm{CH}_{2} \operatorname{Br}(l)\) Just like before, we determine the oxidation states of each atom in the reactants and products. Oxidation states: C: -2 (in both reactants and products) H: +1 (in both reactants and products) Br: 0 (in reactants) and -1 (in products) Similar to Reaction a, Br goes from an oxidation state of 0 to -1, indicating reduction. Thus, Br is the oxidizing agent. The atom being oxidized is C (in the alkene) because it loses one bond; therefore, CH2=CH2 is the reducing agent.

Reaction c analysis

For the given reaction: \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{HBr}(g) \rightarrow \mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{Br}(l)\) We determine the oxidation states of each atom in the reactants and products. Oxidation states: C: -2 (in both reactants and products) H: +1 (in both reactants and products) Br: -1 (in both reactants and products) In this reaction, no changes in oxidation states occur. Therefore, there are no oxidizing or reducing agents in this reaction.

Reaction d analysis

For the given reaction: \(\mathrm{CH}_{2}=\mathrm{CH}_{2}(g)+\mathrm{H}_{2}(g) \rightarrow \mathrm{CH}_{3}-\mathrm{CH}_{3}(g)\) We determine the oxidation states of each atom in the reactants and products. Oxidation states: C: -2 (in both reactants and products) H: +1 (in both reactants and products) In this reaction, no changes in oxidation states occur. Therefore, there are no oxidizing or reducing agents in this reaction.

Key concepts

Oxidation States

In chemical reactions, oxidation states, or oxidation numbers, help in understanding how electrons are exchanged. An oxidation state is a theoretical charge that an atom would have in a molecule if all bonds were 100% ionic.

As we look into alkene reactions, it is important to calculate the oxidation states of each atom in the reactants and products. For instance, in reactions involving double-bonded carbons (alkenes) like ethene every carbon atom typically has an oxidation state of -2, and hydrogen is generally +1.

In reactions a and b, halogens such as chlorine (Cl) and bromine (Br) start from an oxidation state of 0 in their diatomic forms. They shift to an oxidation state of -1 in the products. This change signifies reduction. Whereas, the carbon in the alkene maintains its oxidation state at -2 throughout the reactions, which impacts the identification of oxidizing and reducing agents.

By tracking changes in oxidation states, we effectively decipher which atoms are gaining or losing electrons, paving the way for recognizing oxidizing and reducing agents.

Oxidizing Agents

Oxidizing agents are substances that tend to gain electrons in a chemical reaction. They cause other substances to lose electrons when undergoing a transformation.

In the reactions examined here, oxidizing agents are the elements whose oxidation state decreases, meaning they are reduced. For example, in reactions a ( Cl ) and b ( Br ), the halogens go from an oxidation state of 0 in their diatomic form to -1 when they become single atoms bonded to the carbon chain.

This decrease indicates that they are the oxidizing agents. They accept electrons from the other species, facilitating the oxidation of another substance. Understanding the role of oxidizing agents is crucial for predicting and analyzing potential reaction outcomes.

Reducing Agents

Reducing agents are substances that donate electrons to another substance in a chemical reaction. This donation leads to a decrease in oxidation state for the receiving element and the oxidation of the reducing agent itself.

In the case of reactions a and b, the alkenes ( CH_2=CH_2 ) act as reducing agents. Here, carbon loses one bond to another carbon (a double bond turning into a single bond), facilitating the reduction of the halide atoms. Although the oxidation state of carbon does not change in these specific reactions, it still plays the role of reducing agent by giving electrons to the bonded halogen groups. Recognizing reducing agents is key to balancing chemical reactions, especially those involving electron transfer.

Chemical Reactions Analysis

Chemical reactions are transformations where substances change into new products. Analyzing chemical reactions involves understanding the changes in the molecular and atomic level, particularly electron exchanges.

In alkene reactions like the ones analyzed, understanding the double bonds and their interactions with different agents—oxidizing or reducing—is crucial. Each reaction must be investigated to ensure proper recognition of the changes in electron counts (via oxidation states).

Tools like oxidation state tracking are employed to determine changes. This helps in identifying which elements act as oxidizing or reducing agents. While some reactions, such as reactions c and d, may show no change in oxidation states, emphasizing the diverse nature of reactions.

By unraveling these electron exchanges, one builds a comprehensive grasp of how and why these reactions occur, which is essential knowledge for students studying organic chemistry concepts like alkene reactions.