Problem 105 - Chemistry and Chemical Reactivity

Question

You have a liquid that is either cyclohexene or benzene. When the liquid is exposed to dark-red bromine vapor, the vapor is immediately decolorized. What is the identity of the liquid? Write an equation for the chemical reaction that has occurred.

Step-by-Step Solution

Answer

The liquid is cyclohexene. Reaction: \( \text{C}_6\text{H}_{10} + \text{Br}_2 \rightarrow \text{C}_6\text{H}_{10}\text{Br}_2 \).

Step 1: Understand the Reaction with Bromine Vapor

Bromine vapor is dark-red in color. When a compound such as cyclohexene reacts with bromine, the bromine loses its color and becomes colorless as it forms a dibromo compound. Benzene, however, does not react with bromine under these conditions due to the stability of its aromatic ring.

Step 2: Determine Which Compound Reacts

Cyclohexene is an alkene and reacts with bromine as it has a double bond. Benzene is an aromatic compound with stable pi-electrons, and it does not readily react with bromine without a catalyst. The fact that the bromine vapor is immediately decolorized indicates the presence of cyclohexene.

Step 3: Write the Chemical Equation

The reaction between cyclohexene and bromine forms 1,2-dibromocyclohexane. This can be represented as: \[ \text{C}_6\text{H}_{10} + \text{Br}_2 \rightarrow \text{C}_6\text{H}_{10}\text{Br}_2 \] This equation shows the double bond in cyclohexene reacting with bromine to form a dibromo compound.

Key Concepts

Alkene Reactions

Alkenes are a fascinating class of hydrocarbons characterized by the presence of a carbon-carbon double bond. The double bond is a site of high reactivity because it can easily be broken, allowing the alkene to participate in various chemical reactions. One common reaction involving alkenes is their interaction with halogens like bromine.

When an alkene reacts with bromine (\( \text{Br}_2 \)), the bromine molecules add across the double bond. This reaction is both rapid and visible, as the reddish hue of the bromine vapor disappears, indicating that the bromine is being consumed in the reaction.

In the case of cyclohexene, the reaction can be illustrated as:

• The double bond of cyclohexene breaks.
• Bromine atoms attach to the formerly double-bonded carbons, forming 1,2-dibromocyclohexane.

These steps emphasize the concept of addition reactions, where elements are added to the existing carbon skeleton of an alkene.

Aromatic Compounds

Aromatic compounds, like benzene, are a unique category of organic compounds. The defining feature is their ring structure with delocalized pi-electrons. This delocalization provides exceptional stability, often referred to as 'aromatic stability'. Due to this stability, benzene and similar compounds do not readily participate in addition reactions, which distinguishes them from alkenes.

In the context of the given problem, this stability means benzene does not react with bromine in the absence of a catalyst. The resistance to react is due mainly to the energy cost associated with disrupting the benzene ring's pi-electron cloud.

Therefore, while benzene would maintain its structure in the presence of bromine, an alkene like cyclohexene would react immediately, leading to a color change. This characteristic makes aromatic compounds particularly valuable and interesting in organic chemistry.

Chemical Equations

Chemical equations are vital in representing what occurs during a chemical reaction. They balance the reactants and products using chemical formulas and coefficients, following the law of conservation of mass which states that atoms are neither created nor destroyed.

For the reaction between cyclohexene and bromine, the chemical equation is:

• Reactants: Cyclohexene (\( \text{C}_6\text{H}_{10} \)) and bromine (\( \text{Br}_2 \)).
• Product: 1,2-dibromocyclohexane (\( \text{C}_6\text{H}_{10}\text{Br}_2 \)).

This equation succinctly shows the transformation, with a bromine molecule adding across the double bond of cyclohexene, resulting in the formation of a dibromo compound.

Such equations help chemists not only in understanding but also in predicting the outcomes of reactions, making them essential tools in chemistry.