Problem 28 - Chemistry Matter and Change

Question

What reactant would you use to convert methane to bromomethane?

Step-by-Step Solution

Answer

To convert methane (CH₄) to bromomethane (CH₃Br), the reactant needed is bromine (Br₂) in the presence of UV light or a catalyst. The substitution reaction can be written as: \( CH_4 + Br_2 \xrightarrow [UV\, light \text{ or } Catalyst]{} CH_3Br + HBr \).

Step 1: Identify the Reaction Involved

To convert methane (CH₄) to bromomethane (CH₃Br), we need a substitution reaction, in which one of the hydrogen atoms in methane is replaced by a bromine atom.

Step 2: Choose the Reactant That Can Replace Hydrogen with Bromine

In order to replace one of the hydrogen atoms in methane with a bromine atom, we require a reactant rich in bromine. The most common reagent used for this purpose is bromine (Br₂) in the presence of UV light or a catalyst.

Step 3: Write the Chemical Reaction

The reaction between methane and bromine in the presence of UV light or a catalyst can be summarized as follows: \( CH_4 + Br_2 \xrightarrow [UV\, light \text{ or } Catalyst]{} CH_3Br + HBr \) In this reaction, one of the hydrogen atoms in methane (CH₄) is replaced by a bromine atom, forming bromomethane (CH₃Br) and hydrogen bromide (HBr). So, to convert methane to bromomethane, we would use bromine (Br₂) as the reactant in the presence of UV light or a catalyst.

Key Concepts

Methane

Methane is the simplest alkane, consisting of one carbon atom bonded to four hydrogen atoms. Its chemical formula is \( CH_4 \), and it is an important starting material in many chemical reactions. Being a hydrocarbon, methane is found abundantly in natural gas and is a primary source of energy when burned. In a bromination reaction, methane serves as the substrate whose hydrogen atom is replaced by a bromine atom, leading to the creation of a new compound, bromomethane. This process is a classic example of a substitution reaction, a fundamental concept in organic chemistry.

Bromomethane

Bromomethane, also known as methyl bromide, has the chemical formula \( CH_3Br \). It is produced through the substitution of one hydrogen atom in methane by a bromine atom during a bromination reaction. Bromomethane is notable for its use as a fumigant in agriculture and as a building block in organic synthesis. Although it's effective for its intended industrial applications, safety precautions must be considered due to its toxicity. Understanding its formation from methane is crucial for grasping concepts related to halogen substitution and functional group transformation in chemistry.

Substitution Reaction

A substitution reaction involves replacing one atom or group of atoms in a molecule with another atom or group. In the case of methane and bromine, a hydrogen atom in methane is substituted by a bromine atom, transforming methane into bromomethane. This type of reaction is common in organic chemistry where hydrocarbons undergo modifications to form new compounds. Key characteristics of substitution reactions include:

• The original molecule loses an atom or group.
• A new atom or group takes its place.
• They often require specific conditions, like the presence of light or heat.

This fundamental mechanism underlies many synthetic pathways and facilitates the creation of diverse molecular structures.

Bromine

Bromine, represented by the symbol \( Br_2 \), is a halogen found in Group 17 of the periodic table. In the conversion of methane to bromomethane, bromine acts as the reactant providing the bromine atom needed for substitution. It is a red-brown liquid at room temperature, noteworthy for its reactivity and ability to attack other molecules to form new bonds. Its role in bromination reactions highlights its importance in synthetic chemistry, as bromine can alter the course and outcome of a reaction significantly. Safety is crucial as bromine is corrosive and hazardous to handle, necessitating proper protective measures during experiments.

UV Light

Ultraviolet (UV) light plays a pivotal role in the bromination reaction converting methane to bromomethane. It provides the energy required to break molecular bonds and initiate the reaction process. In this context, UV light breaks the bond between bromine atoms in \( Br_2 \), generating highly reactive bromine radicals. These radicals then participate in the substitution of hydrogen in methane, driving the formation of bromomethane. The need for UV light stems from the stability of the bonds in methane, which require an energy input to overcome the activation barrier and proceed with the reaction. Utilizing UV light is a common practice to expedite and facilitate chemical reactions that would otherwise be unfeasible under standard conditions.