Question

With very reactive agents, such as the halogen elements, alkanes undergo _________ reactions, whereby a new atom replaces one or more hydrogen atoms of the alkane.

Short answer

Alkanes undergo substitution reactions with halogen elements, where one or more hydrogen atoms of the alkane are replaced by a new atom. In the case of methane reacting with chlorine, the balanced chemical equation is CH₄ + Cl₂ → CH₃Cl + HCl. This reaction involves three main steps: initiation, propagation, and termination, as detailed in the step-by-step solution.

Step-by-step solution

Identify the Reaction Type

The reaction between very reactive agents, such as halogen elements, and alkanes is called a "substitution reaction." In this type of reaction, one or more hydrogen atoms of the alkane are replaced by a new atom.

Understand Substitution Reactions

A substitution reaction involves the replacement of one or more atoms in a molecule with a new atom. In the context of this exercise, alkanes (a type of hydrocarbon) undergo substitution reactions with halogen elements. For the reaction to proceed, a molecule of the halogen must break its bond to form a free radical, which is an atom or molecule with unpaired electrons.

Provide an Example

Let's use the example of methane, an alkane with the molecular formula CH₄, reacting with chlorine, a halogen element, with the molecular formula Cl₂.

Write the Balanced Chemical Equation

For the reaction between methane and chlorine, the balanced chemical equation is as follows: CH₄ + Cl₂ → CH₃Cl + HCl

Explain the Steps of the Reaction

There are three main steps in this substitution reaction: 1. Initiation: The chlorine molecule (Cl₂) undergoes homolytic fission, where its bond is broken, and the two electrons are split between the two chlorine atoms. This process produces two chlorine free radicals (chemical species with unpaired electrons). \[ \underline{\ce{Cl2 -> 2Cl}}^\text{•} \] 2. Propagation: The chlorine free radical reacts with the alkane, methane in this case, breaking a C-H bond, and forming a new C-Cl bond and an HCl product. This step also creates a methyl free radical (CH3). \[ \underline{\ce{Cl}}^\text{•}\, + \ce{CH4 -> HCl + CH3}^\text{•} \] The methyl free radical can react with a chlorine molecule to produce chloromethane (CH₃Cl) and a new chlorine free radical. \[ \underline{\ce{CH3}}^\text{•}\, + \ce{Cl2 -> CH3Cl + Cl}^\text{•} \] This newly formed chlorine free radical can continue the reaction with more alkanes. 3. Termination: The reaction ends when two free radicals react together, forming a stable molecule and no more free radicals. This could be when two methyl radicals react to form ethane or when two chlorine radicals react to form a chlorine molecule. \[ \underline{\ce{CH3}}^\text{•}\, + \underline{\ce{CH3}}^\text{•}\,-> \ce{C2H6} \] \[ \underline{\ce{Cl}}^\text{•}\, + \underline{\ce{Cl}}^\text{•}\,-> \ce{Cl2} \] These steps outline the mechanism of a substitution reaction between an alkane and a halogen element.

Key concepts

Halogenation

Halogenation is a chemical reaction where one or more hydrogen atoms in an alkane are replaced by halogen atoms from elements like chlorine or bromine.
This is a common reaction in organic chemistry because it introduces halogen atoms into organic molecules, altering their properties.
In our example, when methane (an alkane) reacts with chlorine (a halogen), a substitution reaction occurs, replacing a hydrogen atom in methane with a chlorine atom. The reaction can be represented by the equation:

• CH₄ + Cl₂ → CH₃Cl + HCl

Halogenation is significant because:

• It transforms alkanes into more reactive compounds, called haloalkanes.
• These haloalkanes are useful in various applications including as solvents, intermediates in chemical synthesis, and in the production of pharmaceuticals.

Understanding halogenation is useful for students as it provides insight into how basic hydrocarbons can be transformed into compounds with varied and useful chemical properties.

Free Radicals

Free radicals are molecules or atoms that have unpaired electrons, making them highly reactive.
In chemical reactions like halogenation, free radicals play a crucial role in initiating the reaction process.
Let's look at how they work in a halogenation reaction: Firstly, a halogen molecule such as Cl₂ undergoes homolytic fission, breaking apart to produce two chlorine free radicals ( \( ext{Cl}^\text{•}\) ).

• This step is called "Initiation" and it sets off the chain reaction.

In the next step known as "Propagation," these chlorine free radicals react with alkanes like methane to form hydrochloric acid (HCl) and a methyl free radical ( \( ext{CH}_3^\text{•}\) ).

• The methyl free radical continues the chain by reacting with more chlorine molecules, perpetuating the formation of new radicals.

Finally, in the "Termination" step, free radicals couple together to form stable molecules, effectively ending the reaction.

• This could be two chlorine radicals forming Cl₂ or two methyl radicals forming ethane (C₂H₆).

Understanding the behavior of free radicals helps students grasp how chemical reactions propagate and terminate, shedding light on the mechanics behind substitution reactions.

Alkanes

Alkanes are a group of hydrocarbons that contain only single bonds between carbon atoms.
They are saturated hydrocarbons and follow the general molecular formula CₙH₂ₙ₊₂.
In the context of substitution reactions, alkanes play a vital role as the starting molecules that undergo transformation. Challenges with alkanes arise from their nonpolar nature and low reactivity, requiring specific conditions to undergo reactions. Consider methane (CH₄), the simplest alkane:

• It reacts with chlorine through halogenation to form dichloromethane, a compound used in manufacturing and solvent applications.

Due to the strength of the C-H bonds, significant energy is needed to initiate their reaction.
This is why halogenation typically requires light or heat to proceed, which helps break down halogen molecules into reactive free radicals. Understanding alkanes and their reactivity patterns is important in organic chemistry, providing insight into how raw hydrocarbons can be converted into more complex and functional organic compounds.