SUBSTITUTION  IN  THE METHANE AND CHLORINE REACTION

 

INTRO:

When a mixture of methane and chlorine is exposed to ultraviolet light - typically sunlight - a substitution reaction occurs and the organic product is chloromethane.

 

CH4 +  Cl2   CH3Cl + HCl

 

https://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFHowever, the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms. That means that you could get any of chloromethane, dichloromethane, trichloromethane or tetrachloromethane.

 

CH4 +  Cl2   CH3Cl + HCl

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2      CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

 

https://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFYou might think that you could control which product you got by the proportions of methane and chlorine you used, but it isn't as simple as that. If you use enough chlorine you will eventually get CCl4, but any other proportions will always lead to a mixture of products.

The mechanisms

The formation of multiple substitution products like di-, tri- and tetrachloromethane can be explained in just the same sort of way as the formation of the Original chloromethane. You just have to look at the likely collisions as the reaction progresses.

Making dichloromethane

methane. In this new case:

 

CH3Cl + Cl  CH2Cl + HCl

 

 

Notice: The dot representing the electron has been moved against the carbon which is the atom with the unpaired electron. It would be potentially confusing to leave it next to the chlorine.

 

The chloromethyl radical formed can then interact with a chlorine molecule in a new propagation step . .

.

 

CH2Cl +  Cl2       CH2Cl2 + Cl

 

. . . and so dichloromethane is formed and a chlorine radical regenerated.

These propagation steps continue until the chain is terminated by any two radicals colliding and combining together.

Making tri- and tetrachloromethane

https://www.chemguide.co.uk/mechanisms/freerad/electron.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/electron.GIFObviously, as time goes on, there is an increasing chance of the dichloromethane being hit by a chlorine radical - producing these propagation steps giving trichloromethane:

 

CH2Cl2 + Cl CHCl2 + HCl

CHCl2 + Cl2 CHCl3 + Cl

 

 

 

 

As the amount of trichloromethane builds up, then you will get these steps giving tetrachloromethane:

 

CHCl3 +  Cl CCl3 + HCl

CCl3 + Cl2    CCl4 + Cl

 

https://www.chemguide.co.uk/mechanisms/freerad/electron.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/electron.GIFThis is why you will always get a mixture of products whatever the reaction proportions of methane and chlorine you use. The whole process is simply governed by chance. Having produced some

methane. In this new case:

 

CH3Cl + Cl  CH2Cl + HCl

 

 

NB: The dot representing the electron has been moved against the carbon which is the atom with the unpaired electron. It would be potentially confusing to leave it next to the chlorine.

 

The chloromethyl radical formed can then interact with a chlorine molecule in a new propagation step . .

.

 

CH2Cl +  Cl2       CH2Cl2 + Cl

 

. . . and so dichloromethane is formed and a chlorine radical regenerated.

These propagation steps continue until the chain is terminated by any two radicals colliding and combining together.

Making tri- and tetrachloromethane

https://www.chemguide.co.uk/mechanisms/freerad/electron.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/arrow.GIFhttps://www.chemguide.co.uk/mechanisms/freerad/electron.GIFObviously, as time goes on, there is an increasing chance of the dichloromethane being hit by a chlorine radical - producing these propagation steps giving trichloromethane:

 

CH2Cl2 + Cl CHCl2 + HCl

CHCl2 + Cl2 CHCl3 + Cl

 

 

 

 

As the amount of trichloromethane builds up, then you will get these steps giving tetrachloromethane:

 

CHCl3 +  Cl CCl3 + HCl

CCl3 + Cl2    CCl4 + Cl

 

chloromethane there is no way that you can prevent it from being hit by chlorine radicals, and similarly for dichloromethane and trichloromethane.

 

Trying to produce mainly one product

If you wanted tetrachloromethane, you could of course get it by using a large excess of chlorine, so that eventually all the hydrogens would be replaced.

If you wanted mainly chloromethane, you could favour this by using a huge excess of methane so that the chances were always greater of a chlorine radical hitting a methane rather than anything else - but even so, you would still get some mixture of products.

There is no obvious way of getting mainly dichloromethane or trichloromethane.chloromethane there is no way that you can prevent it from being hit by chlorine radicals, and similarly for dichloromethane and trichloromethane.

 

 

 

 

Substitution in the Reaction of Methane and Chlorine

 

A Free Radical Substitution Reaction

If a mixture of methane and chlorine is exposed to a flame, it explodes – producing carbon and hydrogen chloride. This isn't a very useful reaction!

The reaction we are going to explore is a gentler one between methane and chlorine in the presence of ultraviolet light – typically sunlight. This is a good example of a photochemical reaction – a reaction brought about by light.

Note: These reactions are sometimes described as examples of photocatalysis – reactions catalysed by light. It is better to use the term

 

"photochemical" and keep the word "catalysis" for reactions speeded up by actual substances rather than light.

CH4Cl2CH3Cl + HCl

The organic product is chloromethane.

One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.

 

The Mechanism

The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end

– and this keeps the process going.

Species: a useful word which is used in chemistry to mean any sort of particle you want it to mean. It covers molecules, ions, atoms, or (in this case) free radicals.

The over-all process is known as free radical substitution, or as a free radical chain reaction.

Note: If you aren't sure about the words free radical or substitution, read the page What is free radical substitution?.

Chain initiation

The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.

Cl22Cl∙

Chain propagation reactions

These are the reactions which keep the chain going.

 

CH4+Cl∙∙CH3+HCl

CH3+HClCH3Cl + Cl∙

Chain termination reactions

These are reactions which remove free radicals from the system without replacing them by new ones.

2Cl∙Cl2∙

.CH3 + Cl. CH3Cl

.CH3 + .CH3 CH3CH3

 

 

 An Overview:

Alkanes are hydrocarbons with the general formula CnH2n+2.Alkanes can have backbones of carbon atoms that are single chains or they can have branches.

There is also a similar class of chemicals called cycloalkanes, which are also hydrocarbons. However, the molecules of cycloalkanes contain one or more closed rings of carbon atoms. We will not be dealing with cycloalkanes specifically here.

A substitution is when something is replaced with something else (like substituting a member of a sports team during a game). So, it should be no surprise that a substitution reaction involves a part of a molecule being substituted with something else. This can involve single atoms or entire functional groups.

Definition: Substitution Reaction

A substitution reaction is a type of reaction where a part of a molecule is removed and replaced with something else.

 

Alkanes are relatively stable—they react with fewer types of chemicals than carboxylic acids, amines, or alkenes, for instance. However, there are a few crucial reactions that alkanes are well known for.

The combustion of alkanes, particularly of those in crude oil, currently drives the majority of the world economy. The substitution of alkanes may be a smaller industry, but the products from this industry are critical for the creation of many useful products.

The most common substitution reaction we perform on alkanes is the reaction of alkanes with elemental halogens. The common halogens are fluorine, chlorine, bromine, and iodine, but for reasons we will come to later, we will not use iodine in this reaction.

When alkanes react with halogens, hydrogen atoms are substituted with halogen atoms and the products are haloalkanes.

These reactions introduce new functional groups into the molecule, which can be useful for carrying out other reactions.

 

 

 

Definition: Functional Group

A functional group is a part of a molecule made of particular atoms (or ions), in a particular bonding arrangement. Functional groups usually behave in specific and predictable ways; for example, a carboxylic acid group (COOH) can make a substance more water soluble and more reactive to metal carbonates.

A haloalkane is a halohydrocarbon (a chemical containing carbon, hydrogen, and halogens only). Any halogens will do. Haloalkanes are also known as halogenoalkanes.

This                           is              the             general reaction equation:alkane+halogenhaloalkane+hydrogenhalide

This is an example:

 

CH4 + F2 → CH3F + HF

Here, we can see that a hydrogen atom is substituted with a fluorine atom.

Example 1: Determining the Number of Products Expected from the Monosubstitution Reaction of Butane and Bromine

How many products are formed in the monosubstitution reaction of butane and bromine?

Answer

Monosubstitution means single substitution. A substitution reaction involves pieces of molecules being replaced with other things; in a monosubstitution reaction, this only happens once.

If an alkane, like butane, reacts with bromine, we expect a hydrogen atom to be replaced with a bromine atom. In this case, we would form bromobutane (either 1-bromobutane or 2-bromobutane) and hydrogen bromide. That means we would have 2 products.

There is a name we use to show that a particular halogen is present in an organic compound, as shown in the following table:

 

Halogen

Name in Organic Halogen Compounds

Fluorine

Fluoro

Chlorine

Chloro

Bromine

Bromo

Iodine

Iodo

 

Using the IUPAC naming system can give haloalkanes unique names. These names are built from the name of the equivalent alkane. The name of the alkane forms the stem of the new name (e.g., methane), and the prefix of any halogen is added before the stem (e.g., fluoromethane).

If there are multiple halogens of the same type, we can use the usual prefixes (di-, tri-, tetra-, etc.) to indicate this:

We combine these names with what we call indexes, which correspond to the carbons in the molecule that the halogens are attached to.

The longest chain of carbon atoms is treated as the main backbone, and the carbon atoms are given a number (1, 2, 3, etc.). Since there are two ends to a chain, we sometimes have to work out the name going one way, then the other, and to pick the name with the lowest numbers.

If there are multiple types of halogens in a compound, the names are put in alphabetical order. (Prefixes, like di, tri, etc., do not affect the order.)

 

Lastly, if there can only logically be one place for the halogen, we can leave out the index (e.g., in fluoromethane).

There are many more layers to the IUPAC naming scheme for organic compounds, but we will leave it there.

Why does this all matter? Well, haloalkanes can be very useful. Various solvents, refrigerants, and fire retardants are haloalkanes. They are also used as intermediates in the synthesis of other chemicals.

Chloroform, CHCl3,was once used as an anesthetic. It knocked people out, but its dose was difficult to regulate and many patients died from overdoses. Halothane, also known as 2-bromo-2-chloro-1,1,1- trifluoroethane, is a more modern, safer alternative.

 

There is a whole class of compounds called freons that includes many chlorofluorocarbons (compounds containing only chlorine, fluorine, and carbon)—for example, tetrafluoromethane (CF4) and dichlorodifluoromethane (CClF22) are both freons. These compounds have historically been used in fridges, freezers, and air conditioners to channel heat from the inside out. Some of these have a greater greenhouse effect than others and have been generally replaced with less environmentally damaging alternatives.

Meanwhile, 1,1,1-trichloroethane is used as a solvent in dry cleaning.

The halogen substitution reactions of alkanes involve the breaking of halogen–halogen and carbon–hydrogen single bonds, followed by the forming of carbon–halogen and hydrogen–halogen bonds.

The energetics for the substitution reactions involving fluorine, chlorine, and bromine are favorable, but that is not the case for iodine—that is why we do not see simple iodine substitution reactions of alkanes. Now, we need to look at rates and activation energies.

The strength of the halogen–halogen bonds is a good indicator of the initial activation energy. Out of fluorine, chlorine, and bromine, the fluorine–fluorine single bond is the weakest (155 kJ/mol)—the weakness of this bond means that the reaction can start spontaneously under ordinary conditions.

Definition: Bond Dissociation Energy

The bond dissociation energy is the energy required to break a bond per mole of bonds.

Here are the relevant halogen–halogen bond dissociation energies:

 

Bond

Dissociation Energy (kJ/mol)

FF

155

 

 

 

 

 

 

 

 

 

 

 

 

ClCl

240

BrBr

190

II

148

 

 
Breaking the halogen–halogen bond is a critical, rate-determining step: the higher the bond dissociation energy, the higher the activation energy of the reaction.

 

 

Definition: Activation Energy

The activation energy is the minimum amount of energy required for a reaction to occur.

The chlorine–chlorine and bromine–bromine bonds are stronger (240 kJ/mol and 190 kJ/mol respectively). They are strong enough to make us need to provide some energy to kick-start the substitution reactions.

We commonly do this by shining ultraviolet light (UV light) through the reaction mixture.

Example 2: Predicting the Products of a Substitution Reaction

When ethane and bromine are mixed and illuminated by ultraviolet light, they react together in a substitution reaction. The incomplete balanced equation for this reaction is shown:

C2H6 + Br2 → X + HBr

Determine the stoichiometry and molecular formula of the product X.

Answer

 

In a substitution reaction, a part of a molecule is replaced (substituted) with something else. In this reaction equation, we can see ethane (C2H6) reacting with bromine (Br2). The only atom that we expect to be substituted in this process is a hydrogen atom.

In the products, we can see hydrogen bromide (HBr)—this hydrogen must have come from ethane. If a hydrogen has been removed from ethane, then—since we know this is a substitution reaction—it must have been replaced with something.

A bromine is missing from the product side (there are 2 on the left, but there is only 1 on the right). Let’s see what we get if we swap a hydrogen in ethane with a bromine

If we want to perform multiple substitutions, we can use more of the halogen:

 

 

Example 3: Identifying the Halogen That Is the Most Reactive in the Substitution Reaction with Methane

In the substitution reaction of methane and a halogen, which of the following halogens is the most reactive?

  1. Chlorine
  2. Fluorine
  3. Bromine
  4. Iodine

Answer

Fluorine is well known for being highly reactive (even more reactive than oxygen). In the halogen substitution reaction of alkanes, there are two main factors to consider: how energetic the reaction is and the magnitude of the activation energy.

 

The reaction of fluorine with methane releases a lot of energy, much more than the reactions of chlorine and bromine. The reaction with iodine, on the other hand, requires energy.

From this, fluorine is already a clear winner.

The activation energy of the reaction can be estimated from the halogen– halogen single bond dissociation energy, since the breaking of the halogen–halogen bond is the first part of the process.

As we go down group 17, the atoms of the halogens get larger. As they get larger, the distance between the nuclei in halogen–halogen bonds goes up. This means that the strength of the halogen–halogen single bond goes down. However, since fluorine atoms are exceptionally small, there is additional repulsion between the core electrons of the atoms of F2 that weakens the fluorine–fluorine bond.

The result is that I2 has the weakest bond, followed by F2,then Br2,and finally Cl2.

So, again, we expect fluorine to be more reactive than chlorine and bromine—the lower the activation energy, the faster the reaction.

Nonetheless, the II bond strength is lower than that of fluorine. So, is iodine even more reactive than fluorine?

No, it is not, because the energetics of the iodine–methane reaction are far less favorable than those of the fluorine–methane reaction.

Fluorine is the most reactive halogen in the halogen substitution reaction of methane.

The answer is B.

Salient facts:

  • A substitution reaction is a type of reaction where a part of a molecule is removed and replaced with something else.

 

  • Alkanes undergo substitution reactions with halogens to produce haloalkanes (also known as halogenoalkanes).
  • Reactions with fluorine are energetically favorable and do not need UV light.
  • Reactions with chlorine and bromine are energetically favorable and are usually done using UV light to overcome the activation energy.
  • Reactions with iodine are not energetically favorable.
  • Haloalkanes are useful as anesthetics, solvents, refrigerants, and fire retardants and in the synthesis of other chemicals.

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