In this explainer, we will learn how to define substitution reactions and write and interpret equations for substitutions of alkanes with halogens.
Alkanes are hydrocarbons with the general formula . 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 () 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:
This is an example:
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, , 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.
This is the structure of a molecule of halothane:
There is a whole class of compounds called freons that includes many chlorofluorocarbons (compounds containing only chlorine, fluorine, and carbon)—for example, tetrafluoromethane () and dichlorodifluoromethane () 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) |
|---|---|
| 155 | |
| 240 | |
| 190 | |
| 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.
Here is the fluorination of methane:
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.
Here is the chlorination of methane:
And here is the bromination of methane:
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: Determine the stoichiometry and molecular formula of the product .
Answer
In a substitution reaction, a part of a molecule is replaced (substituted) with something else. In this reaction equation, we can see ethane () reacting with bromine (). The only atom that we expect to be substituted in this process is a hydrogen atom.
In the products, we can see hydrogen bromide ()—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: .
Let’s look at the whole equation:
This equation is balanced, and the product is what we expect when we treat an alkane like ethane with bromine. The product is bromoethane, with a formula , and we produce one molecule for each molecule of ethane in this reaction.
Our answer is .
If we want to perform multiple substitutions, we can use more of the halogen:
This is the overall process:
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?
- Chlorine
- Fluorine
- Bromine
- 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 that weakens the fluorine–fluorine bond.
The result is that has the weakest bond, followed by , then , and finally .
So, again, we expect fluorine to be more reactive than chlorine and bromine—the lower the activation energy, the faster the reaction.
Nonetheless, the 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.
Key Points
- 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.
