Lesson Video: Substitution Reactions of Alkanes | Nagwa Lesson Video: Substitution Reactions of Alkanes | Nagwa

Lesson Video: Substitution Reactions of Alkanes Chemistry

In this video, we will learn how to define substitution reactions and write and interpret equations for substitutions of alkanes with halogens.


Video Transcript

In this video, we will learn how to define substitution reactions and write and interpret equations for substitutions of alkanes with halogens.

This video is about reactions involving alkanes. Alkanes are a type of hydrocarbon, in other words, their molecules made up of hydrogen atoms and carbon atoms. Specifically, alkanes are a hydrocarbon made up of only single bonds. The simplest alkanes are methane, ethane, and propane, although many other alkanes exist with longer, branched, or cyclic carbon chains. In the structures drawn here, we can confirm there are hydrogen atoms and carbon atoms connected by single bonds. Sometimes, we refer to alkanes as being saturated. Because they contain only single bonds, it allows the carbon atoms in the chain to bond to the maximum number of hydrogen atoms. The carbon atoms are saturated with hydrogen atoms.

If we bring in a similar molecule that is not an alkane for comparison, we can see that replacing the carbon-carbon single bond with a double bond reduces the number of hydrogens in the compound. Doing so turns our saturated alkane into an unsaturated alkene.

This video is also about substitution reactions. In a substitution reaction, two atoms or groups of atoms swap places. In the reaction we’ve written here, a bromine atom can knock off one of the hydrogen atoms of the methane molecule. That hydrogen atom can then bond with the other bromine atom that was left behind. As a result of this substitution or swap, bromomethane and hydrogen bromide are produced. We can also call this specific reaction a halogenation reaction because we are adding a halogen to the alkane. The substitution reactions of alkanes that we’ll look at in this video involve adding atoms and functional groups, such as halogens, to the carbon chain of the alkane.

It’s worth noting that alkanes in general are unreactive. This reaction requires the added energy from ultraviolet light to proceed. On the other hand, haloalkanes are more reactive than alkanes. For this reason, they’re often used as reactants in these kinds of substitution reactions. If we want to attach a new atom or functional group to the carbon chain, it’s easier to knock off a halogen atom than to knock off a hydrogen atom.

One example of such a reaction is this one here that takes bromoethane and water to produce ethanol and hydrogen bromide. In this equation, one of the things being substituted isn’t an atom, but rather a group of two atoms. We call groups of atoms like this functional groups. A functional group is a group of atoms that shows similar chemical properties when it occurs in different compounds. The functional group in this reaction, with an oxygen atom and a hydrogen atom, is called a hydroxy group.

If we add a hydroxy group to a two-carbon alkane, we get the compound ethanol. Ethanol is an example of an alcohol. Alcohols are a group of compounds that all have hydroxy functional groups. Alcohols share many similar chemical properties because they share this hydroxy functional group.

Let’s take a look at what functional groups might be attached to alkanes. We just mentioned that a hydroxy group contains an oxygen atom and a hydrogen atom. There’s also the amino group, which consists of a nitrogen atom and two hydrogen atoms. A nitro group consists of a nitrogen atom and two oxygen atoms. A carboxyl group has this branched structure of a carbon atom, two oxygen atoms, and a hydrogen atom.

Often, a compound’s name will have prefixes or suffixes that indicate what functional groups are present. For example, compounds that contain hydroxy groups use the suffix -ol, for example, methanol. Amino groups use the prefix amino-. If we attach that group to a methane molecule, we get aminomethane. Similarly, the nitro group uses the nitro- prefix to make compounds like nitromethane. For carboxyl groups, the suffix is -oic acid, such as the compound methanoic acid.

When discussing these functional groups in general, you may see them attached to the letter R. The R represents a carbon chain. Since these functional groups can attach to a wide variety of carbon chains, we use the letter R as a placeholder to represent any carbon chain.

In addition to using prefixes and suffixes for halogens and functional groups to name compounds, there are other compounds we need to name more specifically. In the reaction we’ve drawn here, we can add fluorine to propane to create fluoropropane. This is a substitution reaction where one of the fluorine atoms from the fluorine molecule attaches to the carbon chain of the propane. However, there are two different possible arrangements of this product. These two versions of fluoropropane have the same chemical formula but different arrangements. So we call them isomers. The key difference is the location of the fluorine. Either the fluorine atom can attach to one of the carbons on the end of the carbon chain or it can attach to one of the carbons in the middle of the carbon chain.

While the fluorine atom could replace any of the eight hydrogen atoms on the initial propane molecule, each of those eight possible substitutions will produce a product that is a rotation of one of these two options. We can distinguish these two isomers of fluoropropane by numbering the carbons one through three. We can give the isomer on the left the name 1-fluoropropane: one signifying that the fluorine atom is attached to carbon number one. Similarly, if the fluorine atom is attached to carbon number two, we can give that compound the name 2-fluoropropane.

In this situation, we numbered the carbons from left to right, one, two, three. But why didn’t we number them the other way around? By convention, we number the carbons in a way that minimizes the numbers that appear in the name of the compound. For 1-fluoropropane, that means counting the carbons starting at the attachment instead of starting at the other end. In this particular reaction, both isomers are produced. However, depending on the reaction, some isomers may appear more or less frequently than others.

This naming system works the same way for functional groups as it did for the halogen atoms. Attaching an amino group to a propane chain can give us 1-aminopropane or 2-aminopropane. With alcohols, we can put the number in the middle. With a hydroxy group on carbon number one, we get propan-1-ol, with a hydroxy group on carbon number two, propan-2-ol. Sometimes, we will need to name an alkane that has multiple substitutions. In this case, we can simply include multiple prefixes or suffixes and multiple numbers.

As we name these compounds, we should keep in mind that in chemistry there’s rarely just one correct name for a compound. Different naming conventions might use different ways of representing a functional group or a different order for the components of the name, resulting in different names for the same compound.

Now let’s practice naming this polysubstituted alkane. First, we can note that this is a chain of four carbon atoms, so it must be some form of butane. With an attached bromine atom, we can include the prefix bromo-. And since that bromine atom is attached to carbon number one, we can include the number one. The last part to include is this hydroxy group on carbon number two. To include it in the name, we can chop off the e from butane and add the number two as well as the suffix -ol. This is the full name of the compound, 1-bromobutan-2-ol. This name indicates that this is a compound with a four-carbon chain and a bromine atom attached to carbon number one and a hydroxy group attached to carbon number two.

Another thing we might see in names like this is the prefix di-. We’ve altered this compound a bit to make it 1,4-dibromobutan-2-ol. Dibromo- means there’s two bromine atoms attached to the carbon chain. We indicate their location simply by including two numbers: one comma four, one bromine atom on carbon number one and one bromine atom on carbon number four.

It can also be useful practice to do this process in the other direction. Based on this name, can we draw the compound it represents? First, pentane implies a five-carbon chain at the center of the molecule. 2-Amino means there’s an amino group attached to carbon number two. Dichloro- means that there’s two chlorine atoms attached to the carbon chain. The 1,1- in front of dichloro- means that both these chlorine atoms are attached to carbon number one. As a final step, we can fill in the rest of the possible single bonds with hydrogen atoms. Here, we have 1,1-dichloro-2-aminopentane. Although, as we mentioned before, this is just one way to name this molecule.

While these long names might initially seem intimidating, breaking them down into their constituent parts makes them easier to understand. Each name contains information about the carbon chain, the atoms or functional groups attached to that chain, and the location of those attachments along the chain.

Haloalkanes have a variety of everyday uses. The chemical structure of PVC pipes and the Teflon coating on many nonstick pans are polymerized haloalkanes. In other words, they’re the same haloalkane unit drawn here, repeated to make continuous chains. Chloroalkanes are used as powerful solvents in a variety of chemical processes. Fluoroalkanes are used as important building blocks to create pharmaceutical drugs. Chlorofluorocarbons, or in other words alkanes where each of the hydrogen atoms has been replaced by a chlorine atom or a fluorine atom, have been used as refrigerants in air conditioning systems as well as propellants in aerosol cans. However, due to their harmful effects on the ozone layer, chlorofluorocarbons are being phased out in most countries.

Now that we’ve learned about haloalkanes, let’s do a practice problem to review.

What is the name of this molecule? (A) 2-Bromopentane, (B) 5-bromopentane, (C) 1-bromopentane, (D) 2-bromohexane, or (E) 5-bromohexane.

The diagram drawn here is what’s known as a skeletal formula. A skeletal formula represents carbon-carbon bonds as lines. In other words, we can assume that there is a carbon atom at the intersection or end of each of the lines in the diagram. Hydrogen atoms are also assumed to be present but not directly indicated by the skeletal formula. The benefit of a skeletal formula is that by minimizing the visual presence of carbon and hydrogen atoms, it allows us to clearly and quickly visualize which atoms and functional groups are attached to the carbon chain. In this case, we can see that a bromine atom is attached to the carbon chain.

In this question, we are being asked to name the compound that appears in this skeletal structure. The five answer choices give us five possible names that each follow a similar template: a number followed by the prefix bromo- followed by pentane or hexane. Let’s take a look at the three individual parts of the name to figure out which one is represented in the diagram here.

The part of the name that ends in -ane describes the molecule’s carbon chain. Is this alkane, pentane, or hexane? Well, the diagram shows that there are six carbon atoms in the structure. A six-carbon chain is called hexane. Hexa- means six, just like a hexagon has six sides. On the other hand, pentane would have only five carbons. So we can write in hexane as the first part of the final answer.

The prefix bromo- appears in all five of the answer choices. This prefix represents the bromine atom attached to the carbon chain. We can include it in our final answer. The last part of the name to figure out is the number that goes in front of it.

Eliminating the answer choices that use pentane instead of hexane, we’re left with two options. Is this 2-bromohexane or 5-bromohexane? The number in this name will indicate the position of the attached atom or functional group. In this case it’s the attached bromine atom. The bromine atom is attached to this carbon right here. So the number that appears in the name will be the position of this carbon atom one, two, three, four, five, or six along the chain.

We’ve already numbered the carbons right to left, but we could do it another way. We could number them left to right. How do we know which way is correct? As a tiebreaker, we can say that the carbon atoms are numbered to minimize the values that appear in the name. Numbering the carbons from right to left, like we’ve done in pink here, makes the carbon of interest carbon number two, a lower number than if we numbered the carbons from left to right.

Since the bromine atom is attached to carbon number two, the number we include in the name of this compound is two, giving us the full name 2-bromohexane. This corresponds to answer choice (D). The names of compounds like this reveal important information about the carbon chain, the attached atoms or functional groups on the carbon chain, and the location of those attachments. So what is the name of this molecule? That’s choice (D) 2-bromohexane.

Let’s review the key points of the video. Alkanes are hydrocarbons made up of only single bonds. We can create haloalkanes by substituting a halogen atom for one of the hydrogen atoms on the alkane. For example, we can combine methane and bromine under ultraviolet light to create bromomethane and hydrogen bromide. We call reactions like this, where one atom or functional group swaps places with another atom or functional group, substitution reactions.

Functional groups are groups of atoms that show similar properties when they appear in different compounds. Examples include the hydroxy group OH and the amino group NH2.

The name of an alkane reveals its functional groups, their locations, and its carbon chain. For example, let’s take a look at the name 1,4-dichloropentane. The pentane implies that the carbon chain is five carbon atoms long. Dichloro- means that there are two chlorine atoms attached to that chain. And the numbers one and four mean that the chlorine atoms are attached to carbon number one and carbon number four.

Haloalkanes have a variety of common uses, such as in PVC pipes and nonstick coatings. They also have a variety of scientific uses, as a powerful solvent or a building block for pharmaceuticals.

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