Lesson Video: Properties of Alkanes Chemistry • 7th Grade

Video Transcript

In this video, we will learn about alkanes. We will learn how to write their formulas and how to name them correctly. We will also investigate the trends in some of their physical properties.

What are alkanes? Let’s first remind ourselves what organic compounds are. Organic compounds or molecules contain carbon and hydrogen atoms covalently bonded together. And organic molecules may contain other elements like nitrogen, sulfur, oxygen, or chlorine. Now, carbon can form four bonds in one of these three following configurations. In the alkane family of organic compounds, each carbon atom in a molecule is bonded by single bonds. And so each carbon atom is bonded to four other atoms: one, two, three, four.

Alkanes are hydrocarbons, consisting of only hydrogen and carbon. The simplest alkane contains only one carbon atom bonded to four hydrogen atoms. This is its displayed formula. Its molecular formula is CH4 because there is one carbon and four hydrogens. Its IUPAC name is methane. IUPAC or the International Union of Pure and Applied Chemistry is a global body of chemists and scientists that have made a systematic set of naming rules for compounds. Meth- in the name methane refers to one carbon atom and -ane to the fact that it is an alkane consisting of only hydrogen and carbon, where each carbon has four single bonds.

Note that on paper we tend to draw the displayed formula with right angles. But in reality, this molecule has a three-dimensional structure, where the angles are 109.5 degrees, giving this molecule a tetrahedral shape, which looks a bit like this. The next alkane has two carbon atoms, and each carbon atom still makes four single bonds. So each carbon has three hydrogen atoms. This is its displayed formula, and its molecular formula is C2H6. Its IUPAC name is ethane, eth- meaning two carbons and -ane referring to the fact that it is an alkane. We can slightly simplify the displayed formula to get CH3CH3, which is ethane’s structural formula.

Let’s look at one more example. In this alkane, we have three carbon atoms and eight hydrogen atoms. This compound has molecular formula C3H8. And we call it propane, prop- meaning there are three carbon atoms and -ane that it is an alkane. The displayed formula can be simplified to a structural formula, CH3CH2CH3. In the three alkanes we’ve looked at so far, we’ve seen that each carbon has four single bonds, is bonded to four other atoms, and that the only elements present are hydrogen and carbon. There is a special term we use to describe alkanes. We say they are saturated. This means that each carbon atom has the maximum number of atoms bonded to it with single covalent bonds between the carbon atoms. We could simplify this top point and simply say each carbon is saturated.

Now there are many configurations of alkane compounds, depending on the number of carbon atoms. Let’s have a look. Some alkanes, like the ones we have seen, are straight-chain alkanes. We looked at methane, ethane, and propane. Here are ” more: butane, where but- tells us there are four carbons in the chain, and hexane, where hex- refers to the six carbons of the chain. Notice that the root or stem of the name tells us how many carbons there are in the molecule and the suffix -ane tells us we’re dealing with an alkane, a saturated hydrocarbon. The table shows the stem name for an alkane containing from one to 10 carbons. For straight-chain alkanes, we’ve already looked at naming. First, we find the longest carbon chain to get the stem name. And then for the alkanes, we know that the suffix is -ane. But what about this configuration, the branched-chain alkanes?

When a straight-chain alkane has a side chain, we call it a branched-chain alkane. To name this compound, we again start with rules one and ” below. We find the longest carbon chain to get the stem name. But which is the longest carbon chain? Is it this chain in blue in a straight line which has four carbons, or this chain in blue which also has four carbons? It does not matter which one we pick because they both have a four-carbon chain length. And remember, single bonds can rotate. And these molecules when they are long are flexible and can appear in slightly different orientations in reality. So, let’s choose this as our base chain. Since the longest chain contains four carbons, the stem name is but-. The suffix is -ane because here we have a saturated alkane.

We need a third rule for naming the side chain. Rule three: Identify the side chain and locate it on the lowest number carbon of the chain. This means that we must number the base carbon chain either from the left or the right such that the side chain is located on the lowest carbon number in the chain. If we number the carbons from left to right, we see that the side chain is on carbon number three. But if we number the base chain from right to left, the side chain is on a lower carbon number, in this case carbon number ”. In the name we indicate which carbon the side chain is on, in this case carbon two, and then we write a dash. And then we need to identify what that side chain is. The side chain has one carbon atom in it and three hydrogens, which corresponds to the stem meth-. And because it is a side chain, we write -yl, methyl. The name of this compound is 2-methylbutane.

Let’s name this branched-chain alkane. First, we find the longest carbon chain to get the stem name. There are six carbon atoms in this chain, and there are six carbon atoms in this chain. So we could choose either because both are the longest possible number of carbons in a chain. Hex- is therefore the stem name. All carbon-carbon bonds are saturated, so the suffix is -ane. And now in front of the term hexane, we need to write information about the side groups or branches. Here is one branch. It has one carbon atom. So the side chain is called methyl side chain. And here is the other branch. With two carbon atoms, this is an ethyl side chain.

According to the way we’ve numbered the carbon chain, the methyl group is on carbon number two and ethyl on carbon number four. If we’d numbered our carbon chain, our longest carbon chain, like this, the methyl group would have been on carbon number five and the ethyl group on carbon number three. But remember, we need to locate the side groups on the lowest number carbon or carbons in the chain. And so this is not the correct way to number the main chain, but rather as we had it before, one, two, three, four, five, six. This gives our side groups the lowest number carbon attachment. We will indicate that methyl is on carbon number two by doing two dash methyl. And we will indicate that the ethyl side chain is on carbon number four by writing four dash ethyl. But which branch name, 2-methyl or 4-ethyl, do we write first in the name? Rule number four gives us the answer. Order the branch names alphabetically. E comes before M in the alphabet. And so we write 4-ethyl-2-methylhexane.

Let’s look at the third configuration of alkanes, and that is the cycloalkanes. These are ring-like or cyclic structures, for example, this compound here. The naming rules are almost the same as for straight-chain or branched-chain alkanes. Let’s just change some of the wording. Step one: Find the largest carbon ring. There are six carbons in this ring, and so the stem is hex-. Step two: The suffix is still -ane because this is a saturated alkane, but the first prefix is cyclo-, indicating the ring structure. Rules number three and four are the same as before. If there are side chains on the ring structure, we place them or locate them on the lowest number carbon. And if they’re more than one, we order them alphabetically. In this example, there are no side chains. And so this compound is cyclohexane. Here is one last example. There are four carbons in the ring, so the stem is but-. All carbon-carbon bonds are saturated, so the suffix is -ane. And the compound is cyclic, so we have cyclobutane with no side chains.

Now the straight-chain and branched-chain alkanes have the same general formula: C𝑛H2𝑛+2, where 𝑛 indicates the number of carbon atoms. For example, this molecule has eight carbon atoms. So we substitute eight for 𝑛, giving us the molecular formula C8H18, which is octane. Cycloalkanes have the general formula C𝑛H2𝑛. For example, cyclobutane, which we saw earlier, has four carbon atoms. So if we substitute four into the formula for 𝑛, we get the molecular formula for cyclobutane, C4H8.

Now we know about the different types of alkanes and how to name them, as well as their general formulas. Let’s now investigate some of the physical properties of alkanes. Let’s look at boiling point first. The graph shows the trend in boiling points for the first straight-chain alkanes with increasing number of carbon atoms. Note that with increasing chain length, there is a corresponding general increase in the boiling point. This is because shorter chains have a smaller surface area and, thus, weaker Van der Waals attractive forces between them, specifically London dispersion forces.

The weaker the intermolecular forces, the less energy is required to separate the molecules during boiling and the lower the boiling point. For larger molecules or longer carbon chain lengths, the stronger the intermolecular forces of attraction. Because of the larger associated surface areas, more energy is required to separate the molecules during boiling, and thus the higher the boiling points. Note that branched-chain alkanes cannot get as close to each other as straight-chain alkanes. And so they have weaker intermolecular forces and thus, in general, lower boiling points.

Let’s look at density. We won’t look at the first four alkanes since they are all gases at 20 degrees Celsius, but we’ll look at pentane through to decane. The table shows a similar trend as with boiling point. Notice that with increasing chain length or increasing number of carbons, there is a corresponding increase in density. The reasoning is the same. The longer the carbon chain length, the stronger are the intermolecular forces of attraction and, thus, the more dense is the substance.

Since these alkanes are all liquids at 20 degrees Celsius, with increasing density, it makes sense that viscosity is also increased with increasing chain length. The shorter chain lengths are less viscous; they flow easily because of the weaker intermolecular forces. And the longer chain lengths are more viscous, thick and sticky. And when pouring these liquids, they flow slowly because of the stronger intermolecular forces of attraction.

Lastly, let’s have a look at flammability, which follows an opposite trend. Flammability is the ease with which combustible substance can be ignited. In this case, with increasing chain length, there is a decreasing flammability. In other words, shorter chains burn more easily. So where do we find hydrocarbons and what do we use them for? Most hydrocarbon compounds initially come from crude oil reserves underground. These reservoirs are drilled with oil drilling rigs. The crude oil is then taken to a separation plant, where it undergoes fractional distillation.

The oil is separated into its components. The lightest fraction is used for bottled gas. The next fraction that comes off is used for petrol or gasoline. Then we have kerosene or paraffin. A diesel fraction is separated. Then come the even longer chain lengths with lubricating oils and waxes, fuel oils, and lastly the residue, which is collected as bitumen, which is often used as a road coating.

Now let’s summarize what we’ve learned about alkanes. Alkanes are saturated hydrocarbons that contain covalently bonded carbon and hydrogen atoms. We saw that the term “saturated” means that all the carbon-carbon bonds are single bonds, and every carbon atom is bonded to a maximum number of atoms. We learned that there are different types of alkanes: straight-chain alkanes, branched-chain alkanes, and cyclic structures called cycloalkanes. The examples here are represented with skeletal structures.

We saw that straight- and branched-chain alkanes have the same general formula, C𝑛H2𝑛+2. And the cycloalkanes have the general formula C𝑛H2𝑛. We also learned that the longer the carbon chain length in an alkane, the stronger the intermolecular forces and the higher the boiling point, viscosity, and density, but the lower the flammability.