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Lesson Video: Hydrocarbons Chemistry

In this video, we will learn how to identify and name simple hydrocarbons and represent them using different types of formulas.

16:57

Video Transcript

In this video, we will learn how to identify and name simple hydrocarbons and represent them using different types of formulas. We’ll classify basic hydrocarbons, name the first 10 straight-chain alkanes, and learn how to represent organic molecules using displayed, structural, and condensed formulas.

Before we begin to classify and name compounds, it is useful to have a few ways to represent the compounds in question. One representation that we may be familiar with is the molecular formula. A molecular formula is the chemical formula that expresses the exact number and type of atoms of each element in a molecule. For example, the molecular formula of methane gas, the main component of natural gas, is CH4. The molecular formula tells us that each molecule of methane contains one carbon atom and four hydrogen atoms. Formaldehyde, which has been used as a biological specimen preservative, has the molecular formula CH2O. This tells us that each molecule of formaldehyde has one carbon atom, two hydrogen atoms, and one oxygen atom.

Acetylene, a common fuel used for welding, has the molecular formula C2H2, meaning that each molecule of acetylene has two carbon atoms and two hydrogen atoms. A molecular formula tells us the number of each type of atom in the molecule but does not provide any information on how these atoms are bonded together.

To get a more detailed picture of the molecule, we can use a displayed formula. A displayed formula is a drawing representation of a molecule that shows all of the atoms and bonds. Bonds are represented by lines that connect the atoms together. In the displayed formula of methane, single lines connect the hydrogen atoms to a central carbon atom. Each single line represents a single covalent bond, a bond where two electrons are shared between two atoms.

In the displayed formula of formaldehyde, two lines connect the oxygen atom to the carbon atom. This represents a covalent double bond, a bond where four electrons are shared between two atoms. In the displayed formula of acetylene, three lines are drawn between the two carbon atoms. This represents a covalent triple bond, a bond where six electrons are shared between two atoms. Displayed formulas show how the atoms in a molecule are connected, but they are not necessarily the most accurate representation of a molecule.

Displayed formulas represent molecules as flat with 90- or with 180-degree angles between the atoms. In actuality, the atoms in these molecules are positioned so that the electron domains are at the maximum distance from one another as explained by valence-shell-electron-pair-repulsion theory, or VSEPR. We won’t go into VSEPR theory in detail in this video, but it’s worth recognizing that displayed formulas only represent the connectivity of the atoms, not the geometry of the molecule.

Now that we can represent molecules with molecular or displayed formulas, let’s take a look at some hydrocarbons. Organic chemistry is the study of carbon-containing compounds, the simplest of which is a hydrocarbon, a molecule composed of only carbon and hydrogen atoms. Hydrocarbons may be classified as saturated or unsaturated. A saturated hydrocarbon contains only single covalent bonds. This means that each carbon atom will form four single bonds with four different atoms, and each carbon atom will form bonds with the maximum number of hydrogen atoms as possible. For example, a saturated hydrocarbon that contains two carbon atoms must contain six hydrogen atoms. The molecule contains only single bonds, and each carbon atom has formed four total bonds with four different atoms.

Saturated hydrocarbons are also called alkanes. Unsaturated hydrocarbons contain at least one double or triple bond. As each carbon atom will not form four single bonds, an unsaturated hydrocarbon will contain fewer hydrogen atoms than a saturated molecule with the same number of carbon atoms. Shown are two examples of unsaturated hydrocarbons that each contain two carbon atoms. The unsaturated hydrocarbon on the left is an example of an alkene, a hydrocarbon with at least one double bond. The example on the right is an alkyne, a hydrocarbon with at least one triple bond.

Alkanes, alkenes, and alkynes each have a generic molecular formula. For an alkane, the generic formula is C𝑛H2𝑛+2, where 𝑛 represents the number of carbon atoms in the molecule. If an alkane has three carbon atoms, then it must contain two times three plus two hydrogen atoms and have the molecular formula C3H8. The generic formula for an alkene that contains only one double bond is C𝑛H2𝑛. If an alkene has three carbon atoms and only contains one double bond, then it must contain two times three hydrogen atoms and have the molecular formula C3H6. The general formula for an alkyne that contains only one triple bond is C𝑛H2𝑛−2. Therefore, an alkyne with only one triple bond and three carbon atoms will have the molecular formula C3H4.

Hydrocarbons can further be classified as either aromatic or aliphatic. The term aromatic can be used to describe a number of organic molecules. But in order for a hydrocarbon to be considered aromatic, it must contain at least one planar, cyclic, carbon-based structure: planar meaning that that portion of the molecule is flat or sits on one plane and cyclic meaning that the carbon atoms connect to form a ring. The ring structure must contain alternating single and double bonds. This structure gives aromatic compounds unique electronic properties. Benzene, a compound used in the production of plastics, and naphthalene, a compound used for fumigation, are examples of aromatic hydrocarbons. The term aliphatic is used to describe all nonaromatic hydrocarbons. This includes alkanes, alkenes, alkynes, and cycloalkanes.

Now that we have learned how to classify hydrocarbons, let’s focus on alkanes, the simplest hydrocarbon. Each of these displayed formulas represents a different alkane. But as they all contain single-bonded carbon and hydrogen atoms, how do we indicate which alkane is which? The international union of pure and applied chemistry, or IUPAC, established a set of rules for naming compounds, including alkanes. When naming an alkane, we focus on the total number of carbon atoms in a continuous chain. We mentioned earlier in the video that a molecule which contains one carbon atom and four hydrogen atoms is called methane. The prefix meth- means one carbon atom, and the suffix -ane is used to indicate that the compound is an alkane containing only single covalent bonds.

Thus simple alkanes are named by placing a prefix that indicates the number of continuous carbon atoms in front of the suffix -ane. The first 10 prefixes are shown in the table. One carbon atom is meth-. Two carbon atoms in a continuous chain is eth-. Three is prop-. Four is but-. Five is pent-. Six is hex-. Seven is hept-. Eight is oct-. Nine is non-. And 10 is dec-. Let’s use the table to name the alkane below methane. This alkane has five carbon atoms in a continuous chain. We use the prefix pent- for five carbon atoms and add the suffix -ane to indicate that the molecule is an alkane. The third alkane has seven carbon atoms in a line, and we give it the name heptane.

Notice that when using this naming system, the number of carbon atoms is indicated by a prefix, but the number of hydrogen atoms is not indicated in the name at all. Let’s take a look at why it is not necessary to indicate the number of hydrogen atoms with the example hexane.

The prefix hex- means that there are six carbon atoms in a continuous chain, and the suffix -ane indicates that the molecule is an alkane and contains only single covalent bonds. From the name, we can gather that the six carbon atoms are single bonded to one another. We also know that alkanes are saturated. This means that each carbon atom will form four single bonds and will bond with the maximum number of hydrogen atoms as is possible. Therefore, we can complete the displayed formula of hexane without initially knowing how many hydrogen atoms were in the molecule.

Displayed formulas like this one are useful for understanding the connectivity of atoms in a molecule, but they require a lot of space because they are drawn representations of a molecular structure. Instead of using a displayed formula, we can represent the bonding pattern of a molecule in text format by using a structural formula. A structural formula is a text representation of the bonding in a molecule. In this formula, hydrogen atoms are grouped with the atom they are bonded to, forming units. The units are then listed in the order in which they are bonded together. Let’s take another look at the displayed formula of hexane.

The carbon atom labeled with a one is bonded to three hydrogen atoms. This forms the unit CH3. The second carbon atom is bonded to two hydrogen atoms. This forms the unit CH2. The third, fourth, and fifth carbon atoms are each bonded to two hydrogen atoms and can be represented as the unit CH2. The sixth carbon atom is bonded to three hydrogen atoms. This carbon atom forms a CH3 unit. Writing the units in order gives us the structural formula of hexane CH3CH2CH2CH2CH2CH3. Structural formulas are smaller than displayed formulas, but they can still be quite large, depending on the number of carbon atoms in the molecule. A shortened version of a structural formula is called a condensed formula.

In a condensed formula, like repeating units in a structural formula are combined by placing the unit in parentheses, followed by a subscript to indicate the number of times the unit repeats. To see how this works, let’s once again take a look at hexane. Notice that in the structural formula, the unit CH2 appears four times in a row. We can combine these like, repeating units by writing the unit CH2 in parentheses followed by a subscript four indicating that the unit CH2 repeats four times in a row. We write the CH3 unit on either side of this repeating unit to complete the condensed formula. Thus, the condensed formula of hexane is CH3(CH2)4CH3.

We’ve now seen how to write a condensed formula from a structural formula and a structural formula from a displayed formula. But why are these formulas necessary? Couldn’t we have just used the molecular formula all along? We’ve seen that C6H14 is the molecular formula of hexane. But each of these displayed formulas also represent compounds with the molecular formula C6H14. Each of these new molecules may have the same molecular formula as hexane, but their connectivity and therefore their chemical properties are different. These molecules are all structural isomers, molecules that have the same molecular formula but differ from one another in how their atoms are connected.

We won’t go into detail on how to determine the number of structural isomers in this video. But it’s important to understand that many molecules can have the same molecular formula and are therefore best represented by using displayed, structural, and condensed formulas.

Let’s finish this video by summarizing what we’ve learned with the key points. Hydrocarbons are a type of organic compound that only contain carbon and hydrogen atoms. Hydrocarbons can be further classified as saturated, unsaturated, aromatic, or aliphatic. Alkanes are saturated hydrocarbons that have the general formula C𝑛H2𝑛+2. These compounds only contain single covalent bonds. Alkenes are a type of unsaturated hydrocarbon. They have the general formula C𝑛H2𝑛 and contain at least one carbon-carbon double bond. Alkynes are another type of unsaturated hydrocarbon. They have the general formula C𝑛H2𝑛−2 and contain at least one carbon-carbon triple bond.

Straight-chain alkanes are named by attaching a prefix that indicates the number of carbon atoms bonded in a continuous chain to the suffix -ane. For example, CH4 is called methane: meth- meaning one carbon atom and -ane meaning an alkane. We can represent organic molecules using displayed formulas, structural formulas, or condensed formulas.

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