Lesson Explainer: Hydrocarbons Chemistry

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

Organic chemistry is a branch of chemistry that studies carbon-based compounds, the simplest of which is the hydrocarbon. A hydrocarbon is a compound that is composed entirely of covalently bonded carbon and hydrogen atoms. Butane (CH410), a fuel used in lighters, octane (CH818), a component of gasoline, and naphthalene (CH108), a moth repellent, are all examples of hydrocarbons.

Definition: Organic Chemistry

Organic chemistry is a branch of chemistry that studies the structure, properties, composition, reactions, and synthesis of carbon-containing compounds.

Definition: Hydrocarbon

A hydrocarbon is a molecule composed of only carbon and hydrogen atoms.

Example 1: Understanding the Definition of Hydrocarbon

Nonane, CH920, is a hydrocarbon found in JP-8, a type of jet fuel. Why is nonane a hydrocarbon?

  1. It can be used as a fuel.
  2. It contains hydrogen and carbon.
  3. It is made from crude oil.
  4. It contains only hydrogen and carbon.
  5. It contains hydrogen and carbon in the correct ratio.

Answer

By definition, a hydrocarbon is a molecule composed of only carbon and hydrogen atoms. Nonane consists of 9 carbon atoms and 20 hydrogen atoms. Therefore, nonane can be classified as a hydrocarbon because it only contains carbon and hydrogen atoms. The correct answer choice is D.

Hydrocarbons may be classified as either aliphatic or aromatic. Aromatic hydrocarbons describe a class of compounds that include at least one planar, cyclic, carbon-based structure of alternating single and double bonds. Aliphatic hydrocarbons consist of carbon atoms bonded in open chains by either single, double, or triple bonds. They also include cyclic hydrocarbons that may contain single or double bonds.

Examples of aliphatic and aromatic hydrocarbons are shown in the following table.

When all of the bonds between the carbon and hydrogen atoms are single covalent bonds, the hydrocarbon is said to be saturated because the compound contains the maximum number of hydrogen atoms that can bond with the number of carbon atoms present.

Definition: Saturated Hydrocarbon

A saturated hydrocarbon is a hydrocarbon that only consists of single covalent bonds between carbon atoms.

This group of organic compounds are collectively called alkanes. Alkanes have the general formula CH2+2, where 𝑛 is an integer representing the number of carbon atoms in the molecule.

Definition: Alkane

An alkane is a saturated hydrocarbon with the general formula CH2+2.

Examples of alkanes are shown in the table below.

Hydrocarbons may also be unsaturated, meaning that the molecule contains at least one double or triple carbon–carbon bond.

Definition: Unsaturated Hydrocarbon

An unsaturated hydrocarbon is one that contains at least one double or triple carbon–carbon bond.

Alkenes are a group of unsaturated hydrocarbons that contain at least one carbon–carbon double bond. They have the general formula CH2, where 𝑛 is an integer representing the number of carbon atoms in the molecule.

Definition: Alkene

An alkene is an unsaturated hydrocarbon that contains at least one carbon–carbon double bond. Its general formula is CH2.

Examples of alkenes are shown in the table below.

Alkynes are another type of unsaturated hydrocarbon. They contain at least one carbon–carbon triple bond and have the general formula CH22.

Definition: Alkyne

An alkyne is an unsaturated hydrocarbon that contains at least one carbon–carbon triple bond. Its general formula is CH22.

Examples of alkynes are shown below.

Example 2: Calculating the Number of Atoms of Hydrogen in an Alkane

Fill in the blank: The number of hydrogen atoms in an alkane containing 2 carbon atoms is .

Answer

The general formula of an alkane is CH2+2, where 𝑛 is equal to the number of carbon atoms. The number of carbon atoms in this alkane is two. The number of hydrogen atoms can be calculated by substituting two for 𝑛 in the equation 2𝑛+2. Therefore, the number of hydrogen atoms is (2×2)+2, or six hydrogen atoms.

We should fill in the blank with “6 hydrogen atoms.

The simplest alkane contains only one carbon atom. Using the general formula, where 𝑛=1, we can determine that this alkane must contain four hydrogen atoms. The alkane can be represented by the molecular formula CH4. In examining the bonding between the atoms, the carbon atom must be located in the center of the four hydrogen atoms. The Lewis dot diagram (left) and the ball-and-stick model (right) of methane are shown below.

Definition: Molecular Formula

The molecular formula is a chemical formula expressing the exact number and type of atoms of each element in a molecule.

The next largest alkane contains two carbon atoms. Using the general formula, where 𝑛=2, we can determine that this alkane must contain six hydrogen atoms and therefore has the molecular formula CH26. An alkane containing three carbon atoms must contain eight hydrogen atoms and have the molecular formula CH38. In each of these alkanes, the carbon atoms must be bonded together in a continuous chain. The hydrogen atoms then bond with each of the carbon atom’s remaining valence electrons. The ball-and-stick models of these molecules are shown below (left: CH26, right: CH38).

We can give each alkane a name that indicates the number of carbon atoms in the structure.

A straight-chain alkane is an alkane whose carbon atoms are bonded in one continuous chain.

Definition: Straight-Chain Hydrocarbon

A straight-chain hydrocarbon is a hydrocarbon whose carbon atoms are bonded together in one continuous chain.

The table below shows the names of the first ten straight-chain alkanes.

Number of Carbon Atoms in a Straight ChainMolecular FormulaPrefix-Name of Hydrocarbon
1CH4Meth-Methane
2CH26Eth-Ethane
3CH38Prop-Propane
4CH410But-Butane
5CH512Pent-Pentane
6CH614Hex-Hexane
7CH716Hept-Heptane
8CH818Oct-Octane
9CH920Non-Nonane
10CH1022Dec-Decane

The alkane name consists of a prefix, which indicates the number of carbon atoms in the continuous chain, and a suffix, “-ane,” which indicates that the compound is an alkane.

Example 3: Recognizing the First Seven Straight-Chain Alkanes

Complete the alkanes in the correct order: , ethane, , butane, pentane, , heptane.

Answer

Ethane represents a straight-chain alkane (-ane) that contains two carbon atoms (eth-). Butane is a straight-chain alkane that contains four carbon atoms. Pentane has five carbon atoms, and heptane has seven carbon atoms.

This means that we are missing the names for the alkanes containing one, three, and six carbon atoms. The prefix for one carbon atom is “meth-.” The alkane containing one carbon atom is therefore called methane. The prefix for three carbon atoms is “prop-,” giving us the alkane propane, and the prefix for six carbon atoms is “hex-,” giving us the alkane hexane. The missing alkanes in order are methane, propane, and hexane.

When an alkane contains four carbon atoms and therefore ten hydrogen atoms, CH410, there are two possible ways to connect the atoms together. The carbon atoms can either bond together in one continuous chain, called butane, or three carbon atoms can form a chain with the remaining carbon atom bonding to the second carbon atom in the chain. This second structure is called 2-methylpropane. “Propane” indicates that there are three carbon atoms in a continuous chain and “2-methyl” indicates that there is a single carbon connected to the second carbon in the continuous chain. Below are the ball and stick models of butane (left) and 2-methylpropane (right).

Each of these possible structures represents a structural isomer of CH410. Structural isomers are molecules that have the same molecular formula but differ in the way in which their atoms are bonded. As the number of atoms in a molecule increases, so does the number of structural isomers.

Definition: Structural Isomer

Structural isomers are molecules that have the same molecular formula but differ from one another in how their atoms are connected.

As butane and 2-methylpropane have the same molecular formula, we must have ways to distinguish them from one another. We could, of course, use models like the ball-and-stick molecular models or give each structure a different name. We can also use a variety of other representations, namely, displayed formulas, structural formulas, and condensed formulas.

A displayed formula shows all of the atoms and bonds in the molecule with lines representing the bonds. Each line corresponds to two shared electrons. As such, single bonds are represented by one line, double bonds with two lines, and triple bonds with three lines.

Definition: Displayed Formula

A displayed formula is a representation of a molecule showing all of the atoms and bonds. Bonds are represented by lines.

Shown below is the displayed formula of butane (left) and 2-methylpropane (right).

C C C C H H H H H H H H H H C C C C H H H H H H H H H H

Notice that the displayed formula represents the molecule as being flat with 90 angles between the carbon and hydrogen atoms. In actuality, these molecules are three-dimensional, and the angle between the carbon and hydrogen atoms is closer to 109. Shown below is the displayed formula of methane and two three-dimensional representations of methane with the correct 109.5 angles between the atoms.

Example 4: Determining the General Formula of a Compound from a Displayed Formula

What is the general formula for the group of compounds to which the following displayed formula is related?

Answer

Looking at the displayed formula, we can see that two of the carbon atoms are connected to one another with three lines.

Each line in a displayed formula represents two shared electrons. Three lines, therefore, represent six shared electrons. This is a triple bond.

Hydrocarbons that contain at least one triple bond are called alkynes. They have the general formula CH22, where 𝑛 represents the number of carbon atoms in the molecule. We can verify that this is the correct general formula. There are six carbon atoms in the displayed formula. We can substitute six for 𝑛 in the general formula to determine the molecular formula: CHCH·62(6)2610

The molecular formula for an alkyne with six carbon atoms shows that the alkyne must have ten hydrogen atoms. There are indeed ten hydrogen atoms in the displayed formula. The general formula is CH22.

There are also formulas that allow us to represent isomers in text rather than with a picture. A structural formula lists atoms in the order in which they are connected. Hydrogen atoms are often grouped with the carbon atoms to which they are attached to form units such as CH3, CH2, and CH. Let us look at the displayed formula of butane, shown below, and see how to derive the structural formula.

(1)(2)(3)(4)CCCCHHHHHHHHHH

Looking at the displayed formula, we can see that butane consists of four carbon atoms bonded in a continuous chain. The leftmost carbon atom, labeled carbon number one, is bonded to three hydrogen atoms. This can be written as the unit CH3. Carbon atom number two is bonded to two hydrogen atoms. This can be written as the unit CH2. Carbon atom number three can also be represented as a CH2 unit, and carbon atom number four can be represented as a CH3 unit. We write the units in a line in the order in which they are connected to form the structural formula CHCHCHCH3223.

Definition: Structural Formula

The structural formula is a text representation of the bonding in a molecule whereby the atoms are written in the order in which they are connected.

Structural formulas can often become quite lengthy. We can shorten the formula by combining like consecutive units. We do so by writing the repeating unit in parentheses, followed by a subscript value indicating the number of times the unit repeats. This shortened formula is called the condensed formula.

Definition: Condensed Formula

The condensed formula is a text representation of the bonding in a molecule whereby the atoms are written in the order in which they are connected. Like repeating units are combined in parentheses followed by a subscript value to indicate the number of times the unit repeats.

Let us once again look at butane. The structural formula is CHCHCHCH3223. Notice that the unit CH2 appears twice in a row. We can combine these repeating units by writing the unit in parentheses, followed by a subscript two to indicate that the unit CH2 appears twice. This gives us the condensed formula of CH(CH)CH3223. Shown below is the process of deriving the condensed and structural formulas from the displayed formula of butane.

Let us practice writing the displayed, structural, and condensed formula of hexane. First, we need to recognize that “hex-” means that there are six carbon atoms in a continuous chain, and “-ane” means that all of the bonds in the molecule are single covalent bonds. When building the displayed formula, we can begin by connecting the six carbon atoms in a row using single lines.

CCCCCC

Next, we need to recognize that hexane is a hydrocarbon. This means that the structure will also contain hydrogen atoms in addition to the carbon atoms shown. We also need to recall that carbon atoms have four valence electrons that can form four single bonds. We should complete the displayed formula by ensuring that each carbon atom is surrounded by four single lines and that a hydrogen atom is shown at the end of each newly drawn line.

CCCCCCHHHHHHHHHHHHHH

We should double-check that the displayed formula has the correct number of carbon and hydrogen atoms. The displayed formula has six carbon atoms and 14 hydrogen atoms. The general formula of an alkane is CH2+2, where 𝑛 is the number of carbon atoms. An alkane with 6 carbon atoms should contain (2×6)+2, or 14, hydrogen atoms. The molecular formula of hexane, CH614, and the displayed formula contain the same number of carbon and hydrogen atoms.

We can break the displayed formula into carbon–hydrogen units to write the structural formula.

(1)(2)(3)(5)(6)(4)CCHHHHHCCHHHHCCHHHHH

Carbon atom numbers one and six are attached to three hydrogen atoms each. They can be represented by the unit CH3. Carbon atom numbers two, three, four, and five are attached to two hydrogen atoms each. They can be represented by the unit CH2. We place the units in order from carbon atom number one to carbon atom number six to give us the structural formula CHCHCHCHCHCH322223.

To write the condensed formula, we look for repeating units in the structural formula. We can see that CH2 appears four times in a row. We can combine these like units together by writing the unit CH2 in parentheses, followed by a subscript four to indicate that the unit repeats four times. This gives us the condensed formula CH(CH)CH3243.

We should also be able to determine the structural formula, displayed formula, and alkane name from a condensed formula. Let us practice by looking at the condensed formula, CH(CH)CH3233. Notice that there is a subscript three outside of the parentheses. This means that the CH2 unit should appear three times in the structural formula. Expanding the condensed formula gives us the structural formula CHCHCHCHCH32223.

To draw the displayed formula, we follow along with the structural formula. On the leftmost side, there should be a carbon atom connected to three hydrogen atoms. This first carbon atom should then be connected to another carbon atom that is itself connected to two hydrogen atoms. By reading through the structural formula, we can create the following displayed formula.

CCHHHCHHHCHHCHHHH

Finally, we see that this structure contains five carbon atoms that are single bonded to one another in a straight chain. The prefix for five carbon atoms is “pent-” and the suffix to indicate a hydrocarbon that only contains single bonds is “-ane.” This molecule has the name pentane.

Example 5: Determining the Structural Formula of an from an Alkane Name

What is the structural formula of propane?

Answer

To begin, we must first decipher the name propane. The prefix “prop-” indicates that the molecule contains three carbon atoms. The suffix “-ane” indicates that the molecule is a hydrocarbon that only contains single bonds.

We can begin to draw a displayed formula by single bonding the three carbon atoms in a row.

CCC

Next, we can recognize that each carbon atom can form four single bonds. We draw additional lines so that each carbon atom has four total lines around it. We add hydrogen atoms to each newly drawn line. This gives us the following displayed formula.

CCCHHHHHHHH

We can derive the structural formula, a text representation of the bonding in a molecule whereby the atoms are written in the order in which they are connected, by breaking the displayed formula into discrete carbon–hydrogen units. The leftmost carbon atom is connected to three hydrogen atoms. We can represent this as the unit CH3. The middle carbon atom is connected to two hydrogen atoms. We can represent this as the unit CH2. The rightmost carbon atom is connected to three hydrogen atoms and can be represented as the unit CH3. We put the units in order from left to right to write the structural formula. The structural formula of propane is CHCHCH323.

Key Points

  • A hydrocarbon is a compound containing only carbon and hydrogen atoms.
  • Hydrocarbons can further be classified as alkanes, alkenes, or alkynes.
  • Straight-chain alkanes can be named by attaching a prefix indicating the number of carbon atoms to the suffix “-ane.”
  • A displayed formula is a pictorial representation of a molecule where lines are drawn to represent the bonds between each atom.
  • A structural formula is a text representation of a molecule where the atoms are listed in the order in which they are bonded.
  • A condensed formula is a shorthand version of a structural formula whereby like units are combined together in parentheses.

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