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25.2: Hydrocarbons

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Lesson Objectives

The student will:

  • define structural isomers and be able to draw isomers for selected compounds
  • differentiate between alkanes, alkenes, and alkynes
  • name alkanes, alkenes, and alkynes when given a structure
  • draw alkanes, alkenes, and alkynes when given a name
  • explain the difference between saturated and unsaturated hydrocarbons

Vocabulary

  • alkane
  • alkene
  • alkyne
  • functional group
  • saturated compound
  • structural isomers
  • unsaturated compound

Introduction

The basic skeleton of any organic molecule consists primarily of carbon and hydrogen atoms bounded to one another in various arrangements. In this section, we will explore various types of organic compounds that include only carbon and hydrogen. In a later section, we will consider the additional possibilities that exist when other atoms are bound to a hydrocarbon backbone.

Alkanes

Alkanes are the simplest hydrocarbons. All carbon atoms are sp3 hybridized, and each carbon atom makes four sigma bonds. Depending on the size of the chain, alkanes can exist as solids, liquids, or gases at room temperature. Alkanes are non-polar molecules, so they are not soluble in water.

At low temperatures, alkanes are generally not very reactive compared to most other organic molecules. However, when provided with an initial source of energy, alkanes can react with oxygen (a combustion reaction) to produce carbon dioxide and water. Because of this property, many alkanes are used as fuels, including methane (natural gas), propane (used in gas grills), butane (found in lighters), and octane (found in gasoline). Larger alkanes exist as waxy solids. Since they burn less quickly than their smaller counterparts, they are used to make candles.

Naming Alkanes

Due to the immense variety of possible organic structures that can exist, it is necessary to have a systematic method for naming these compounds so that chemists can communicate with one another. There are a set of fairly straightforward rules that will allow you to name most of the organic compounds you will encounter.

The basic starting point for learning about organic nomenclature is to look at the straight-chain alkanes. The first ten are compiled in Table below. Note that although the structures are drawn in a linear format, the shape around each carbon atom is still tetrahedral. Thus, the actual three-dimensional structure of these compounds would look more like a zig-zag.

First Ten Alkanes
Name Formula Structural Formula
methane \mathrm{CH}_4
ethane \mathrm{C}_2\mathrm{H}_6
propane \mathrm{C}_3\mathrm{H}_8
butane \mathrm{C}_4\mathrm{H}_{10}
pentane \mathrm{C}_5\mathrm{H}_{12}
hexane \mathrm{C}_6\mathrm{H}_{14}
heptane \mathrm{C}_7\mathrm{H}_{16}
octane \mathrm{C}_8\mathrm{H}_{18}
nonane \mathrm{C}_9\mathrm{H}_{20}
decane \mathrm{C}_{10}\mathrm{H}_{22}

The names of these simple compounds consist of two parts. As you may be able to deduce from the examples, the suffix “-ane” indicates that these compounds are alkanes. The first part of the name indicates how many carbon atoms are in the chain. The prefixes associated with each number are shown in Table below. These should be committed to memory.

Numerical Prefixes for Straight Chain Organic Compounds
Number of C atoms in the longest chain Numerical Prefix
1 meth-
2 eth-
3 prop-
4 but-
5 pent-
6 hex-
7 hept-
8 oct-
9 non-
10 dec-

Example:

Name the following molecule:

\mathrm{CH}_3 - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_2 - \mathrm{CH}_3

Solution:

This is a straight chain hydrocarbon with all single bonds. Therefore, it is an alkane. Since the longest straight chain contains 5 carbon atoms, it is pentane.

You should recognize that the actual molecule does not look like this straight chain picture. The bonding around every carbon atom is a tetrahedron. Therefore, these molecules look more like saw teeth than they do straight chains. They are drawn in straight lines to make it easier to draw them. If the same molecule is drawn with a bend in it, it is still a straight chain (see below).

This molecule is still a 5-carbon straight chain and is still pentane.

Various side chains of carbon atoms can be attached to the straight chain. When a hydrocarbon side chain is attached to the straight chain, the side chain is named according to the following Table below.

Alkyl Groups Branches
Alkyl Groups Group Name
-\mathrm{CH}_3 methyl
-\mathrm{CH}_2\mathrm{CH}_3 ethyl
-\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3 propyl
-\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3 butyl
-\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3 pentyl

A side chain can be attached to any carbon but the end carbons in a straight chain. If the chain were attached to an end carbon, it would be part of the straight chain. Here is a straight chain hydrocarbon with a methyl side chain attached.

When naming this compound, we must first identify both the parent chain and the side chain. The parent chain is the longest straight chain of carbon atoms. Be careful, because this chain is not always drawn in a straight line. Additionally, we must indicate the carbon atom to which the side chain is attached. In order to identify the carbon atom to which the side chain is attached, we number the carbon atoms in the parent straight chain. Since it is possible to begin numbering at either end of the straight chain, there is a rule about how the carbon atoms are numbered. The rule is that you must number the carbon atoms in the straight chain so that the side chain is attached to the lowest possible numbered carbon atom. Here is the compound again with two possible numbering sequences. In the numbering on the left, the side chain is attached to carbon number 3, whereas on the right, the side chain is attached to carbon number 5. Therefore, the numbering sequence on the right is wrong.

To name this compound, we indicate the number of the carbon atom to which the side chain is attached, insert a hyphen after the number (no spaces in between), then name the side chain (again no spaces), and finally name the parent straight chain (again no spaces). Therefore, the correct name of this compound is 3-methylheptane.

If two or more of the same type of side chain is present, this is indicated with a slightly different set of numerical prefixes. “Mono-” is not used because the prefix is omitted when only one of that particular side chain is present. “Eth-,” “prop-,” and “but-” are replaced with “di-,” “tri-,” and “tetra-,” respectively. The remaining prefixes are the same as those shown above in Table above.

If multiple different types of side chains are present, they are written in alphabetical order. When alphabetizing names, we ignore the secondary prefix that indicates how many of each side chain there are. Therefore, “ethyl” comes before “dimethyl.”

Example:

Name the following molecule:

Step 1: Find the parent chain.

The longest chain in this molecules contains 8 carbon atoms. Therefore, the parent chain is octane.

Step 2: Name the branches and determine the carbon numbers to which they are attached. Looking at the molecule, there are two branches that are not part of the parent chain. Using Table above, these branches can be identified as a methyl group and an ethyl group.

Step 3: Add the name of the branches and their positions to the parent chain name.

5-ethyl-3-methyloctane

Notice that there are no spaces anywhere in the name.

Drawing Alkanes

So far, we have been given the structure of the molecule and asked to provide a name. However, there are also cases where we will be given a name and asked to draw the structure. Converting names to structures is generally easier than converting structures to names.

Example:

Draw the structure for butane, \mathrm{C}_4\mathrm{H}_{10}.

Solution:

Step 1: Start by drawing the four carbon atoms in a straight chain with all the atoms connected by single bonds.

\mathrm{C} - \mathrm{C} - \mathrm{C} - \mathrm{C}

Step 2: Add single bonds until each carbon atom has four possible bonding sites.

Step 3: Add one hydrogen atom to the end of each of the single bonds formed in Step 2.

Prior to this chapter, we have mainly been considering compounds that can be uniquely identified simply by writing their chemical formula. However, this is not the case for organic molecules. In the previous example, you were asked to draw the structure for butane, \mathrm{C}4\mathrm{H}10. However, this is not the only way that those atoms could be arranged. Drawn below is 2-methylpropane. It is still composed of four carbon atoms and ten hydrogen atoms, but they are arranged in a different way. Two molecules that have the same formula but different bonding arrangements are referred to as structural isomers.

Structural isomers have the same formula, but because they have different structures, they also have different chemical properties. As the number of carbon atoms increases, the number of possible isomers increases. For example, decane has 75 structural isomers, each with its own properties. A compound such as \mathrm{C}_{30}\mathrm{H}_{62} has 4 \times 10^8 isomers, many of which have not even been isolated yet. With this many possible compounds, you can understand the need for this complex naming system.

Example:

Draw the structural isomers of pentane, \mathrm{C}_5\mathrm{H}_{12}.

Solution:

Alkenes

Alkanes provide the basic skeleton for organic molecules. Although there are vast numbers of alkanes that could potentially be constructed, the chemistry available to alkanes is quite similar and is relatively limited. Generally, the reactivity of an organic molecule is determined by its functional groups. Functional groups include double bonds, triple bonds, and groups that contain atoms other than carbon and hydrogen. The first functional group we will consider is an alkene, which is defined as a double bond between two carbon atoms.

When a molecule has only one functional group, sometimes the name of the functional group is used to refer to the entire molecule. For example, the entire 2-hexene molecule can be referred to as an alkene, even though the only double bond is between carbons 2 and 3.

Because the carbons in an alkene are not surrounded by the maximum number of other atoms, alkenes are referred to as unsaturated. Conversely, alkanes are referred to as saturated, since no additional atoms can be added to any of the carbons without removing some of the ones already present.

As we stated earlier, simple alkanes are generally not very reactive unless enough energy is added to start a combustion reaction. This can be explained by the fact that alkanes contain only C-C and C-H sigma bonds, both of which are relatively strong. In contrast, alkenes contain pi bonds, which are not as strong. Much of the chemistry of alkenes involves breaking this relatively weak bond to create two new sigma bonds between the carbons of the alkene and atoms from other molecules.

Naming Alkenes

The naming of alkenes is very similar to that for alkanes, except for a few key differences:

  1. The end of the name is changed from “-ane” to “-ene.”
  2. The parent chain should always include the double bond, even if it is no longer the longest chain of carbon atoms.
  3. Numbering begins at the end of the parent chain closest to the double bond
  4. The position of the double bond is indicated by inserting the number of the carbon closest to the end just before the name of the parent chain.

Example:

Name the following compound.

\mathrm{H}_3\mathrm{C} - \mathrm{CH} = \mathrm{CH} - \mathrm{CH}_2 - \mathrm{CH}_3

Solution:

The parent chain has five (5) carbon atoms. Using Table above, the prefix for this compound is “pent-.”

There are no branches, but the double bond is between carbons 2 and 3. Therefore the name of the molecule is 2-pentene (using the smaller of the two numbers on the carbon atoms).

Drawing Alkenes

Drawing alkenes also follows the same rules as drawing alkanes. Remember to place the double bond where the name indicates its placement.

Example:

Draw the structural formula for 4–methyl–2– heptene.

Solution:

Looking at the name reveals a great deal about how to draw the structure.

Therefore, the molecule has the structure:

When numbering alkene chains, you always begin with the carbon end nearest to the double bond regardless of what this does to the side chain number.

Alkynes

An alkyne is a carbon-carbon triple bond. It can also refer to a molecule containing a carbon-carbon triple bond. Like alkenes, alkynes are considered unsaturated because the carbons in the triple bond are not surrounded by the maximum number of other atoms.

Naming Alkynes

Alkynes are names in the same way as alkenes, except the suffix at the end of the parent chain is changed to “-yne.” For these compounds, the position of the triple bond is indicated in the name of the structure.

Example:

Name the following structure.

Solution:

The parent chain has six carbon atoms. Using Table above, the prefix is “hex-.”

There is a triple bond, so the suffix is “-yne.”

The triple bond is between carbons 2 and 3.

There is one branch on carbon 4 that is a methyl group.

Therefore the name of the molecule is 4-methyl-2-hexyne.

Drawing Alkynes

To draw alkynes, again follow the same rules as for alkanes and alkenes. Remember to place the triple bond where the name indicates its placement.

Example:

Draw the structural formula for 5-ethyl-3-octyne.

Solution:

Looking at the name reveals a great deal about how to draw the structure.

Therefore, the molecule has the structure:

This video is an introduction to process of naming hydrocarbons (10d), see http://www.youtube.com/watch?v=cmzHMHw6oeA (9:02).

Lesson Summary

  • Alkanes are relatively unreactive hydrocarbons that contain only C-C and C-H single bonds.
  • Alkenes contain a carbon-carbon double bond.
  • Alkynes contain a carbon-carbon triple bond.
  • Organic molecules containing only single bonds are referred to as saturated, while those containing double or triple bonds are referred to as unsaturated.

Further Reading / Supplemental Links

The learner.org website allows users to view the Annenberg series of chemistry videos. You are required to register before you can watch the videos, but there is no charge to register. The video called “Molecular Architecture” examines isomers (primarily organic molecules)and how the electronic structure of a molecule's elements and bonds affects its shape and physical properties.

Review Questions

  1. Define the terms alkane, alkene and alkyne.
  2. What is the difference between a saturated and an unsaturated compound?
  3. Which of the following organic compounds is unsaturated?
    1. ethylcyclobutane
    2. 3-ethyl-2-methyl-1-pentene
    3. 2-bromobutane
    4. 2-methyl-1-chlorohexane
  4. Which compound is a structural isomer of the compound shown below?
    1. butane
    2. methane
    3. pentane
    4. hexane
  5. Which structures are isomers of one of the other structures?
    1. I, II, III
    2. II, III
    3. I, II, IV
    4. They are all isomers.
  6. Which of the following structures has the shortest parent chain? (There can be more than one correct answers.)
  7. Draw each of the following compounds.
    1.  2,3,4-trimethylpentane
    2. 2-chloro-1-propene
    3. 1-bromo-2-methylbutane
    4. Ethyne
    5. 1-bromo-5,5-dimethylheptane
  8. Name each of the following structures.
  9. Name the isomers for \mathrm{C}_6\mathrm{H}_{14}.

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