Chemistry 350 Organic Chemistry I

Study Guide :: Unit 3

Organic Compounds: Alkanes and Their Stereochemistry

Unit Preview

This unit begins with an introduction to the concept of the functional group, a concept that facilitates the systematic study of organic chemistry. Next, we introduce the fundamentals of organic nomenclature (i.e., the naming of organic chemicals) through examination of the alkane family of compounds. We then discuss, briefly, the occurrence and properties of alkanes, and the unit closes with a description of cis-trans isomerism in cycloalkanes.

Note: Appendices A, B and C provide information about thermodynamics, kinetics, and selected physical constants, respectively.

Unit Objectives

After you have completed Unit 3, you should be able to

  1. fulfill the detailed objectives listed under each section.
  2. identify some of the commonest functional groups.
  3. write the structures and names of the first ten straight-chain alkanes.
  4. recognize and name the simple alkyl substituents, and give the systematic names for branched-chain alkanes.
  5. briefly describe some of the processes used during the refining of petroleum.
  6. briefly describe the physical properties of alkanes.
  7. draw a number of possible conformations of some simple alkanes and alkane-like compounds, and represent the energies of such conformations on energy versus rotation diagrams.
  8. define, and use in context, the new key terms identified in the LibreText readings.

3.1  Functional Groups

Objectives

After completing this section, you should be able to

  1. explain why the properties of a given organic compound are largely dependent on the functional group or groups present in the compound.
  2. identify the functional groups present in each of the following compound types: alkenes, alkynes, arenes, (alkyl and aryl) halides, alcohols, ethers, aldehydes, ketones, esters, carboxylic acids, (carboxylic) acid chlorides, amides, amines, nitriles, nitro compounds, sulfides and sulfoxides.
  3. identify the functional groups present in an organic compound, given its structure.
  4. identify to which of the compound types listed under Objective 2, above, an organic compound containing a single functional group belongs, given its structure.
  5. draw the structure of a simple example of each of the compound types listed in Objective 2.

Learning Activities

Read 3.1 Functional Groups and do any associated exercises.

3.2  Alkanes and Alkane Isomers

Objectives

After completing this section, you should be able to

  1. draw the Kekulé structure, condensed structure and shorthand structure of each of the first ten straight-chain alkanes.
  2. name each of the first ten straight-chain alkanes, given its molecular formula, Kekulé structure, condensed structure or shorthand structure.
  3. explain the difference in structure between a straight- and a branched-chain alkane, and illustrate the difference using a suitable example.
  4. explain why the number of possible isomers for a given molecular formula increases as the number of carbon atoms increases.
  5. draw all the possible isomers that correspond to a given molecular formula of the type CnH2n+2, where n is ≤ 7

Learning Activities

Read 3.2 Alkanes and Alkane Isomers and do any associated exercises.

3.3  Alkyl Groups

Objectives

After completing this section, you should be able to

  1. recognize and name any alkyl group that can be considered to have been formed by the removal of a terminal hydrogen atom from a
  2. straight-chain alkane containing ten or fewer carbon atoms.
  3. explain what is meant by a primary, secondary, tertiary or quaternary carbon atom.
  4. represent the various types of organic compounds using the symbol “R” to represent any alkyl group.

Learning Activities

Read 3.3 Alkyl Groups and do any associated exercises.

3.4  Naming Alkanes

Objectives

After completing this section, you should be able to

  1. provide the correct IUPAC name for any given alkane structure (Kekulé, condensed or shorthand).
  2. draw the Kekulé, condensed or shorthand structure of an alkane, given its IUPAC name.

Learning Activities

Read 3.4 Naming Alkanes and do any associated exercises.

3.5  Properties of Alkanes

Objectives

After completing this section, you should be able to

  1. arrange a number of given straight-chain alkanes in order of increasing or decreasing boiling point, melting point or density.
  2. arrange a series of isomeric alkanes in order of increasing or decreasing boiling point.
  3. explain the difference in boiling points between a given number of alkanes.

Learning Activities

Read 3.5 Properties of Alkanes and do any associated exercises.

3.6  Conformations of Ethane

Objectives

After completing this section, you should be able to

  1. explain the concept of free rotation about a carbon-carbon single bond.
  2. explain the difference between conformational isomerism and the other types of isomerism which you have encountered.
  3. represent the conformers of ethane by both sawhorse representation and Newman projection.
  4. sketch a graph of energy versus bond rotation for ethane, and discuss the graph in terms of torsional strain.

Learning Activities

  1. Read 3.6 Conformations of Ethane and do any associated exercises.

  2. Construct a model of ethane using the molecular model kit. If you are not familiar with the ball-and-stick models, review Sections 1.4, 1.7 and 2.13, and practise making models of simple organic compounds. To construct a model of ethane, first join two black, four-hole (carbon) balls with a rod. Next, insert a total of six $\ce{\sf{C-H}}$ bonds into the six vacant holes in the “carbon” atoms. Finally, attach a hydrogen atom (one-hole ball) to each of the $\ce{\sf{C-H}}$ bonds. Hold one of the carbon atoms in one hand while rotating the second carbon in the other. Notice the free rotation about the carbon-carbon bond. If you rotate the second carbon atom very slowly, you should realize that an infinite number of conformers is possible; however, only the lowest-energy conformer (the staggered conformer) and the highest-energy conformer (the eclipsed conformer) are of interest to us at this stage. In the staggered conformer, as you view the model from the end (holding it in such a way that the front carbon obliterates the rear one), the $\ce{\sf{C-H}}$ bonds at the front appear to bisect the $\ce{\sf{H-C-H}}$ bond angles of the rear carbon atom.

    You can obtain a model of the eclipsed conformer of ethane by taking the model of the staggered conformer and rotating the front carbon atom 60° in either direction. As you view the model from the end, you will understand the origin of the name for this conformer—the front atoms obliterate or eclipse those at the rear.

3.7  Conformations of Other Alkanes

Objectives

After completing this section, you should be able to

  1. depict the staggered and eclipsed conformers of propane (or a similar compound) using sawhorse representations and Newman projections.
  2. sketch a graph of energy versus bond rotation for propane (or a similar compound) and discuss the graph in terms of torsional strain.
  3. depict the anti, gauche, eclipsed and fully eclipsed conformers of butane (or a similar compound), using sawhorse representations and Newman projections.
  4. sketch a graph of energy versus ($\ce{\sf{C{2}-C{3}}}$) bond rotation for butane (or a similar compound), and discuss it in terms of torsional and steric repulsion.
  5. assess which of two (or more) conformers of a given compound is likely to predominate at room temperature from a semi-quantitative knowledge of the energy costs of the interactions involved.

Learning Activities

  1. Read 3.7 Conformations of Other Alkanes and do any associated exercises.

  2. Construct a model of propane by removing one of the $\ce{\sf{C-H}}$ rods and its attached hydrogen ball from the model of ethane used in the previous section, and replacing it with a methyl group formed by a rod, a black (carbon) ball, three $\ce{\sf{C-H}}$ rods and three hydrogen balls. Look at the model from the perspective shown in Figure 3.8 on page 96 of the textbook, and observe the eclipsed and staggered conformers.

    As in the two previous sections, your molecular model kit will assist you in understanding the material discussed here. Construct a model of butane using the appropriate rods and balls—you should know which ones to use by now. Begin with the anti conformation shown on the left-hand side of the figure at the bottom of page 96 of Organic Chemistry, 8th ed. and rotate the rear carbon atom 60 degrees at a time, while holding the front carbon still (i.e., rotate around the $\ce{\sf{C{2}-C{3}}}$ bond).

3.8  Gasoline: A Deeper Look

Objectives

After completing this section, you should be able to

  1. describe the general nature of petroleum deposits, and recognize why petroleum is such an important source of organic compounds.
  2. explain, in general terms, the processes involved in the refining of petroleum.
  3. define the octane number of a fuel, and relate octane number to chemical structure.

Learning Activities

Read 3.8 Gasoline: A Deeper Look and do any associated exercises.

Summary

In this unit you began your study of organic chemistry in earnest, although if you have completed Athabasca University’s Chemistry 218, you will already be familiar with a number of the topics discussed. You should review the contents of this unit. The self test at the end of Unit 5 will give you an idea of your progress through the initial part of the course.