Chemistry 350 Organic Chemistry I

Study Guide :: Unit 5

Stereochemistry at Tetrahedral Centres

Unit Preview

This unit introduces the concept of chirality, and discusses the structure of compounds containing one or two chiral centres. A convenient method of representing the three-dimensional arrangement of the atoms in chiral compounds is explained. Considerable emphasis is placed on the use of molecular models to assist in the understanding of the phenomenon of chirality.

The unit examines stereochemistry—the three-dimensional nature of molecules. The subject is introduced using the experimental observation that certain substances have the ability to rotate plane-polarized light. Finally, certain reactions of alkenes are re-examined in light of the new material encountered in this unit.

Unit Objectives

After you have completed Unit 5, you should be able to

  1. fulfill all of the detailed objectives listed under each individual section.
  2. use molecular models in solving problems on stereochemistry.
  3. solve road-map problems that include stereochemical information.
  4. define, and use in context, the new key terms.

5.1  Enantiomers and the Tetrahedral Carbon

Objectives

After completing this section, you should be able to

  1. use molecular models to show that only a tetrahedral carbon atom satisfactorily accounts for the lack of isomerism in molecules of the type CH2XY, and for the existence of optical isomerism in molecules of the type CHXYZ.
  2. determine whether two differently oriented wedge-and-broken-line structures are identical or represent a pair of enantiomers.

Learning Activities

  1. Read 5.1 Enantiomers and the Tetrahedral Carbon and do any associated exercises.
  2. Use a set of molecular models to do the following manipulations.
    1. Construct the two mirror isomers of CH2XY. Convince yourself that a tetrahedral carbon atom gives rise to only one isomer of CH2XY.
    2. Construct a model of CHXYZ. Now construct a second model that is its mirror image. Convince yourself that the models are not superimposable.

Hint: To do so, try placing the second model on top of the first in such a way that atom X of one model is on top of atom X in the second model. You should find that it is impossible to superimpose the two models.

5.2  The Reason for Handedness in Molecules: Chirality

Objectives

After completing this section, you should be able to

  1. determine whether or not a compound is chiral, given its Kelulé, condensed or shorthand structure, with or without the aid of molecular models.
  2. label the chiral centres (carbon atoms) in a given Kelulé, condensed or shorthand structure.

Learning Activities

Read 5.2 The Reason for Handedness in Molecules: Chirality and do any associated exercises.

5.3  Optical Activity

Objectives

After completing this section, you should be able to

  1. describe the nature of plane-polarized light.
  2. describe the features and operation of a simple polarimeter.
  3. calculate the specific rotation of a compound, given the relevant experimental data.

Learning Activities

Read 5.3 Optical Activity and do any associated exercises.

5.4  Pasteur’s Discovery of Enantiomers

Objective

After completing this section, you should be able to discuss how the results of work carried out by Biot and Pasteur contributed to the development of the concept of the tetrahedral carbon atom.

Learning Activities

Read 5.4 Pasteur’s Discovery of Enantiomers and do any associated exercises.

5.5  Sequence Rules for Specifying Configuration

Objectives

After completing this section, you should be able to

  1. assign Cahn-Ingold-Prelog priorities to a given set of substituents.
  2. determine whether a given wedge-and-broken-line structure corresponds to an R or an S configuration, with or without the aid of molecular models.
  3. draw the wedge-and-broken-line structure for a compound, given its IUPAC name, complete with R or S designation.
  4. construct a stereochemically accurate model of a given enantiomer from either a wedge-and-broken-line structure or the IUPAC name of the compound, complete with R or S designation.

Learning Activities

Read 5.5 Sequence Rules for Specifying Configuration and do any associated exercises.

5.6  Diasteromers

Objectives

After completing this section, you should be able to

  1. calculate the maximum number of stereoisomers possible for a compound containing a specified number of chiral carbon atoms.
  2. draw wedge-and-broken-line structures for all possible stereoisomers of a compound containing two chiral carbon atoms, with or without the aid of molecular models.
  3. assign R,S configurations to wedge-and-broken-line structures containing two chiral carbon atoms, with or without the aid of molecular models.
  4. determine, with or without the aid of molecular models, whether two wedge-and-broken-line structures containing two chiral carbon atoms are identical, represent a pair of enantiomers, or represent a pair of diastereomers.
  5. draw the wedge-and-broken-line structure of a specific stereoisomer of a compound containing two chiral carbon atoms, given its IUPAC name and R,S configuration.

Learning Activities

  1. Read 5.6 Diasteromers and do any associated exercises.
  2. To obtain maximum benefit from this section, build models of the four stereoisomers of 3‑bromo-2‑butanol shown in the LibreText.

5.7  Meso Compounds

Objectives

After completing this section, you should be able to

  1. determine whether or not a compound containing two chiral carbon atoms will have a meso form, given its Kekulé, condensed or shorthand structure, or its IUPAC name.
  2. draw wedge-and-broken-line structures for the enantiomers and meso form of a compound such as tartaric acid, given its IUPAC name, or its Kekulé, condensed or shorthand structure.
  3. make a general comparison of the physical properties of the enantiomers, meso form and racemic mixture of a compound such as tartaric acid.

Learning Activities

  1. Read 5.7 Meso Compounds and do any associated exercises.
  2. Make models of each of the two compounds that are “rotated” in the reading, to convince yourself that these pairs of stereoisomers are indeed identical.

5.8  Racemic Mixtures and the Resolution of Enantiomers

Objective

After completing this section, you should be able to explain why racemic mixtures do not rotate plane-polarized light.

Learning Activities

Read 5.8 Racemic Mixtures and the Resolution of Enantiomers and do any associated exercises.

5.9  A Review of Isomerism

Objective

After completing this section, you should be able to explain the differences among constitutional (structural) isomers and stereoisomers (geometric isomers).

Learning Activities

Read 5.9 A Review of Isomerism and do any associated exercises.

5.10  Chirality at Nitrogen, Phosphorus, and Sulfur

Objectives

After completing this section, you should be able to

  1. recognize that atoms other than carbon can be chiral centres.
  2. explain why enantiomers of compounds such as ethylmethylamine cannot normally be isolated.

Learning Activities

Read 5.10 Chirality at Nitrogen, Phosphorus, and Sulfur and do any associated exercises.

5.11  Prochirality

Objectives

After completing this section, you should be able to

  1. identify a compound as being prochiral.
  2. identify the Re and Si faces of prochiral sp2 centre.
  3. identify atoms (or groups of atoms) as pro-R or pro-S on a prochiral sp3 centre.

Learning Activities

Read 5.11 Prochirality and do any associated exercises.

5.12  Chirality in Nature and Chiral Environments

Objective

After completing this section, you should be able to explain how chiral molecules in nature can have such dramatically different biological properties.

Learning Activities

Read 5.12 Chirality in Nature and Chiral Environments and do any associated exercises.

Summary

Stereochemistry is a very difficult topic for a distance education student to master—be sure to make full use of molecular models. When you are confident that you understand the material presented in Units 3-5, please complete Self Test 1.