II STRUCTURE, ISOMERISM AND NOMENCLATURE OF ORGANIC COMPOUNDS I. OBJECTIVES AND BACKGROUND This exercise will give you an opportunity to experience the three-dimensional nature of molecules and to visualize how structural formulas are used to represent three-dimensional molecules. You will become better acquainted with structural and stereoisomerism among organic compounds. Additionally, the exercise will demonstrate how organic nomenclature specifies the composition and structure of molecules, and even the spatial arrangement of atoms within a molecule. Before coming to laboratory, you should consult your lecture notes and carefully read the appropriate sections of your textbook to become familiar with the following concepts: structures of alkanes, alkenes, and cycloalkanes complete and condensed structural formulas general rules of organic nomenclature structural isomerism cis and trans stereoisomerism II. CEMICALS AND EQUIPMENT Each pair of students will be supplied with a set of molecular models. Each set should contain the following: 7 tetrahedral carbons 14 hydrogens 21 single bonds (rigid tubes) 2 double bonds (flexible tubes) III. PROCEDURE Construct a model for each of the molecules represented on the report sheet. Provide names and structures for the molecules as instructed.
Report Sheet: Experiment 2 Name Partner's Name Date Instructor's Initials A. Alkanes 1. Construct a model of ethane, C 2 6. Note the tetrahedral geometry of the electron pairs about the carbon atoms, and that the two methyl groups can be rotated about the carboncarbon single bond without breaking the bond. Write the complete and condensed structural formulas for ethane. 2. Construct a model of propane, C 3 8. Note that because of the tetrahedral geometry of the carbon atoms, the carbon skeleton is not linear, but has a zig-zag appearance. Write the complete and condensed structural formulas for propane.
B. Structural Isomerism of Alkanes 1. Construct models for the three molecules that have the molecular formula C 5 12. These molecules are structural isomers. Write condensed structural formulas and give IUPAC names for these molecules. C. Geometric Isomerism of Cycloalkanes 1. Construct a model of cyclopentane, C 5 10. Note that the ring is essentially planar. Now attach a methyl group to each of two adjacent carbons to create a molecule of 1,2- dimethylcyclopentane, C 7 14. Note that the methyl groups can be attached in two different ways; 1) both methyl groups on the same side of the cycloalkane ring, and 2) methyl groups on opposite sides of the cycloalkane ring. These are the cis and trans isomers, respectively. Write condensed structural formulas for these molecules, and label the structures as cis or trans as appropriate.
D. Alkenes 1. Examine the model of ethene, C 2 4. This molecule can be constructed using the flexible tubing to represent each electron pair of the double bond. Note the trigonal planar geometry of the electron domains about the carbon atoms, and that the carbon-carbon double bond cannot be rotated. Write the complete structural formula for ethene. 2. Construct a model of propene, C 3 6. Note the trigonal planar geometry of the two carbon atoms participating in the double bond, and the tetrahedral geometry of the third carbon atom. Write the complete structural formula for propene. E. Structural and Geometric Isomerism of Alkenes 1. Start with a model of ethene, C 2 4, and replace a hydrogen atom on each of the two carbons with a methyl group to create a molecule of 2-butene, C 4 8. Note that the methyl groups can be attached in two different ways: 1) both methyl groups on the same side of the double bond, and 2) methyl groups on opposite sides of the double bond. These are the cis and trans isomers, respectively. Write structural formulas for these molecules, and label the structures as cis or trans as appropriate.
2. Two additional non-cyclic molecules have the molecular formula C 4 8, and can be constructed by trading the positions of a methyl group on the end of one of the 2-butene molecules and a hydrogen on one of the other carbon atoms. These molecules are structural isomers of one another and of the stereoisomers in part E.1. Construct models of these molecules, write their condensed structural formulas and give the IUPAC name for each molecule. F. Additional Exercises in Structure and Nomenclature 1. Write a condensed structural formula for each of the following compounds. You may find it helpful to build a model of each molecule. a. 2,4-dimethylhexane b. 2,3,5-trimethyl-4-propyl-l-heptene c. 3-ethyl-4-isopropylheptane
2. Give the IUPAC name for each of the following compounds. Be certain that you have identified the longest carbon chain in each molecule. You may find it helpful to construct a model of each molecule. a. C C 2 C 2 C 2 C 2 b. C 2 C C C 2 c. C C C 2 C C 2 C 3 C 2 C d. C 2 e. C 2 C C C C 2 C C 2 C 2 C 2 C C 2 f. C C C C 2 C 3 C C