Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

5. Chirality

Constitutional Isomers vs. Stereoisomers

Organic Chemistry

5. Chirality

Constitutional Isomers vs. Stereoisomers

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2:50 minutes

Problem 16

Textbook Question

How many stereoisomers are possible (a) for 4-methylnona-2,7-diene? (b) For 6-chloronona-2,4,7-triene?

Verified step by step guidance

Step 1: Understand the concept of stereoisomers. Stereoisomers are compounds with the same molecular formula and connectivity of atoms but differ in the spatial arrangement of atoms. For alkenes, stereoisomerism arises due to restricted rotation around the double bond, leading to cis-trans (E/Z) isomerism. Additionally, chiral centers can also contribute to stereoisomerism.

Step 2: Analyze the structure of 4-methylnona-2,7-diene. This compound contains two double bonds (at positions 2 and 7) and a methyl group at position 4. Each double bond can exhibit cis-trans (E/Z) isomerism. Check if there are any chiral centers in the molecule, as they can further increase the number of stereoisomers.

Step 3: Calculate the number of stereoisomers for 4-methylnona-2,7-diene. For each double bond, there are two possible configurations (E or Z). Multiply the possibilities for each double bond. If there are no chiral centers, the total number of stereoisomers is simply 2^n, where n is the number of double bonds capable of E/Z isomerism.

Step 4: Analyze the structure of 6-chloronona-2,4,7-triene. This compound contains three double bonds (at positions 2, 4, and 7) and a chlorine atom at position 6. Each double bond can exhibit cis-trans (E/Z) isomerism. Check if the chlorine atom creates a chiral center (a carbon atom bonded to four different groups). If so, this will add to the total number of stereoisomers.

Step 5: Calculate the number of stereoisomers for 6-chloronona-2,4,7-triene. For the three double bonds, there are 2^3 = 8 possible configurations (E/Z). If the chlorine atom creates a chiral center, multiply the number of configurations by 2 to account for the additional stereoisomerism. Combine these factors to determine the total number of stereoisomers.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stereoisomerism

Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. This can lead to different physical and chemical properties. The two main types of stereoisomers are enantiomers, which are non-superimposable mirror images, and diastereomers, which are not mirror images of each other.

Cis-Trans Isomerism

Cis-trans isomerism is a specific type of stereoisomerism that occurs in alkenes and cyclic compounds, where the relative positioning of substituents around a double bond or ring structure can differ. In cis isomers, similar groups are on the same side, while in trans isomers, they are on opposite sides. This distinction can significantly affect the physical properties and reactivity of the compounds.

Counting Stereoisomers

The number of possible stereoisomers for a compound can be determined using the formula 2^n, where n is the number of stereocenters in the molecule. Additionally, for compounds with double bonds, the presence of cis-trans isomerism must also be considered. This requires careful analysis of the structure to identify all stereocenters and geometric isomers, leading to a comprehensive count of stereoisomers.

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Related Practice

Textbook Question

For each of the stereocenters (circled) in Figure 5-5,

a. draw the compound with two of the groups on the stereocenter interchanged.

b. give the relationship of the new compound to the original compound.

Textbook Question

One of the crowning achievements of natural products synthesis was Bryostatin 1, published by Professor Gary Keck (University of Utah; Journal of the American Chemical Society, 2011, 133, 744–747). The Bryostatins are a family of compounds isolated from aquatic invertebrates known as Bryozoans. The compounds are of interest for a variety of biological effects, including anti-cancer activity and reversing brain damage in rodents.

(d) How many chiral centers are in this molecule?

(e) Using the number of chiral centers you reported in part (d), calculate the number of stereoisomers possible at these chiral centers. (Ignore stereoisomers at double bonds.)

Textbook Question

One of the crowning achievements of natural products synthesis was Bryostatin 1, published by Professor Gary Keck (University of Utah; Journal of the American Chemical Society, 2011, 133, 744–747). The Bryostatins are a familyof compounds isolated from aquatic invertebrates known as Bryozoans. The compounds are of interest for a variety of biological effects, including anti-cancer activity and reversing brain damage in rodents.(d) How many chiral centers are in this molecule?(e) Using the number of chiral centers you reported in part(d), calculate the number of stereoisomers possible atthese chiral centers. (Ignore stereoisomers at double bonds.)

Textbook Question

Draw the enantiomer, if any, for each structure.

(a)

(b)

Textbook Question

Draw all the stereoisomers for each of the following:

e. 3,4-dichlorohexane

f. 1,2-dichlorocyclobutane

Textbook Question

For each of the following chiral molecules, obtain the enantiomer (i) by drawing the nonsuperimposable mirror image and (ii) by switching the spatial orientation at each asymmetric center. Confirm (possibly using models) that the structures you drew for (i) and (ii) are the same.

(d)

Textbook Question

For many centuries, the Chinese have used extracts from a group of herbs known as ephedra to treat asthma. A compound named ephedrine has been isolated from these herbs and found to be a potent dilator of air passages in the lungs.

a. How many stereoisomers does ephedrine have?

b. The stereoisomer shown here is the one that is pharmacologically active. What is the configuration of each of the asymmetric centers?

Textbook Question

How many stereoisomers are possible for each of the following molecules?

(a)

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