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

Previous problemNext problem

3:24 minutes

Problem 38d

Textbook Question

3,4-Dimethylpent-1-ene has the formula CH₂=CH—CH(CH₃)—CH(CH₃)₂. When pure (R)-3,4-dimethylpent-1-ene is treated with hydrogen over a platinum catalyst, the product is (S)-2,3-dimethylpentane.
d. How useful is the (R) or (S) designation for predicting the sign of an optical rotation? Can you predict the sign of the rotation of the reactant? Of the product? (Hint from Juliet Capulet: “What’s in a name? That which we call a rose/By any other name would smell as sweet.”)

Verified step by step guidance

Understand the context: The (R) and (S) designations refer to the absolute configuration of chiral centers in a molecule, based on the Cahn-Ingold-Prelog priority rules. These designations do not directly correlate with the sign of optical rotation (i.e., whether the compound is dextrorotatory (+) or levorotatory (-)). Optical rotation must be determined experimentally.

Analyze the reactant: The reactant, (R)-3,4-dimethylpent-1-ene, has a defined absolute configuration at its chiral center. However, the (R) designation does not provide information about whether the compound rotates plane-polarized light to the right (+) or to the left (-). Therefore, the sign of the optical rotation of the reactant cannot be predicted solely from the (R) configuration.

Analyze the product: The product, (S)-2,3-dimethylpentane, has a different absolute configuration at its chiral center compared to the reactant. However, as with the reactant, the (S) designation does not indicate the direction of optical rotation. The sign of the optical rotation of the product must also be determined experimentally.

Consider the relationship between reactant and product: The reaction involves hydrogenation over a platinum catalyst, which changes the double bond in the reactant to a single bond in the product. This process also inverts the configuration at the chiral center, converting (R) to (S). However, this inversion does not allow us to predict the optical rotation of the product based on the reactant.

Conclusion: The (R) or (S) designation is useful for describing the absolute configuration of chiral centers but is not sufficient for predicting the sign of optical rotation. The optical rotation of both the reactant and the product must be determined experimentally, as there is no direct correlation between configuration and optical activity.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above

Video duration:

Play a video:

Was this helpful?

Key Concepts

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

Optical Activity

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. Chiral molecules have non-superimposable mirror images, leading to two enantiomers that can rotate light in opposite directions. The direction and degree of rotation depend on the specific structure of the molecule and its concentration. Understanding optical activity is crucial for predicting the behavior of (R) and (S) enantiomers in reactions.

Chirality and Enantiomers

Chirality is a property of a molecule that makes it non-superimposable on its mirror image, resulting in two enantiomers with distinct spatial arrangements. The (R) and (S) designations are used to specify the configuration of chiral centers based on the Cahn-Ingold-Prelog priority rules. These designations help in predicting the optical activity of the compounds, as enantiomers will rotate light in opposite directions, making it essential for understanding the optical rotation of reactants and products.

Catalytic Hydrogenation

Catalytic hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) to unsaturated compounds, typically using a metal catalyst like platinum. This process converts alkenes into alkanes, often resulting in the formation of new chiral centers. The stereochemistry of the product can be influenced by the reaction conditions and the nature of the catalyst, making it important to analyze how these factors affect the optical activity of the resulting compound.

Watch next

Master Determining when molecules are different. with a bite sized video explanation from Johnny

Start learning

Related Videos

Related Practice

Textbook Question

For each of the compounds described by the following names,

1. draw a three-dimensional representation.

2. star (*) each chiral center.

3. draw any planes of symmetry.

4. draw any enantiomer.

5. draw any diastereomers.

6. label each structure you have drawn as chiral or achiral.

c. (2R,3S)-2,3-dibromohexane

d. (1R,2R)-1,2-dibromocyclohexane

Textbook Question

Natural products are organic compounds produced by living organisms, including plants, fungi, and animals. Often referred to as secondary metabolites because they are not required for survival of the organism, natural products have found broad utility as drugs themselves or as lead compounds used in the development of medicines. One such example is paclitaxel, which has been used as a cancer drug. Paclitaxel is isolated from the yew tree, where it is produced as a single stereoisomer (shown). Based on its structure, how many stereoisomers are possible for paclitaxel?

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.

(b)

Textbook Question

Draw a three-dimensional structure for each compound, and star all asymmetric carbon atoms. Draw the mirror for each structure, and state whether you have drawn a pair of enantiomers or just the same molecule twice. Build molecular models of any of these examples that seem difficult to you.

(c)

(d) 1-bromo-2-methylbutane

Textbook Question

To show that (R)-2-butyl (R,R)-tartrate and (S)-2-butyl (R,R)-tartrate are not enantiomers, draw and name the mirror images of these compounds.

Textbook Question

To show that (R)-2-butyl (R,R)-tartrate and (S)-2-butyl (R,R)-tartrate are not enantiomers, draw and name the mirror images of these compounds.

Textbook Question

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

(d)

Textbook Question

Using the wedge-and-dash notation, draw the nine stereoisomers of 1,2,3,4,5,6-hexachlorocyclohexane.

Show more practices