Ch.5 - Stereochemistry

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.5 - Stereochemistry

Problem 38c

Chapter 5, Problem 38c

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.
c. The reactant is named (R), but the product is named (S). Does this name change imply a change in the spatial arrangement of the groups around the chiral center? So why does the name switch from (R) to (S)?

Verified step by step guidance

Identify the chiral center in the reactant, (R)-3,4-dimethylpent-1-ene. The chiral center is the carbon atom bonded to four different groups.

Understand that the hydrogenation reaction over a platinum catalyst will add hydrogen atoms to the double bond, converting the alkene into an alkane.

Recognize that the addition of hydrogen atoms will change the priority of the groups around the chiral center, potentially altering the stereochemistry from (R) to (S).

Apply the Cahn-Ingold-Prelog priority rules to determine the configuration of the chiral center in the product, (S)-2,3-dimethylpentane. Assign priorities based on atomic number and connectivity.

Conclude that the change from (R) to (S) indicates a change in the spatial arrangement of the groups around the chiral center due to the hydrogenation process, which alters the priority of the substituents.

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Key Concepts

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

Chirality and Stereochemistry

Chirality refers to the geometric property of a molecule having a non-superimposable mirror image, often due to the presence of a chiral center, typically a carbon atom with four different substituents. Stereochemistry involves the study of spatial arrangements of atoms in molecules and their effects on chemical reactions. The R/S nomenclature is used to denote the configuration of chiral centers, with (R) and (S) indicating the arrangement of substituents around the chiral center.

Hydrogenation Reaction

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) across a double bond, typically in the presence of a catalyst such as platinum. This process converts alkenes into alkanes by saturating the carbon-carbon double bond, which can lead to changes in the stereochemistry of the molecule, as seen in the conversion of (R)-3,4-dimethylpent-1-ene to (S)-2,3-dimethylpentane.

R/S Configuration Change

The change from (R) to (S) configuration indicates a shift in the spatial arrangement of groups around the chiral center during the reaction. This can occur due to the hydrogenation process, which alters the priority of substituents based on the Cahn-Ingold-Prelog rules. The switch in configuration suggests that the relative positions of the groups have changed, affecting the molecule's stereochemistry and resulting in a different optical isomer.

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R and S Naming- Step 4

Related Practice

Textbook Question

The original definition of meso is 'an achiral compound that has chiral diastereomers.' Our working definition of meso is 'an achiral compound that has chiral centers (usually asymmetric carbon atoms).' The working definition is much easier to apply, because we don't have to envision all possible chiral diastereomers of the compound. Still, the working definition is not quite as complete as the original definition.

a. Show how cis-cyclooctene is defined as a meso compound under the original definition, but not under our working definition. (Review Figure 5-19)

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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.

a. Draw the equation for this reaction. Show the stereochemistry of the reactant and the product.

b. Has the chiral center retained its configuration during this hydrogenation, or has it been inverted?

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Textbook Question

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.”)

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Textbook Question

A graduate student was studying enzymatic reductions of cyclohexanones when she encountered some interesting chemistry. When she used an enzyme and NADPH to reduce the following ketone, she was surprised to find that the product was optically active. She carefully repurified the product so that no enzyme, NADPH, or other contaminants were present. Still, the product was optically active.

c. If this reaction could be accomplished using H₂ and a nickel catalyst, would the product be optically active? Explain.

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Textbook Question

Free-radical bromination of the following compound introduces bromine primarily at the benzylic position next to the aromatic ring. If the reaction stops at the monobromination stage, two stereoisomers result.

d. What is the relationship between the two isomeric products?

e. Will these two products be produced in identical amounts? That is, will the product mixture be exactly 50:50?

f. Will these two stereoisomers have identical physical properties such as boiling point, melting point, solubility, etc.? Could they be separated (theoretically, at least) by distillation or recrystallization?

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Textbook Question

a. Propose a mechanism to show why free-radical halogenation occurs almost exclusively at the benzylic position.

b. Draw the two stereoisomers that result from monobromination at the benzylic position.

c. Assign R and S configurations to the asymmetric carbon atoms in the products.

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