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Ch.5 - Stereochemistry
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh.5 - StereochemistryProblem 38a,b
Chapter 5, Problem 38a,b

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?

Verified step by step guidance
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Step 1: Analyze the given reactant, (R)-3,4-dimethylpent-1-ene (CH2=CH—CH(CH3)—CH(CH3)2). This molecule contains a double bond at the 1-position and a chiral center at the 3-position. The stereochemistry of the chiral center is specified as (R).
Step 2: Understand the reaction conditions. The molecule is treated with hydrogen gas (H2) in the presence of a platinum catalyst. This is a typical catalytic hydrogenation reaction, which converts alkenes into alkanes by adding hydrogen atoms across the double bond.
Step 3: Predict the product. The double bond in the reactant will be reduced, resulting in a saturated hydrocarbon. The product is specified as (S)-2,3-dimethylpentane, indicating that the chiral center at the 3-position has undergone inversion of configuration from (R) to (S).
Step 4: Draw the reaction mechanism. The hydrogenation process involves syn addition of hydrogen atoms to the double bond. This addition does not directly affect the stereochemistry of the chiral center, but the inversion occurs due to the specific spatial arrangement during the reaction.
Step 5: Conclude whether the chiral center retains its configuration or is inverted. Based on the given information, the chiral center at the 3-position has been inverted from (R) to (S) during the hydrogenation process.

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Key Concepts

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

Hydrogenation

Hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) to an unsaturated compound, typically an alkene or alkyne, in the presence of a catalyst, such as platinum or palladium. This process converts double or triple bonds into single bonds, resulting in a saturated compound. In this case, the hydrogenation of (R)-3,4-dimethylpent-1-ene leads to the formation of (S)-2,3-dimethylpentane.
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Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In the context of this question, it is crucial to understand the stereochemical configuration of the reactant and product. The (R) and (S) designations refer to the specific three-dimensional arrangement of substituents around the chiral centers, which can influence the properties and reactivity of the molecules.
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Chirality and Inversion

Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image, often due to the presence of chiral centers. In this reaction, it is important to determine whether the chiral center in (R)-3,4-dimethylpent-1-ene retains its configuration or undergoes inversion during hydrogenation. The outcome of this reaction can affect the optical activity of the product, (S)-2,3-dimethylpentane, indicating a change in chirality.
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Related Practice
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.”)

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Textbook Question

For each structure,

1. draw all the stereoisomers.

2. label each structure as chiral or achiral.

3. give the relationships between the stereoisomers (enantiomers, diastereomers).

(a)

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

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)?

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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

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.

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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