Ch. 4 - Isomers: The Arrangement of Atoms in Space

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 4 - Isomers: The Arrangement of Atoms in Space

Problem 32

Chapter 5, Problem 32

The reaction of (R)-1-iodo-2-methylbutane with hydroxide ion forms an alcohol without breaking any bonds to the asymmetric center. The alcohol rotates the plane of polarization of plane-polarized light counterclockwise. What is the configuration of (+)-2-methyl-1-butanol?

Verified step by step guidance

Step 1: Analyze the reaction mechanism. The reaction involves the substitution of the iodine atom in (R)-1-iodo-2-methylbutane with a hydroxide ion (OH⁻) to form an alcohol. This substitution occurs via an SN2 mechanism, which results in inversion of configuration at the carbon where substitution occurs.

Step 2: Determine the stereochemistry of the starting material. The starting material, (R)-1-iodo-2-methylbutane, has the (R)-configuration at the asymmetric center. Since the SN2 reaction causes inversion, the product will have the opposite configuration, which is (S)-2-methyl-1-butanol.

Step 3: Understand the relationship between optical activity and stereochemistry. The problem states that the alcohol rotates plane-polarized light counterclockwise, which corresponds to a negative rotation (denoted as (-)). Optical rotation is not directly related to the R/S configuration but is experimentally determined.

Step 4: Clarify the configuration of (+)-2-methyl-1-butanol. The (+) sign indicates that the compound rotates plane-polarized light clockwise. Since the product of the reaction is (S)-2-methyl-1-butanol, the configuration of (+)-2-methyl-1-butanol must be (R), as the optical rotation is independent of the reaction mechanism.

Step 5: Conclude that the configuration of (+)-2-methyl-1-butanol is (R). This conclusion is based on the inversion of configuration during the SN2 reaction and the experimental observation of optical rotation.

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Key Concepts

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

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and how this affects their chemical behavior. In this context, the configuration of chiral centers is crucial, as it determines the optical activity of the compound. The terms 'R' and 'S' denote specific configurations based on the Cahn-Ingold-Prelog priority rules, which help in identifying the three-dimensional orientation of substituents around a chiral carbon.

Optical Activity

Optical activity refers to the ability of a chiral compound to rotate the plane of polarized light. The direction of rotation can be either clockwise (dextrorotatory, +) or counterclockwise (levorotatory, -). In this question, the alcohol formed from the reaction is noted to rotate light counterclockwise, indicating it is a levorotatory compound, which is essential for determining its configuration as (+)-2-methyl-1-butanol.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. In this case, hydroxide ion acts as the nucleophile that attacks the carbon atom bonded to the iodine in (R)-1-iodo-2-methylbutane, leading to the formation of an alcohol. Understanding the mechanism of this reaction, whether it proceeds via an SN1 or SN2 pathway, is vital for predicting the stereochemical outcome of the product.

Related Practice

Textbook Question

a. Is (R)-lactic acid dextrorotatory or levorotatory?

b. Is (R)-sodium lactate dextrorotatory or levorotatory?

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Textbook Question

(+)-Mandelic acid has a specific rotation of +158. What would be the observed specific rotation of each of the following mixtures?

c. 75% (-)-mandelic acid and 25% (+)-mandelic acid

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Textbook Question

What is the configuration of the following compounds? (Use the given structures to answer the question.)

c. ( $+$ )-lactic acid

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Textbook Question

The observed rotation of 2.0 g of a compound in 50 mL of solution in a polarimeter tube 20 cm long is +138°. What is the specific rotation of the compound?

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What is the configuration of the following compounds? (Use the given structures to answer the question.)

a. ( $-$ )-glyceric acid

b. ( $+$ )-isoserine

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Textbook Question

Naproxen, a nonsteroidal anti-inflammatory drug that is the active ingredient in Aleve (p. 115), has a specific rotation of +66. One commercial preparation results in a mixture with a 97% enantiomeric excess.

a. Does naproxen have the R or the S configuration?

b. What percent of each enantiomer is obtained from the commercial preparation?