Ch.5 - Stereochemistry

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

Wade 9th Edition

Ch.5 - Stereochemistry

Problem 6

Chapter 5, Problem 6

A solution of pure (S)-2-iodobutane ([α] = +15.90° ) in acetone is allowed to react with radioactive iodide, 131I-, until 1.0% of the iodobutane contains radioactive iodine. The specific rotation of this recovered iodobutane is found to be +15.58°. a. Determine the percentages of (R)- and (S)-2-iodobutane in the product mixture.

Verified step by step guidance

Identify the initial specific rotation of pure (S)-2-iodobutane, which is given as +15.90°.

Recognize that the specific rotation of the recovered iodobutane is +15.58°, indicating a mixture of (R)- and (S)-2-iodobutane.

Use the formula for enantiomeric excess (ee): ee = (observed specific rotation / specific rotation of pure enantiomer) * 100% to find the enantiomeric excess of the mixture.

Calculate the percentage of the major enantiomer (S)-2-iodobutane using the enantiomeric excess: %S = (ee + 100) / 2.

Determine the percentage of the minor enantiomer (R)-2-iodobutane: %R = 100 - %S.

Verified video answer for a similar problem:

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

Video duration:

Was this helpful?

Key Concepts

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

Optical Activity and Specific Rotation

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. The specific rotation, denoted as [α], quantifies this rotation and is defined as the observed rotation divided by the concentration and path length. In this question, the specific rotation values of the pure and recovered iodobutane are crucial for determining the composition of the product mixture.

Chirality and Enantiomers

Chirality is a property of molecules that are non-superimposable on their mirror images, leading to the existence of enantiomers—two compounds that are mirror images of each other. In this case, (S)-2-iodobutane and (R)-2-iodobutane are enantiomers, and their relative proportions in the product mixture can be inferred from the specific rotation measurements and the extent of racemization during the reaction.

Racemization and Reaction Kinetics

Racemization is the process by which one enantiomer is converted into its mirror image, resulting in a mixture of both enantiomers. The reaction kinetics, influenced by factors such as solvent and temperature, can affect the rate of racemization. In this scenario, the introduction of radioactive iodide and the resulting specific rotation provide insights into the extent of racemization and the final enantiomeric composition of the product.

Recommended video:

Guided course

08:51

Kinetic vs. Thermodynamic Control

Related Practice

Textbook Question

b. Draw the six stereoisomers of octa-2,4,6-triene. Explain why there are only six stereoisomers, rather than the eight we might expect for a compound with three stereogenic double bonds.

views

Textbook Question

Star (*) each asymmetric carbon atom in the following examples, and determine whether it has the (R) or (S) configuration.

(i)

views

Textbook Question

For each compound, determine whether the molecule has an internal mirror plane of symmetry. If it does, draw the mirror plane on a three-dimensional drawing of the molecule. If the molecule does not have an internal mirror plane, determine whether the structure is chiral.

(a) methane

(b) cis-1,2-dibromocyclobutane

views

Textbook Question

(d) 1,2-dichloropropane

(e)

views

Textbook Question

Star (*) each asymmetric carbon atom in the following examples, and determine whether it has the (R) or (S) configuration.

(a)

(b)

views

Textbook Question

(h)

views