Ch.5 - Stereochemistry

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

Wade 9th Edition

Ch.5 - Stereochemistry

Problem 10

Chapter 5, Problem 10

A chiral sample gives a rotation that is close to 180°. How can one tell whether this rotation is +180° or -180°?

Verified step by step guidance

Understand that optical rotation is a property of chiral compounds, and it is measured using a polarimeter. The rotation can be either clockwise (denoted as +) or counterclockwise (denoted as -).

Recall that the sign of the rotation (+ or -) is determined by the direction in which the plane of polarized light is rotated when it passes through the chiral sample.

To determine whether the rotation is +180° or -180°, observe the direction of rotation of the plane of polarized light. If the light rotates clockwise, the rotation is +180°. If it rotates counterclockwise, the rotation is -180°.

Use the polarimeter to carefully observe the direction of rotation. The polarimeter will indicate whether the rotation is positive or negative based on the direction of the observed rotation.

Ensure that the measurement is taken under standard conditions (e.g., specific wavelength of light, temperature, and concentration of the sample) to avoid any ambiguity in determining the sign of the rotation.

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Key Concepts

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

Chirality

Chirality refers to the geometric property of a molecule that makes it non-superimposable on its mirror image. Chiral molecules typically have a carbon atom bonded to four different substituents, leading to two distinct enantiomers. These enantiomers can rotate plane-polarized light in opposite directions, which is a key characteristic used to distinguish them.

Optical Activity

Optical activity is the ability of chiral substances to rotate the plane of polarized light. The direction and degree of this rotation are measured in degrees, with positive values indicating clockwise rotation (+) and negative values indicating counterclockwise rotation (-). The specific rotation can be influenced by factors such as concentration and path length, making it essential to control these variables during measurement.

Polarimeter

A polarimeter is an instrument used to measure the angle of rotation of polarized light as it passes through a chiral sample. By analyzing the rotation, one can determine whether the sample is a specific enantiomer. The readings from a polarimeter can indicate whether the rotation is +180° or -180°, allowing chemists to identify the configuration of the chiral compound.

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Specific rotation vs. observed rotation.

Related Practice

Textbook Question

Write structural formulas for the following compounds (includes both old- and new-style names).(j) vinylacetylene(k) (S)-3-methyl-1-penten-4-yne

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Textbook Question

A chemist finds that the addition of (+)-epinephrine to the catalytic reduction of butan-2-one (Figure 5-17 ) gives a product that is slightly optically active, with a specific rotation of +0.45°. Calculate the percentages of (+)-butan-2-ol and (−)-butan-2-ol formed in this reaction.

Textbook Question

When optically pure (R)-2-bromobutane is heated with water, butan-2-ol is the product. The reaction forms twice as much (S)-butan-2-ol as (R)-butan-2-ol. Calculate the e.e. and the specific rotation expected for the product.

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Textbook Question

A solution of 2.0 g of (+)-glyceraldehyde, HOCH₂CHOHCHO, in 10.0 mL of water was placed in a 100-mm cell. Using the sodium D line, a rotation of +1.74° was found at 25 °C. Determine the specific rotation of (+)-glyceraldehyde

Textbook Question

If you had the two enantiomers of carvone in unmarked bottles, could you use just your nose and a polarimeter to determine

a. whether it is the (+) or (−) enantiomer that smells like spearmint

b. whether it is the (R) or (S) enantiomer that smells like spearmint?

c. With the information given in the drawings of carvone above, what can you add to your answers to (a) and (b)?

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Textbook Question

A solution of 0.50 g of (−)-epinephrine (see Figure 5-16) dissolved in 10.0 mL of dilute aqueous HCl was placed in a 20-cm polarimeter tube. Using the sodium D line, the rotation was found to be −5.1° at 25 °C. Determine the specific rotation of epinephrine.

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