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

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Step 1: Understand the terms 'dextrorotatory' and 'levorotatory'. Dextrorotatory compounds rotate plane-polarized light to the right (clockwise, denoted as '+'), while levorotatory compounds rotate it to the left (counterclockwise, denoted as '-'). These properties are determined experimentally and are not directly related to the R/S configuration.

Step 2: Analyze the chirality of (R)-lactic acid. The R/S configuration is determined based on the Cahn-Ingold-Prelog priority rules, but it does not predict optical rotation. Experimental data shows that (R)-lactic acid is levorotatory (rotates light to the left, '-').

Step 3: Consider the effect of ionization on optical rotation. When lactic acid is converted to sodium lactate, the removal of the acidic proton and the addition of a sodium ion can alter the electronic environment of the molecule, potentially affecting its optical rotation.

Step 4: Experimental data indicates that (R)-sodium lactate is dextrorotatory (rotates light to the right, '+'). This change in optical rotation is due to the altered electronic structure upon ionization.

Step 5: Summarize the findings: (R)-lactic acid is levorotatory ('-'), while (R)-sodium lactate is dextrorotatory ('+'). Optical rotation must always be determined experimentally and cannot be inferred solely from the R/S configuration.

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Key Concepts

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

Optical Activity

Optical activity refers to the ability of chiral compounds to rotate plane-polarized light. This property is a result of the asymmetrical arrangement of atoms around a chiral center, leading to two enantiomers: one that rotates light to the right (dextrorotatory) and one that rotates it to the left (levorotatory). The direction of rotation is determined experimentally and is crucial for identifying the specific optical isomer.

Chirality and Enantiomers

Chirality is a property of a molecule that makes it non-superimposable on its mirror image, similar to how left and right hands are mirror images but not identical. Enantiomers are pairs of chiral molecules that are mirror images of each other, often exhibiting different biological activities. The designation of (R) or (S) is used to specify the configuration of the chiral center, which is essential for understanding their optical activity.

Sodium Lactate and Its Properties

Sodium lactate is the sodium salt of lactic acid, which retains the chiral properties of its parent compound. The optical activity of sodium lactate can be influenced by the configuration of the lactic acid from which it is derived. Understanding the relationship between lactic acid and sodium lactate is important for predicting their behavior in biological systems and their interactions with polarized light.

Related Practice

Textbook Question

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?

Textbook Question

What is the configuration of each of the following?

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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

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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