Chemistry/Chapter 7: Organic Chemistry
7.1.3 Properties of Optical Isomers
The main idea is that optical isomers have identical physical and chemical properties
Explanation level
In the previous section, we learned that optical isomers (enantiomers) are non-superimposable mirror images of each other. We also learned that they can rotate the plane of polarized light. This section explores the physical, chemical, and biological properties of optical isomers, highlighting their similarities and the crucial differences that make them important in fields like medicine and biology.
Main Idea
- They rotate the plane of polarized light in opposite directions.
- They interact differently with other chiral molecules, which leads to different biological and physiological effects.
Core Concept
The core concept is that the chirality of a molecule determines its interaction with a chiral environment. While the two enantiomers are mirror images, they are not identical in a chiral environment because the "receptor" they interact with is also chiral. This is analogous to a hand fitting into a glove: your right hand fits a right-handed glove, but not a left-handed glove. Similarly, one enantiomer of a drug may fit a biological receptor (like an enzyme or a protein) perfectly, while the other enantiomer does not.
Detailed Explanation
The textbook describes several important properties of optical isomers.
1. Similar Physical and Chemical Properties
The physical and chemical properties of optical isomers are similar. For example, the optical isomers of the same compound possess the same density and melting points. They also have the same solubility in achiral solvents, the same boiling points, and the same chemical reactivity with achiral reagents.
2. Different Interaction with Plane-Polarized Light
Optical isomers of a compound can be differentiated based on their ability to rotate the plane of plane-polarized light in opposite directions. This is the defining property of optical isomers, as we saw in the previous section.
- Dextrorotatory (d or +): Rotates the plane of polarized light clockwise.
- Levorotatory (l or -): Rotates the plane of polarized light counterclockwise.
3. Different Physiological Effects
However, they show different physiological effects in the human body, highlighting the fact that biological sensors and biochemical reactions are dependent on the shapes of molecules. For instance:
- Asparagine: One enantiomer of the amino acid asparagine has a bitter taste, whereas the other counterpart gives a sweet taste.
- Limonene: The limonene molecule has chirality, and the body can distinguish between the taste and odour of its two enantiomers. Lemons and oranges contain the same isomer, (+)-limonene. In contrast, (-)-limonene is found in pine needles, peppermint, and spearmint and is different in smell or taste to the (+)- isomer.
4. Importance in Drug Design (Pharmacokinetics and Pharmacodynamics)
Therapeutic drugs may also show optical isomerism as they are chiral molecules. One of the two enantiomers has the desired pharmacokinetic and pharmacodynamic properties.
- Pharmacokinetics: Studies absorption, distribution, metabolism, and excretion.
- Pharmacodynamics: The study of the actions of an administered drug on the system of the body and the way the drug binds to the target site.
Because the two enantiomers can have different biological activities, it is important to separate them or synthesize only the desired one. For example:
- Ibuprofen: A common painkiller that is a chiral molecule, whose one enantiomer is active while the other is inactive.
- Propoxyphene: One enantiomer is an analgesic (pain-relieving), whereas the other has anti-coughing properties, although the commercial names are mirror images of one another.
- Thalidomide: A drug that was prescribed as a sedative and for treating morning sickness. Later, it was found that this drug caused birth defects. Research proved that the (+)-enantiomer of thalidomide is active and used as a sedative with no side effects, whereas the (-)-enantiomer is a teratogenic, causing birth defects in babies whose mothers took the drug. Unfortunately, the two enantiomers of thalidomide soon interconvert, making the administration of a single intended enantiomer ineffective to prevent the harmful effects.
Intermediate Level
Optical isomers (enantiomers) have identical physical and chemical properties in an achiral environment, such as melting point, boiling point, density, and solubility. They also have the same chemical reactivity with achiral reagents. However, they differ in two key ways. First, they rotate the plane of polarized light in opposite directions (one dextrorotatory, one levorotatory). Second, and more importantly, they interact differently with other chiral molecules, such as biological receptors (enzymes, proteins). This leads to different biological and physiological effects. For example, one enantiomer of a drug may be therapeutically active, while the other may be inactive or even harmful, as in the case of thalidomide. The separation of enantiomers is therefore crucial in the pharmaceutical industry.
Important Points
- - Similar Physical Properties: Enantiomers have identical melting points, boiling points, densities, and solubilities in achiral solvents.
- - Different Optical Activity: They rotate the plane of polarized light in opposite directions.
- - Different Biological Activity: They interact differently with chiral biological molecules, leading to different physiological effects.
- - Examples:
- - Asparagine: one enantiomer is bitter, the other is sweet.
- - Limonene: (+)-limonene smells like oranges; (-)-limonene smells like pine/peppermint.
- - Ibuprofen: one enantiomer is active, the other is inactive.
- - Thalidomide: one enantiomer is sedative, the other is teratogenic.
- - Importance: Enantiomers must be separated or synthesized selectively in the pharmaceutical industry.
Remember
- - Same physical properties (except optical rotation).
- - Opposite optical rotation (dextrorotatory vs. levorotatory).
- - Different biological effects (due to chiral receptors).
- - Examples: Asparagine (bitter/sweet), limonene (orange/pine), thalidomide (safe/teratogenic).
- - Pharmacokinetics: What the body does to the drug.
- - Pharmacodynamics: What the drug does to the body.
- - Racemization can occur in the body.
Common Mistakes
- - Assuming that enantiomers have different physical properties (other than optical rotation). They have identical melting points, boiling points, etc.
- - Thinking that the biological activity of enantiomers is always the same. It is often very different, which is why enantiomer separation is important.
- - Forgetting that enantiomers can interconvert (racemize) in the body. This is important for drugs like thalidomide.
- - Confusing pharmacokinetics and pharmacodynamics. Pharmacokinetics is about the drug's movement in the body; pharmacodynamics is about the drug's effect on the body.
- - Not recognizing the importance of chirality in drug design. Many drugs are chiral, and their enantiomers can have vastly different effects.
Quick Revision
Optical isomers (enantiomers) have identical physical and chemical properties in an achiral environment, such as melting point, boiling point, and density. However, they differ in their ability to rotate the plane of polarized light (opposite directions) and in their interactions with chiral biological molecules, leading to different physiological effects. For example, one enantiomer of asparagine is bitter while the other is sweet; (+)-limonene smells like oranges while (-)-limonene smells like pine. In the pharmaceutical industry, these differences are critical: one enantiomer of a drug may be therapeutic while the other may be inactive or harmful (as in thalidomide). Therefore, the separation or selective synthesis of enantiomers is essential for drug safety and efficacy.
60-Second Revision
Enantiomers have identical physical properties (MP, BP, density) but rotate polarized light in opposite directions. They also have different biological effects because they interact differently with chiral receptors. Examples: asparagine (bitter/sweet), limonene (orange/pine), thalidomide (sedative/teratogenic). This is why enantiomer separation is crucial in drug design.
Frequently Asked Questions (FAQs)
- They rotate the plane of polarized light in opposite directions.
- They interact differently with other chiral molecules, leading to different biological and physiological effects.
Q2. How can optical isomers be differentiated based on their properties? Answer: Optical isomers can be differentiated by their interaction with plane-polarized light. One enantiomer (dextrorotatory) rotates the plane of polarized light clockwise, while the other (levorotatory) rotates it counterclockwise. They can also be differentiated by their biological activity, as one enantiomer may have a different taste, smell, or therapeutic effect than the other.
- Q3. Give an example of two optical isomers with different tastes.
- Answer: One enantiomer of the amino acid asparagine has a bitter taste, while the other enantiomer has a sweet taste.
Q4. Explain the difference in the smell of (+)-limonene and (-)-limonene. Answer: (+)-Limonene is found in lemons and oranges and has a citrus smell. (-)-Limonene is found in pine needles, peppermint, and spearmint and has a different smell (piney or minty). The body can distinguish between the two enantiomers due to the chiral nature of olfactory receptors.
Q5. Why do optical isomers have different physiological effects? Answer: Optical isomers have different physiological effects because biological systems (enzymes, receptors) are chiral. A chiral receptor will interact differently with the two enantiomers of a drug, just as a right hand fits a right glove but not a left glove. This can lead to one enantiomer being therapeutically active while the other is inactive or harmful.
Q6. What is the difference between pharmacokinetics and pharmacodynamics? Answer: Pharmacokinetics is the study of what the body does to a drug, including its absorption, distribution, metabolism, and excretion (ADME). Pharmacodynamics is the study of what the drug does to the body, including its mechanism of action and the relationship between drug concentration and effect.
Q7. What is the significance of the thalidomide tragedy in the context of optical isomerism? Answer: The thalidomide tragedy highlighted the importance of optical isomerism in drug design. Thalidomide was prescribed as a sedative, but it caused birth defects. Research showed that the (+)-enantiomer was a safe sedative, while the (-)-enantiomer was teratogenic. Unfortunately, the two enantiomers interconvert in the body, so administering only the safe enantiomer did not prevent the harmful effects. This led to stricter regulations and a greater emphasis on testing individual enantiomers.
Q8. Why is it important to separate optical isomers in drug manufacturing? Answer: It is important to separate optical isomers because the two enantiomers of a drug can have different biological activities. One enantiomer may be therapeutically effective, while the other may be inactive or even harmful. By using only the desired enantiomer, the drug's efficacy can be maximized while minimizing side effects.
- Chiral chromatography: Using a chiral stationary phase that interacts differently with the two enantiomers.
- Crystallization: Using a chiral resolving agent that forms diastereomeric salts with the enantiomers, which have different solubilities.
- Enzymatic resolution: Using enzymes that selectively react with one enantiomer.
Multiple Choice Questions (MCQs)
Q1. Which of the following properties is different for the two enantiomers of a chiral molecule? A. Melting point B. Boiling point C. Density D. Direction of rotation of plane-polarized light Correct Answer: D Explanation: Enantiomers have identical physical properties (melting point, boiling point, density) but rotate the plane of polarized light in opposite directions.
- Q2. Which of the following is an example of a molecule that has different tastes for its two enantiomers?
- A. Limonene
- B. Asparagine
- C. Ibuprofen
- D. Thalidomide
- Correct Answer: B
- Explanation: One enantiomer of asparagine is bitter, and the other is sweet.
- Q3. What is the term used to describe the study of what the body does to a drug?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: B
- Explanation: Pharmacokinetics is the study of the absorption, distribution, metabolism, and excretion of a drug.
- Q4. Which enantiomer of limonene is found in lemons and oranges?
- A. (+)-limonene
- B. (-)-limonene
- C. Both
- D. Neither
- Correct Answer: A
- Explanation: (+)-Limonene is found in lemons and oranges and has a citrus smell.
- Q5. What is the term used to describe the study of the actions of an administered drug on the body?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: A
- Explanation: Pharmacodynamics is the study of the actions of a drug on the body and the way it binds to the target site.
- Q6. Which of the following statements is true about the physical properties of enantiomers?
- A. They have different melting points.
- B. They have different boiling points.
- C. They have identical physical properties (except for optical rotation).
- D. They have different densities.
- Correct Answer: C
- Explanation: Enantiomers have identical physical properties (melting point, boiling point, density) except for their interaction with plane-polarized light.
- Q7. Which of the following is an example of a drug that has one active and one inactive enantiomer?
- A. Thalidomide
- B. Ibuprofen
- C. Asparagine
- D. Limonene
- Correct Answer: B
- Explanation: Ibuprofen is a chiral molecule where one enantiomer is active as a painkiller, and the other is inactive.
- Q8. What is the term used to describe the interconversion of enantiomers in the body?
- A. Resolution
- B. Racemization
- C. Hybridization
- D. Isomerization
- Correct Answer: B
- Explanation: Racemization is the interconversion of enantiomers, which can occur in the body.
- Q9. Why is the (-)-enantiomer of thalidomide dangerous?
- A. It is a sedative.
- B. It is teratogenic and causes birth defects.
- C. It is inactive.
- D. It has a bitter taste.
- Correct Answer: B
- Explanation: The (-)-enantiomer of thalidomide is teratogenic, meaning it causes birth defects.
- Q10. Which of the following is a method used to separate enantiomers?
- A. Simple distillation
- B. Chiral chromatography
- C. Filtration
- D. Evaporation
- Correct Answer: B
- Explanation: Chiral chromatography uses a chiral stationary phase to separate enantiomers based on their different interactions with the chiral environment.
Multiple Choice Questions (MCQs)
- Q1. Which of the following properties is different for the two enantiomers of a chiral molecule?
- A. Melting point
- B. Boiling point
- C. Density
- D. Direction of rotation of plane-polarized light
- Correct Answer: D
- Explanation: Enantiomers have identical physical properties but rotate the plane of polarized light in opposite directions.
- Q2. Which of the following is an example of a molecule that has different tastes for its two enantiomers?
- A. Limonene
- B. Asparagine
- C. Ibuprofen
- D. Thalidomide
- Correct Answer: B
- Explanation: One enantiomer of asparagine is bitter, and the other is sweet.
- Q3. What is the term used to describe the study of what the body does to a drug?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: B
- Explanation: Pharmacokinetics is the study of the absorption, distribution, metabolism, and excretion of a drug.
- Q4. Which enantiomer of limonene is found in lemons and oranges?
- A. (+)-limonene
- B. (-)-limonene
- C. Both
- D. Neither
- Correct Answer: A
- Explanation: (+)-Limonene is found in lemons and oranges.
- Q5. What is the term used to describe the study of the actions of an administered drug on the body?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: A
- Explanation: Pharmacodynamics is the study of the actions of a drug on the body.
- Q6. Which of the following statements is true about the physical properties of enantiomers?
- A. They have different melting points.
- B. They have different boiling points.
- C. They have identical physical properties (except for optical rotation).
- D. They have different densities.
- Correct Answer: C
- Explanation: Enantiomers have identical physical properties.
- Q7. Which of the following is an example of a drug that has one active and one inactive enantiomer?
- A. Thalidomide
- B. Ibuprofen
- C. Asparagine
- D. Limonene
- Correct Answer: B
- Explanation: Ibuprofen has one active enantiomer and one inactive enantiomer.
- Q8. What is the term used to describe the interconversion of enantiomers in the body?
- A. Resolution
- B. Racemization
- C. Hybridization
- D. Isomerization
- Correct Answer: B
- Explanation: Racemization is the interconversion of enantiomers.
- Q9. Why is the (-)-enantiomer of thalidomide dangerous?
- A. It is a sedative.
- B. It is teratogenic and causes birth defects.
- C. It is inactive.
- D. It has a bitter taste.
- Correct Answer: B
- Explanation: The (-)-enantiomer of thalidomide is teratogenic.
- Q10. Which of the following is a method used to separate enantiomers?
- A. Simple distillation
- B. Chiral chromatography
- C. Filtration
- D. Evaporation
- Correct Answer: B
- Explanation: Chiral chromatography is used to separate enantiomers.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a difference between the two enantiomers of a chiral molecule?
- A. They have different physical properties.
- B. They rotate the plane of polarized light in opposite directions.
- C. They have different chemical properties.
- D. They have different molecular formulas.
- Correct Answer: B
- Explanation: Enantiomers rotate the plane of polarized light in opposite directions.
- Q2. Which of the following pairs of enantiomers have different smells?
- A. (+)-limonene and (-)-limonene
- B. (+)-asparagine and (-)-asparagine
- C. (+)-ibuprofen and (-)-ibuprofen
- D. (+)-thalidomide and (-)-thalidomide
- Correct Answer: A
- Explanation: (+)-Limonene smells like oranges, while (-)-limonene smells like pine.
- Q3. What is the term for the study of the absorption, distribution, metabolism, and excretion of a drug?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: B
- Explanation: Pharmacokinetics is the study of ADME.
- Q4. Which of the following is a chiral molecule?
- A. Methane
- B. Ethane
- C. 2-butanol
- D. Benzene
- Correct Answer: C
- Explanation: 2-Butanol has a chiral center.
- Q5. Which enantiomer of thalidomide is teratogenic?
- A. (+)-thalidomide
- B. (-)-thalidomide
- C. Both
- D. Neither
- Correct Answer: B
- Explanation: The (-)-enantiomer of thalidomide is teratogenic.
- Q6. Why is it important to separate enantiomers in drug manufacturing?
- A. Because they have different physical properties.
- B. Because they can have different biological activities.
- C. Because they have different molecular formulas.
- D. Because they have different colors.
- Correct Answer: B
- Explanation: Enantiomers can have different biological activities.
- Q7. What is the term used to describe a mixture of equal amounts of two enantiomers?
- A. Racemic mixture
- B. Meso compound
- C. Chiral mixture
- D. Enantiomeric mixture
- Correct Answer: A
- Explanation: A racemic mixture consists of equal amounts of two enantiomers.
- Q8. Which of the following is an example of a molecule with different tastes for its enantiomers?
- A. Limonene
- B. Asparagine
- C. Ibuprofen
- D. Thalidomide
- Correct Answer: B
- Explanation: Asparagine enantiomers have different tastes.
- Q9. Which of the following is a method for separating enantiomers?
- A. Distillation
- B. Chiral chromatography
- C. Filtration
- D. Evaporation
- Correct Answer: B
- Explanation: Chiral chromatography is used to separate enantiomers.
- Q10. What is the term for the interconversion of enantiomers in the body?
- A. Resolution
- B. Racemization
- C. Hybridization
- D. Isomerization
- Correct Answer: B
- Explanation: Racemization is the interconversion of enantiomers in the body.
Multiple Choice Questions (MCQs)
- Q1. Which of the following is a difference between the two enantiomers of a chiral molecule?
- A. They have different physical properties.
- B. They rotate the plane of polarized light in opposite directions.
- C. They have different chemical properties.
- D. They have different molecular formulas.
- Correct Answer: B
- Explanation: Enantiomers rotate the plane of polarized light in opposite directions.
- Q2. Which of the following pairs of enantiomers have different smells?
- A. (+)-limonene and (-)-limonene
- B. (+)-asparagine and (-)-asparagine
- C. (+)-ibuprofen and (-)-ibuprofen
- D. (+)-thalidomide and (-)-thalidomide
- Correct Answer: A
- Explanation: (+)-Limonene smells like oranges, while (-)-limonene smells like pine.
- Q3. What is the term for the study of the absorption, distribution, metabolism, and excretion of a drug?
- A. Pharmacodynamics
- B. Pharmacokinetics
- C. Pharmacology
- D. Toxicology
- Correct Answer: B
- Explanation: Pharmacokinetics is the study of ADME.
- Q4. Which of the following is a chiral molecule?
- A. Methane
- B. Ethane
- C. 2-butanol
- D. Benzene
- Correct Answer: C
- Explanation: 2-Butanol has a chiral center.
- Q5. Which enantiomer of thalidomide is teratogenic?
- A. (+)-thalidomide
- B. (-)-thalidomide
- C. Both
- D. Neither
- Correct Answer: B
- Explanation: The (-)-enantiomer of thalidomide is teratogenic.
- Q6. Why is it important to separate enantiomers in drug manufacturing?
- A. Because they have different physical properties.
- B. Because they can have different biological activities.
- C. Because they have different molecular formulas.
- D. Because they have different colors.
- Correct Answer: B
- Explanation: Enantiomers can have different biological activities.
- Q7. What is the term used to describe a mixture of equal amounts of two enantiomers?
- A. Racemic mixture
- B. Meso compound
- C. Chiral mixture
- D. Enantiomeric mixture
- Correct Answer: A
- Explanation: A racemic mixture consists of equal amounts of two enantiomers.
- Q8. Which of the following is an example of a molecule with different tastes for its enantiomers?
- A. Limonene
- B. Asparagine
- C. Ibuprofen
- D. Thalidomide
- Correct Answer: B
- Explanation: Asparagine enantiomers have different tastes.
- Q9. Which of the following is a method for separating enantiomers?
- A. Distillation
- B. Chiral chromatography
- C. Filtration
- D. Evaporation
- Correct Answer: B
- Explanation: Chiral chromatography is used to separate enantiomers.
- Q10. What is the term for the interconversion of enantiomers in the body?
- A. Resolution
- B. Racemization
- C. Hybridization
- D. Isomerization
- Correct Answer: B
- Explanation: Racemization is the interconversion of enantiomers in the body.
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7.1.3 Properties of Optical Isomers
Sticky Notes · Quick Revision
Key Point 1
- Same physical properties: MP, BP, density, solubility.
Key Point 2
- Different optical rotation: d vs. l.
Key Point 3
- Different biological activity: Due to chiral receptors.
Key Point 4
- Asparagine: Bitter vs. sweet.
Key Point 5
- Limonene: Orange vs. pine/peppermint.
Key Point 6
- Ibuprofen: One active, one inactive.
Key Point 7
- Thalidomide: Sedative vs. teratogenic.
Key Point 8
- Pharmacokinetics: ADME (absorption, distribution, metabolism, excretion).
Key Point 9
- Pharmacodynamics: Drug effect and binding.
Key Point 10
- Racemization: Interconversion of enantiomers in the body.
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