Plane of Symmetry in Molecules

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the condition for a molecule to be optically inactive?

A molecule is optically inactive if it possesses a plane of symmetry, meaning its mirror image is superimposable.

What happens to the mirror image of a molecule when it has a chiral carbon atom?

The mirror image is not superimposable when a molecule has a chiral carbon atom.

What is the effect of a racemic mixture on optical rotation?

The net optical rotation of a racemic mixture is zero.

What is the difference in physical properties between enantiomers?

There is no difference in physical properties like melting point, boiling point, and refractive index between enantiomers. Signup and view all the answers

What is unique about the biological properties of enantiomers?

Enantiomers show marked differences in biological properties like smell, taste, and drug action. Signup and view all the answers

What is the difference in chemical properties between enantiomers?

Enantiomers show some differences in reactivity with optically active reagents, but not with optically inactive reagents. Signup and view all the answers

What is the significance of chiral carbon atoms in a molecule?

A molecule with one chiral carbon atom has 2 optical isomers related as mirror images. Signup and view all the answers

What is an example of a molecule with a chiral carbon atom?

Lactic acid is an example of a molecule with a chiral carbon atom. Signup and view all the answers

What is the condition for a carbon atom to be asymmetric, and how many optical isomers are possible if a molecule has two asymmetric carbon atoms with different terminal groups?

A carbon atom is asymmetric if all four groups attached to it are different. If a molecule has two asymmetric carbon atoms with different terminal groups, 2^2 = 4 optical isomers are possible. Signup and view all the answers

What is the difference between enantiomeric pairs and diastereomeric pairs?

Enantiomeric pairs are non-superimposable mirror images, while diastereomeric pairs are not mirror images of each other. Signup and view all the answers

What is a meso form, and how is it different from an enantiomeric pair?

A meso form is an optically inactive molecule with two asymmetric carbon atoms having identical terminal groups. It is different from an enantiomeric pair, which consists of two non-superimposable mirror images. Signup and view all the answers

What are the conditions for a molecule to be optically active?

A molecule is optically active if it has one or more asymmetric carbon atoms and all four groups attached to each asymmetric carbon are different. Signup and view all the answers

How many isomers are possible for a molecule with two asymmetric carbon atoms having the same terminal groups?

Three isomers are possible: one enantiomeric pair and one meso form. Signup and view all the answers

What is the difference between an enantiomer and a diastereomer?

An enantiomer is a non-superimposable mirror image, while a diastereomer is a non-mirror image stereoisomer. Signup and view all the answers

What is a racemic mixture, and how is it related to enantiomers?

A racemic mixture is a 1:1 mixture of two enantiomers, which cancels out the optical activity of each other. Signup and view all the answers

How many asymmetric carbon atoms are present in CH3 − (CHOH)3 − CH3, and what is the total number of optical isomers possible?

There are two asymmetric carbon atoms, and 2^2 = 4 optical isomers are possible. Signup and view all the answers

What is the condition for optical activity in a molecule?

Non-superimposability of mirror image of a molecule with the original molecule Signup and view all the answers

What is the term for optically active isomers that are not related as mirror images?

Diastereomers Signup and view all the answers

What is the term for a molecule that rotates plane polarized light clockwise?

Dextrorotatory (d or +) Signup and view all the answers

What is the term for a molecule that has a non-superimposable mirror image?

Chiral molecule Signup and view all the answers

What is the term for optically active isomers that are related as mirror images?

Enantiomers Signup and view all the answers

What is the term for a mixture of equal amounts of dextrorotatory and leavorotatory isomers?

Racemic mixture Signup and view all the answers

How do enantiomers differ from each other?

Enantiomers have the same degree of rotation but opposite sign Signup and view all the answers

What is the importance of checking for a plane of symmetry in determining optical activity?

To determine non-superimposability of mirror image Signup and view all the answers

Study Notes

Stereochemistry and Optical Isomerism

A molecule with a plane of symmetry is optically inactive, and its mirror image is superimposable.
Planar molecules are optically inactive because they possess a plane of symmetry.

Chiral Carbon Atoms

A molecule with one chiral carbon atom has 2^(1) = 2 optical isomers, which are related to each other as mirror images.
Example: Lactic acid, which has a specific rotation of +2.67°, and its mirror image (-) lactic acid has a specific rotation of -2.67°.

Properties of Enantiomers

Physical properties like melting point, boiling point, and refractive index are the same for enantiomers.
Optical rotation is opposite for enantiomers.
Chemical properties are the same with optically inactive reagents, but show differences in reactivity with optically active reagents.
Biological properties like smell, taste, and drug action show marked differences between enantiomers.

Molecules with Two Asymmetric Carbon Atoms

If a molecule has two asymmetric carbon atoms with different terminal groups, 2^(2) = 4 optical isomers are possible.
Example: 2,3-dihydroxy butanoic acid, which has two asymmetric carbon atoms and four optical isomers.

Molecules with Two Asymmetric Carbon Atoms with Same Terminal Groups

If a molecule has two asymmetric carbon atoms with the same terminal groups, only one enantiomeric pair and an optically inactive form (meso) exist.
Example: Tartaric acid, which has two chiral carbon atoms and three optical isomers (one enantiomeric pair and one meso form).

Problem 4.1.1

CH3 - (CHOH)3 - CH3 has two asymmetric carbon atoms, and 2^(2) = 4 optical isomers are possible.

Problem 4.1.2

CH3 - CH(NH2) - CHO and 2-butanol are optically active because they have asymmetric carbon atoms with different groups attached.

Problem 4.1.3

C6H14 has several possible isomers.

Geometrical Isomerism in Rings

Geometrical isomerism is also possible in ring structures like cyclopropane, cyclobutane, and cyclohexane due to restricted rotation of C-C bonds involved in rings.
In molecules like cyclohexane, although bond rotation is possible, the 'cis' isomer never transforms into the 'trans' isomer, and the isomers can be separated.

Optical Isomerism

Certain substances possess the property to rotate plane-polarized light and are called optically active substances.
The substance that rotates plane-polarized light clockwise is called dextrorotatory (indicated as d or +), while the one that rotates plane-polarized light anticlockwise is called levorotatory (l or -).
The d and l isomers have the same degree of rotation but opposite signs.
These molecules are related to each other as their mirror images and are called enantiomers.
The optically active isomers that are not related as mirror images are called diastereomers.

Condition for Optical Activity

Non-superimposability of the mirror image of a molecule with the original molecule is the only condition for optical activity.
A molecule having a chiral carbon atom (all four groups attached to the C-atom are different) has non-superimposable mirror image and thus is optically active.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Identify the characteristics of molecules with plane of symmetry and their optical activity