Plane of Symmetry in Molecules

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.