Chemistry Isomers Overview

Isomers

Isomers are compounds with the same molecular formula but different arrangements of atoms.

• Constitutional isomers have different IUPAC names, different functional groups, different physical properties, and different chemical properties.
• Stereoisomers differ only in the way the atoms are oriented in space.
• Stereoisomers have the same IUPAC names (except for a prefix like cis or trans).
• Stereoisomers always have the same functional groups.

Geometrical Isomers

• Geometrical isomers are isomers that differ in the spatial arrangement of atoms around a double bond or ring structure.
• The π-bond in an alkene does not permit rotation, which leads to different spatial arrangements.
• Cis isomers have substituents on the same side of the double bond.
• Trans isomers have substituents on opposite sides of the double bond.
• Trans isomers are usually more stable than cis isomers.

Identifying Cis and Trans Isomers

• Example: 3-methyl-2-pentene can exist as both cis and trans isomers.

Cis-trans Isomerism in Cycloalkanes

• Cis isomers have substituents on the same side of the ring.
• Trans isomers have substituents on opposite sides of the ring.

Optical Activity

• Optical activity is the ability of a chiral molecule to rotate the plane of plane-polarized light.
• Plane-polarized light vibrates in a single plane.
• A polarimeter measures the angle of rotation of plane-polarized light.

Dextrorotatory and Levorotatory

• Dextrorotatory (+) compounds rotate plane-polarized light clockwise.
• Levorotatory (-) compounds rotate plane-polarized light counterclockwise.

Specific Rotation

• Specific rotation [α]D is the angle of rotation of plane-polarized light by a 1.00 gram per cm-3 sample in a 1 dm tube.
• Specific rotation depends on the nature of the substance, wavelength of light, concentration, thickness of the layer, solvent, and temperature.

Chiral Centers

• A chiral center is a tetrahedral atom bonded to four different groups.
• Chiral molecules have nonsuperimposable mirror images (enantiomers).
• Achiral molecules have superimposable mirror images.

Configuration

• Configuration refers to the specific arrangement of atoms in a molecule, particularly with regard to stereochemistry.
• The R/S system and the D/L system are used to assign configurations.

Enantiomers

• Enantiomers are non-superimposable mirror image isomers.
• Enantiomers have identical physical properties but differ in their optical activity.
• They have different biological properties.
• A racemic mixture is formed when equal quantities of enantiomers are mixed.

Diastereomers

• Diastereomers are stereoisomers that are not mirror images of each other.
• They have different physical properties.
• Diastereomers have different specific rotations and may have the same or opposite sign of rotation.

Meso Compounds

• A meso compound is a molecule with multiple stereocenters that is superimposable on its mirror image.
• Meso compounds are achiral despite having chiral centers.
• They have an internal plane of symmetry that divides the molecule into two symmetrical halves with opposite stereochemistry.

This quiz covers the concept of isomers, their classifications, and specific types such as geometrical isomers. You will learn about constitutional isomers, stereoisomers, and how to identify cis and trans isomers based on their spatial arrangement. Test your understanding of these fundamental chemical principles!