Stereochemistry: Isomerism and Chirality

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STEREOCHEMISTRY

Isomerism & Chirality

Stereochemistry is the study of the spatial arrangement of atoms in molecules and its impact on their chemical behavior. This topic covers the concepts of isomerism, geometric isomers, enantiomers, chirality, and the rules for assigning absolute configuration.

Intended Learning Outcomes

Define the concept of isomerism.
Distinguish structural isomers from stereoisomers.
Identify geometric isomers.
Apply the CIP rule to determine the cis/trans conformation of alkenes and cyclic molecules.
Appreciate the difference between cis/trans and E/Z nomenclature.
Identify optically active compounds and stereogenic centers.
Apply the CIP rule to assign R and S descriptors to stereogenic centers.
Apply the CORN law to assign L and D descriptors to amino acids.

ISOMERISM

Definition and Types

Isomers are compounds with the same atomic composition but different structural or stereochemical formulas, resulting in different physical and chemical properties.

Structural Isomers: Molecules with the same molecular formula but different atom connectivity. Example: C3H6O can be either an aldehyde (propanal) or a ketone (acetone), each with distinct boiling points and properties.
- Propanal: , bp = 49°C
- Acetone: , bp = 56°C
Stereoisomers: Molecules with the same structural formula and atom connectivity, but different spatial orientation of atoms.
- Geometric Isomers (cis/trans or E/Z)
- Enantiomers & Diastereomers

GEOMETRIC ISOMERS

Alkenes and Cyclic Molecules

Geometric isomers arise due to restricted rotation around a double bond or within cyclic structures, leading to different spatial arrangements (cis/trans or E/Z).

Alkenes: Restricted rotation around the double bond leads to cis (Z) and trans (E) isomers.
- cis (Z): Similar groups on the same side of the double bond.
- trans (E): Similar groups on opposite sides of the double bond.
Cyclic Molecules: Geometric isomerism can also occur in rings, e.g., cis- and trans-1,2-dimethylcyclohexane.

E/Z Nomenclature: The E/Z system is based on the Cahn-Ingold-Prelog (CIP) priority rules, assigning priorities to substituents attached to the double bond.

Cahn-Ingold-Prelog Convention

Each group attached to the double bond is assigned a priority based on atomic number.
If the two highest priority groups are on the same side, the isomer is Z (zusammen, together).
If on opposite sides, the isomer is E (entgegen, opposite).

Example: In 2-butene, methyl groups on the same side = cis (Z); on opposite sides = trans (E).

ENANTIOMERS

Definition and Properties

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. They have identical physical properties except for their interaction with plane-polarized light and reactions in chiral environments.

A stereogenic center is a carbon atom bonded to four different substituents.
Presence of a stereogenic center is necessary but not sufficient for chirality.
Chiral molecules lack a plane of symmetry and cannot be superimposed on their mirror images.

Example: The two enantiomers of lactic acid rotate plane-polarized light in opposite directions.

SUCROSE

Optical Activity

Chiral molecules, such as sucrose, rotate the plane of polarized light. This property is called optical activity.

Example: Sucrose is a disaccharide composed of glucose and fructose, both of which are chiral.

POLARIMETER

Measurement of Optical Activity

A polarimeter is an instrument used to measure the rotation of plane-polarized light by chiral compounds.

Light passes through a polarizer, sample tube, and analyzer.
The angle of rotation is measured to determine the optical activity of the sample.

Equation: The observed rotation () is related to concentration (), path length (), and specific rotation ():

ABSOLUTE STEREOCHEMISTRY

R and S Configuration (CIP Convention)

The absolute configuration at a stereogenic center is defined by the descriptors R (Rectus, right) and S (Sinister, left), assigned according to the Cahn-Ingold-Prelog priority rules.

Identify the stereocenter.
Assign priorities (1 = highest, 4 = lowest) to the groups attached, based on atomic number.
Orient the molecule so the lowest priority group (4) is pointing away.
If the sequence 1 → 2 → 3 is clockwise, assign R; if counterclockwise, assign S.

Example: Assigning R/S to the stereocenter in alanine.

THE CORN LAW

L and D Notation for Amino Acids

The CORN law is used to establish whether amino acids are L or D isomers, based on the arrangement of substituents around the chiral center.

(D) and (L) notation originated from the direction in which enantiomers of glyceraldehyde rotate plane-polarized light.
In amino acids, the CORN law uses the orientation of COOH, NH2, R group, and H to assign L or D.
CORN stands for COOH, R, NH2 (in clockwise order).
If the CORN sequence is clockwise when the hydrogen is pointed away, the amino acid is L; if counterclockwise, it is D.

Example: L-alanine and D-alanine differ in the spatial arrangement of their substituents.