Chapter 3.2
Optical Isomers
Optical Isomerism: A phenomenon occurring when an asymmetric carbon atom has four different atoms or groups attached to it.
Chiral Center: A carbon atom with four different atoms or groups attached.
Optically Active Compounds: Organic molecules that can rotate the plane of polarized light (PPL).
When a compound rotates PPL to the right (clockwise), it is referred to as:
(+) or Dextrorotary (d)
When it rotates to the left (anti-clockwise), it is called:
(-) or Levorotatory (l)
A compound becomes optically active if it contains a chiral center or chiral axis and does not possess an element of symmetry.
Polarimeter
Polarimeter: An instrument used to determine the optical activity of compounds.
Components of a Simple Polarimeter:
Light Source: Typically a sodium lamp that emits light for analysis.
Polarizer: Converts a beam of mixed polarized light into a beam with well-defined polarization.
Sample Tube: Holds the sample under examination.
Analyzer: Detects the rotation of polarized light.
Measuring Scale: Determines the number of degrees of rotation.
Rotation Properties of Enantiomers: The two enantiomers of a chiral compound rotate PPL in equal amounts but in opposite directions, allowing for their distinction.
Specific Rotation
Specific Rotation ($eta$): Defined as the number of degrees of rotation observed when light passes through an optically active compound placed in a tube having a path length of 10 cm and a concentration of 1 gram per milliliter.
The formula for calculating specific rotation is: eta = rac{ ext{observed rotation}}{c imes l} Where:
$eta$ = Specific rotation
$c$ = Concentration in g/mL
$l$ = Length of tube in dm
Chiral Center
Chiral Center: Known as Stereocenter or Stereogenic center.
Defined as an atom (often carbon) that is connected to four different groups.
Chiral Carbon: A tetrahedral (sp³) carbon connected to four different groups.
Chirality and Chiral Molecules
Chiral compounds possess:
One or more chiral centers
No elements of symmetry
The compound's mirror image is non-superimposable, leading to the formation of:
Enantiomers: A pair of chiral molecules that are non-superimposable mirror images.
Elements of Symmetry
Plane of Symmetry: An internal mirror plane that divides a molecule into two equal halves, where each half is a reflection of the other.
Can pass through atoms or between atoms.
Characteristic of achiral molecules.
Achiral Compounds: Lacking chiral centers and possessing elements of symmetry, making them superimposable on their mirror images.
Examples include:
2-Chloropropane: Has a plane of symmetry; thus, it is achiral.
2-Chlorobutane: Lacks a plane of symmetry; thus, it is chiral.
Center of Symmetry
Center of Symmetry: A point in the center of a molecule from which a line can be extrapolated such that when extended an equal distance, it meets identical atoms located an equal distance in the opposite direction.
Achiral Compounds
Characteristics:
No chiral center.
May contain a chiral center in some contexts, but the compound and its mirror image are superimposable.
Possess elements of symmetry (plane or center symmetry).
Optically inactive.
Comparison of Chiral and Achiral Compounds
Chiral Compounds:
Has a chiral center.
Non-superimposable mirror images.
No elements of symmetry.
Optically active.
Achiral Compounds:
No chiral centers.
May contain chiral centers, but they are superimposable with their mirror images.
Possess elements of symmetry.
Optically inactive.
Enantiomers
Definition: Stereo isomer compounds with the same connectivity but different arrangements in space. They possess:
One or more chiral centers.
No planes of symmetry.
Non-superimposable mirror images.
Identical physical and chemical properties.
Identical rotations of PPL but in opposite directions.
Racemic Mixture
Definition: A mixture consisting of equal amounts of two enantiomers.
Optically inactive ($eta = 0$) as the activities cancel each other.
Can be classified as either achiral or a racemic mixture, indicated by the prefix (±).
Nomenclature of Enantiomers (R,S System)
Steps for Determining the Configuration of Chiral Centers:
Rank the atoms bonded to the chiral center by atomic number according to C.I.P (Cahn-Ingold-Prelog) rules.
Example ranking: H-(1) < C-(6) < N-(7) < O-(8) < F-(9) < Cl-(17) < Br-(35) < I-(53)
Determine the sequence at the first point of difference if groups are not all different.
Assign priority to the substituents: the highest is ranked 1, the next highest is 2, and so on.
Position the lowest priority group (ranked 4) at the back.
Draw a curve from the highest priority group (1) through the second (2) to the third (3).
Determine R (right turn) or S (left turn).
Examples of Nomenclature
Example: For the chiral carbon with four attached atoms:
Atoms ranked by atomic number.
Oxygen receives priority 1 (highest), while hydrogen gets 4 (lowest).
If the lowest priority group is attached, the R/S assignment is reversed.
Double and Triple Bonds: Treated as bonds to duplicate atoms while assigning priority.
Diastereomers
Definition: Stereoisomers that are not mirror images and possess different physical and chemical properties.
Types of Diastereomers:
Cis-trans Isomers: Based on the spatial arrangement of substituents around a double bond.
E/Z Isomers: Specific nomenclature for compounds based on priority of substituents.
Conformational Isomers: Include R/S configurations relating to the free rotation of single bonds.
Properties: Diastereomers contain more than one chiral center and differ from enantiomers.
Exercises on Diastereomers
Challenges involving the stereo representations of 1,2,3-butanetriol, requiring students to:
Write IUPAC names with R/S configurations.
Identify pairs of enantiomers and diastereomers.
Answers to Optional Exercises
IUPAC Naming for Butanetriol:
(1) (2S, 3S)-1,2,3-Butanetriol
(2) (2S, 3R)-1,2,3-Butanetriol
(3) (2R, 3S)-1,2,3-Butanetriol
(4) (2R, 3R)-1,2,3-Butanetriol
Enantiomers Identified:
(1) and (4); (2) and (3).
Diastereomers Identified:
Pairs include (1) and (2), (1) and (3), (2) and (4), and (3) and (4).