Chapter 4 Organic Chemistry: Isomers

Constitutional Isomers (Structural Isomers): Differ in their bonding sequences; their atoms are connected differently.
Stereoisomers (Configurational Isomers): Have the same bonding sequence but differ in the orientation of their atoms in space.

Constitutional Isomers: Isomers that differ in connectivity.
Stereoisomers:
- Enantiomers: Mirror-image isomers.
- Diastereomers: Non-mirror-image stereoisomers.
- Configurational Diastereomers: Stereoisomers that are not interconvertible by rotation around single bonds.
- Cis-Trans Diastereomers: A specific type of diastereomer.

Rotation in Alkanes: Rotation occurs around the carbon–carbon bond in ethane; however, no rotation is possible around the carbon–carbon bonds in cyclopropane without breaking the ring.
Flexibility: Cycloalkanes are less flexible than open-chain alkanes.
When viewed edge-on, cycloalkanes have two faces: a “top” face and a “bottom” face, which allows isomerism in substituted cycloalkanes.
Stereochemical Isomers: Compounds with the same atom connectivity but differing in their 3-D orientation.
Stereochemistry: Refers to the three-dimensional aspects of chemical structure and reactivity.

Cis–Trans Isomers: A subclass of stereoisomers defined by the prefixes:
- Cis-: Latin for “on the same side”.
- Trans-: Latin for “across”.

Comparison of Hands: Your left and right hands are similar but not superimposable; they represent chiral objects as they have nonsuperimposable mirror images.
Chirality in Molecules:
- Chiral: Molecules without a plane of symmetry that are not superimposable on their mirror image.
- Achiral: Molecules with a plane of symmetry that are superimposable on their mirror image.

Enantiomers: Nonsuperimposable mirror-image molecules; a chiral compound always has a corresponding enantiomer.
Achiral Compound: A mirror image that is superimposable on the original.

Definition: A carbon atom bonded to four different groups, referred to as an asymmetric carbon atom or chiral carbon (denoted by an asterisk *).
Example: 5-bromodecane has four different substituents at its chiral center.

Non-Chiral Centers: -CH3, -CX3, -CH2-, -CX2- carbons, and aromatic ring carbons are not chirality centers.

Example: Alanine has distinguishable enantiomers that can be metabolized differently by enzymes.
Cahn–Ingold–Prelog Rules: Assigns (R) or (S) to chiral centers.

Step 1: Rank the four atoms attached to the chirality center by atomic number.
Step 2: Arrange the molecule so the lowest priority group is facing away.
Step 3: Clockwise order = (R), Counterclockwise order = (S).
Rule 3: Multiple-bonded atoms count as equivalent to the same number of single-bonded atoms.

Swapping two groups on a chiral center will lead to its opposite configuration.
Swapping two groups twice or rotating three groups returns the original configuration but allows for the priority 4 group to point away.

Enantiomers have identical physical properties in a non-chiral environment, but their chemical reactivity can differ in a chiral environment.

R and S Thalidomide Example:
- (R)-thalidomide: Drug that helped many patients.
- (S)-thalidomide: Known teratogen that caused severe birth defects in numerous embryos.

Molecules with multiple chirality centers can have more than two stereoisomers: Up to $2^n$ stereoisomers, where n is the number of chirality centers.
Epimers: A type of diastereomer that differs at only one chirality center.

Morphine: Identify the chirality centers (5) and possible stereoisomers ($2^5 = 32$ total; many too strained to exist).

Achiral compounds that contain chirality centers, such as tartaric acid, which has both a chiral and an achiral form.

Stereoisomer	Melting Point (°C)	[α] D	Density (g/cm³)	Solubility at 20 °C (g/100 mL H2O)
(+)	168-170	+12	1.7598	139.0
(-)	168-170	-12	1.7598	139.0
Meso	146-148	0	1.6660	125.0