Enantiomers, Diastereomers, and Meso Compounds

Stereoisomers

• Molecules with the same molecular formula and structure, but different arrangements of atoms in 3D space.
  • Example: Three molecules of four bromo three chlorocyclopentine.

Constitutional Isomers

• Isomers with the same molecular formula but a different molecular structure.
  • Example: Two molecules with the same formula, but the bromine is on a different carbon in the second molecule.

Enantiomers

• Stereoisomers that are non-superimposable, non-identical mirror images of one another.
• Molecules with enantiomers are called chiral molecules.
  • A chiral molecule lacks a plane or point of symmetry.
• Identifying Enantiomers:
  • One molecule can be rotated to view it as a mirror image of the other.
  • All configurations of all stereocenters are inverted (e.g., from RR to SS).

Example

A molecule with two chirality centers, configurations R and R (3R, 4R, four bromo, three chlorocyclopentene) has an enantiomer with configurations S and S (3S, 4S, four bromo, three chlorocyclopentene).

Diastereomers

• Stereoisomers that are not mirror images of one another.
• Identified when at least one, but not all, of the R/S configurations have changed.
• Examples:
  • E and Z isomers of substituted alkenes (e.g., E-2-butene and Z-2-butene).

Key Differences

• Enantiomers: Mirror images with all R/S configurations swapped.
• Diastereomers: Not mirror images with at least one, but not all, R/S configurations swapped.

Practice Examples

1. Example 1: Two molecules with a chain of five carbons, a chlorine, and two bromines. The chlorine atom changes from a dash bond to a wedged bond. The configurations of the two bromines are the same in both molecules, therefore, these compounds are diastereomers of one another.
2. Example 2: Two substituted cyclohexane rings with two methyl groups. After rotating the second molecule, the configurations of both chirality centers have been swapped. In the first molecule, they're both on dashes pointing away, and in the second molecule, they're both on wedges pointing towards us, therefore, these are enantiomers of one another.
3. Example 3: Two molecules with a chirality center that both have five carbons in a chain, but the Oh is on the second carbon in the left molecule and the first carbon in the right molecule, therefore, these are constitutional isomers.

Number of Stereoisomers

• A molecule with n chirality centers can have up to 2^n stereoisomers.

Example

A molecule with two chirality centers:

• Original: R and S
• Enantiomer: S and R (swap both)
• Diastereomers: RR (switch on the OH carbon), SS (enantiomer of RR)

Meso Compounds

• Stereoisomers that have a plane of symmetry and are achiral.
• Result in fewer total stereoisomers than the predicted 2^n number.
• A meso compound and its mirror image are identical.
• Meso describes an achiral molecule that has chirality centers.

Example

A five-membered ring with two methyl groups:

• RR isomer and SS isomer are enantiomers.

• RS isomer is a diastereomer, but has a plane of symmetry, making it a meso compound.

  • Rotating the SR isomer results in the same molecule as the RS isomer.
  • These two compounds are identical, that's their relationship, and it is a meso compound.

  Another meso compound has an internal plane of symmetry, so it must be achiral, and these two compounds are identical, that's their relationship, and it is a meso compound.

Practice Problem

• Sorting molecule pairs according to their relationship:
  • Possible relationships: enantiomers, identical molecules, diastereomers, or constitutional isomers.
  • Top two molecules: seven-membered ring with a methyl group on a wedge or dash; since the molecule is not chiral, these are identical.
  • Next two molecules: 2R,3R,4S-2-fluoro-4-methyl-3-hexanol and 2S,3S,4R-2-fluoro-4-methyl-3-hexanol; all chirality centers are swapped, so these are enantiomers.
  • Next two molecules: alkenes with different configurations are diastereomers of one another.
  • Last two molecules: five-membered rings where the methyl group and double bond are at different distances; these are constitutional isomers.