Isomers and Stereochemistry

Isomers

  • Molecules with the same molecular formula but different arrangement of atoms.

Types of Isomers

  • Constitutional Isomers: Same molecular formula but different connections between atoms.

  • Stereoisomers: Same connections between atoms but different spatial orientations.

  • Enantiomers: Stereoisomers that are mirror images of each other.

  • Diastereoisomers: Stereoisomers that are not mirror images (covered in less detail).

Constitutional Isomers

  • Molecules with the same chemical formula, regardless of atom arrangement.

  • Example: C3H7F with the F atom in different positions.

  • Focus is primarily on stereoisomers.

Stereoisomers (Spatial Isomers)

  • Two types: Diastereoisomers and Enantiomers

Cis/Trans Diastereoisomers

  • Single bonds can rotate, double bonds cannot.

  • Stereoisomers are formed due to the rigidity of double bonds.

  • Example: Two molecules with the same atoms attached (X and H), but different arrangements due to the double bond.

  • Cis Isomer (Z isomer): Groups on the same side of the double bond.

  • Trans Isomer (E isomer): Groups on opposite sides of the double bond.

Identifying Cis/Trans Isomers

  • Draw a line through the double bond.

  • Determine the priority of groups based on atomic number.

  • Higher atomic number = higher priority.

  • If atoms on one side of the double bond are the same, look at the next attached atom.

  • A single atom of a higher atomic number has a greater priority than multiple smaller atoms.

Example

  • Draw a line to separate sides.

  • Left side: Carbon has higher priority than Hydrogen.

  • Right side: Carbon has higher priority than Hydrogen.

  • If priority groups are on the same side: cis isomer.

  • If priority groups are on opposite sides: trans isomer.

Practice Question Example

  • Draw a line down the middle.

  • Compare atomic numbers: Br vs Cl

  • Br has a higher atomic number than Cl.

  • Compare atomic numbers: Cl vs H

  • Cl has a higher atomic number than H.

More Complex Example

  • If the atom attached to the carbon is same (e.g. carbon), go to the next functional group attached.

  • Determine priority based on the atomic number of this functional group

Key Takeaways

  • Know how to assign priority and determine if a molecule is a cis, trans, Z, or E isomer.

Enantiomers

  • Mirror images of one another.

  • Same atoms are attached to the carbon atoms.

  • Differentiated from one another by a few properties.

Chirality

  • For something to be an enantiomer, it needs to be a chiral molecule.

  • Chiral molecule: Molecule with four different atoms or groups attached to it.

  • Look for carbons that are attached to four different things.

Enantiomer Properties and Significance

  • Generally have the same properties (melting point, boiling point, etc.).

  • Differ in how they rotate plane-polarized light.

Rotation of Polarized Light

  • When light travels through a solution of one type of enantiomer, the light rotates a certain angle.

  • The other enantiomer will rotate the light the same angle in the opposite direction.

  • If one enantiomer rotates light 45 degrees clockwise, the other rotates it 45 degrees counterclockwise.

  • Racemic Mixture: Equal mix of enantiomers is almost always present, unless we cause it to be otherwise

  • Because approximately half of the molecules are rotating light clockwise and the other half are rotating light counterclockwise, the overall rotation is zero.

  • Labeled as plus minus followed by the name of the molecule (e.g., plus minus propanol).

Naming Enantiomers (R/S Configuration)

  • Need a way to name each enantiomer because chiral carbons have two different ways they can be arranged

  • Find the chiral molecule.

  • Determine if it's chiral - it needs to have four different groups attached

  • Assign priorities to the attached groups based on atomic number.

  • If there is a hydrogen atom present, it is always going to be number four in priority.

  • Rotate the molecule so that the group with the least priority points into the page.

  • Draw a circle from priority one to two to three.

  • If the circle is counterclockwise, it is an S configuration. If it is clockwise, it is an R configuration.

Summary

  • Know functional groups.

  • Isomers: Constitutional, Diastereoisomers (cis/trans or Z/E), and Enantiomers.

  • Diastereoisomers: cis/trans isomers or zed/e isomers.

  • Enantiomers: Arrangement of atoms around a chiral carbon (carbon attached to four different groups or atoms).