Isomers in Chemistry

Isomers

• Isomers are compounds with the same molecular formula but different structures and properties.
• Three types of isomers: structural, geometric, and enantiomers.

Structural Isomers

• Same molecular formula, different connectivity of atoms.
• Typically exhibit different physical and chemical properties.
• Example: Acetone (C₃H₆O) and Propanol (C₃H₆O) have different boiling points and melting points.
  • Acetone (2-propanone): bp 56°C, mp -94°C
  • Propanol (propionaldehyde): bp 48°C, mp -81°C
• Other examples include isomers of aminophenols (e.g., 2-aminophenol, 3-aminophenol, 4-aminophenol).

Geometric Isomers

• Same molecular formula and connectivity, different spatial arrangements.
• Require an inflexible bond (e.g., a double bond).
• Each carbon of the double bond must have two different substituent groups.
• Determining cis/trans:
  • Assign priority to substituents based on molecular mass (longer carbon chain = higher priority).
  • Classify as cis or trans based on the position of highest priority groups on each carbon of the double bond.
  • Same for ring structures, based on priority of substituent groups.

Enantiomers

• Molecules that are mirror images of each other.

• Left and right hands are an example.

• Enantiomers are not identical.

• Chiral molecules:

  • Asymmetric carbon atom (4 different groups bonded).
  • Lack a plane of symmetry.
  • Mirror image is non-superimposable.

• Achiral molecules:

  • Lack chiral properties.
  • Possess a plane of symmetry.
  • Mirror image is superimposable.

• Enantiomers only exist when all four groups attached to the central carbon atom are different.

• This central carbon is a chiral carbon, making the molecule chiral.

• Example: Amino acids, building blocks of proteins. Many amino acids exist as enantiomers.

Application: Enzymes

• Enzymes are chiral.
• Their binding sites are specific to one enantiomer.
• Binding sites won't fit the wrong enantiomer.

Racemic Mixture: Thalidomide

• Racemic mixture: A solution with equal quantities of both enantiomers.
• Thalidomide example: A drug used to combat morning sickness in pregnant women.
• Had a chiral carbon, with one enantiomer being effective while the other caused birth defects.

Racemic Mixture: Advil (Ibuprofen)

• Ibuprofen is a chiral molecule (with a chiral carbon).
• Production results in a racemic mixture (equal amounts of both enantiomers).
• One enantiomer is effective as an anti-inflammatory.
• The other enantiomer has no significant effect.