Isomers are molecules with the same formula but different structures. They come in two main types: constitutional isomers have different bonding arrangements, while stereoisomers have different spatial arrangements. Understanding isomers is crucial for predicting molecular properties and reactivity.

Stereoisomers include enantiomers (mirror images), diastereomers (non-mirror images), and cis-trans isomers (geometric isomers). Chirality, the property of non-superimposable mirror images, is key in stereochemistry. Fischer and Newman projections help visualize 3D structures in 2D representations.

Types of Isomers

Constitutional vs stereoisomers

• Constitutional isomers have the same molecular formula but different bonding arrangements or atomic connectivity (butane and 2-methylpropane, ethanol and dimethyl ether)
• Stereoisomers have the same molecular formula and bonding arrangements but different spatial arrangements of atoms ((R)-2-butanol and (S)-2-butanol, cis-2-butene and trans-2-butene)

Types of constitutional isomers

• Skeletal isomers have the same molecular formula but different carbon chain arrangements (pentane and 2-methylbutane)
• Functional isomers have the same molecular formula but different functional groups (propanal and propanone (acetone))
• Positional isomers have the same molecular formula and functional group but different positions of the functional group on the carbon chain (1-propanol and 2-propanol)

Stereoisomers

Categories of stereoisomers

• Enantiomers are non-superimposable mirror images of each other with opposite configurations at all stereogenic centers ((R)-2-butanol and (S)-2-butanol)
  • Enantiomers exhibit optical activity, rotating plane-polarized light in opposite directions
• Diastereomers are not mirror images of each other and have the same configuration at one or more stereogenic centers, but opposite configurations at other stereogenic centers ((2R,3S)-2,3-butanediol and (2R,3R)-2,3-butanediol)
• Cis-trans isomers (geometric isomers) differ in the spatial arrangement of substituents across a double bond or ring
  • Cis isomers have substituents on the same side of the double bond or ring
  • Trans isomers have substituents on opposite sides (cis-2-butene and trans-2-butene)
• Relationship between stereoisomers
  1. Enantiomers are a type of stereoisomer with a specific mirror-image relationship
  2. Diastereomers are all other stereoisomers that are not enantiomers
  3. Cis-trans isomers are a specific type of diastereomer

Chirality and Representation

• Chirality refers to the property of molecules that are non-superimposable on their mirror images
• Meso compounds are optically inactive despite having stereogenic centers due to an internal plane of symmetry
• Fischer projections are two-dimensional representations of three-dimensional molecules, commonly used for depicting stereochemistry of sugars and amino acids
• Newman projections are useful for visualizing different conformations of molecules, particularly along single bonds