Chemistry

Isomers

Objectives

  • Distinguish between the two main categories of isomers, structural isomers and stereoisomers.
  • Differentiate between cis- and trans- geometric isomers.
  • Recognize different structural isomers given a structural formula.
  • Recognize different structural isomers given a structural formula.

Structural Isomers

Isomers: 2 or more compounds with same molecular formula but different molecular structures Bonded in different orders
  • Different physical and chemical properties (example: Boiling point)
  • Boiling point increases as molecular structure becomes more linear
  • As # of C atoms increases so do the # of isomers
  • C7H16=9
  • C20H42=316,319


C5H12 Pentane
  • Boiling Point: 36˚C

pentane molecule

Structural Isomers

C5H12 2-Methylbutane
  • Boiling Point: 28˚C

Methylbutane molecule

Structural Isomers

C5H12 2,2-Dimethylpropane
  • Boiling Point: 9˚C

dimethylpropane molecule

Stereoisomers

How do they differ from isomers?
  • Bonded in same order but arranged differently in space.
  • C atoms with single bond can rotate freely in space.
  • Two carbon atoms bonded = no free rotation

stereoisomerism

Stereoisomers

Ethane C2H4: Two Carbon atoms double bonded = no free rotation.

ethane molecule

Stereoisomers

  • Cis-2-Butene vs. Trans-2-Butene
  • Cis= methyl groups on same side
  • Cis= methyl groups on same side
  • Two carbon atoms bonded = no free rotation
  • Boiling and melting points
  • Trans-fatty acids= really bad for you
  • Cis form of same are not as bad

Cis-2-butene

Stereoisomers

Cis-2-Butene: C4H8
  • Melting Point: -139˚C
  • Boiling Point: 3.7˚C

Cis-2-butene

Stereoisomers

Trans-2-Butene: C4H8
  • Melting Point: -106˚C
  • Boiling Point: 0.8˚C

Trans-2-butene

Chirality

Molecules are chiral another, identical, molecule exists as a non-superimposable mirror image.
  • Dextro-rotary = Right handed
  • Levo-rotary = Left handed
  • Determined by polarized light
  • One form is found in protein of living organisms
    • Dextro or Levo
    • Only one will fit active site on enzyme

Chirality

Objectives in Review

  • Distinguish between the two main categories of isomers, structural isomers and stereoisomers.
  • Differentiate between cis- and trans- geometric isomers.
  • Recognize different structural isomers given a structural formula.
  • Recognize different structural isomers given a structural formula.

The End