introduction to stereoisomers - university of california, irvine

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Introduction to Stereoisomers UCI Chem 51A Dr. Link

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Page 1: Introduction to Stereoisomers - University of California, Irvine

Introduction to Stereoisomers

UCI Chem 51ADr. Link

Page 2: Introduction to Stereoisomers - University of California, Irvine

GoalsAfter this lesson you should be able to:

Distinguish between constitutional isomers and stereoisomersIdentify stereocenters in a moleculeDetermine whether a molecule is chiral (except multiple stereocenters)Recognize pairs of enantiomersDraw the enantiomer of a chiral molecule

Page 3: Introduction to Stereoisomers - University of California, Irvine

Types of IsomersConstitutional Isomers: Molecules with the same molecular formula whose structures vary by arrangement of atoms & bonds (different connections)

Stereoisomers: Molecules that have the same molecular formula and same connections but vary by spatial arrangement

Page 4: Introduction to Stereoisomers - University of California, Irvine

Types of StereoisomersEnantiomers: Isomers that are mirror images of each other but are not superimposable on each otherDiastereomers: Not quite enantiomers, contain multiple stereocenters (we’ll get to these in another lesson)Cis/trans isomers: Isomers that are not enantiomers or diastereomers but differ in cis/trans (most common with alkenes)

Page 5: Introduction to Stereoisomers - University of California, Irvine

ChiralityA molecule is chiral if it IS NOT superimposable on its mirror image.

A molecule is achiral if it IS superimposable on its mirror image.

1 2 1

(rotated)

Page 6: Introduction to Stereoisomers - University of California, Irvine

EnantiomersEnantiomers are a pair of isomers that are mirror images of each other and are NOT superimposable on each other.

_______________________________________________

1 2

Page 7: Introduction to Stereoisomers - University of California, Irvine

Stereocenters Most (but not all) chiral molecules have at least one stereocenter.

Stereocenter:

Page 8: Introduction to Stereoisomers - University of California, Irvine

Cyclic Molecules & Stereocenters

Watch out for symmetry!

stereocenter?

stereocenter?

Page 9: Introduction to Stereoisomers - University of California, Irvine

How to Tell If It’s Chiral or Achiral

1. Look for stereocenters.If one stereocenter, chiral.Multiple stereocenters? Maybe chiral.

2. Look for planes of symmetry.Plane of symmetry = achiral

Page 10: Introduction to Stereoisomers - University of California, Irvine

How to Draw the Enantiomer

Option 1. Literally draw the mirror image.Option 2. Switch any two groups.

mirrorplane

Option 1 Option 2

red & white switched

rotate

Page 11: Introduction to Stereoisomers - University of California, Irvine

Chiral Without A Stereocenter?!

DNA is chiral!Helices have a “handedness”

HexaphenylbenzeneAlso a helix

Page 12: Introduction to Stereoisomers - University of California, Irvine

Wrapping UpPractice recognizing types of isomersPractice identifying stereocentersPractice drawing and identifying enantiomersPractice looking for planes of symmetry in moleculesPractice determining whether a molecule is chiral or achiral