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CHAPTER 15 The 3-D Shape of Molecules General, Organic, & Biological Chemistry Janice Gorzynski Smith. CHAPTER 15: The 3-D Shape of Molecules. Learning Objectives: Understand the difference between Chiral and Achiral molecules Identify chiral centers

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slide1

CHAPTER 15

The 3-D Shape of Molecules

General, Organic, & Biological Chemistry

Janice GorzynskiSmith

slide2

CHAPTER 15: The 3-D Shape of Molecules

  • Learning Objectives:
  • Understand the difference between Chiral and Achiral molecules
  • Identify chiral centers
  • Draw the enantiomers & diastereomers of a chiral molecule (using dashes & wedges to show geometry)
  • Draw fischer projections of chiral molecules
  • Explain the relevance of chiral centers in Biochemistry

Smith. General Organic & Biological Chemistry 2nd Ed.

slide3

ISOMERS

Same chemical formula

Stereoisomers

Spatial Isomers

Constitutional

Isomers

Structural Isomers

Different connectivity between atoms

Different spatial arrangement of atoms but with the same connectivity

Enantiomers

Diastereomers

Mirror images of a chiral molecule that are not superimposable

All stereoisomers that are not mirror images of one another

Cis/Trans

Isomers

Conformational

Isomers

Rotation around C-C single bonds

R

R

R

R

slide4

Stereo-

isomers

Chiral & Achiral

  • Left hands and right hands are mirror images of
  • each other.
  • A molecule that is not superimposableon its mirror
  • image is chiral.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide5

Stereo-

isomers

Chiral & Achiral

  • Two socks from a pair are mirror images that are
  • superimposable.
  • A molecule that is superimposable on its mirror
  • image is achiral.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide6

Stereo-

isomers

Chiral & Achiral

To test whether a molecule is chiral or achiral:

Draw the molecule in 3 dimensions.

Draw its mirror image.

Try to align all bonds and atoms.

  • To superimpose a molecule and its mirror image
  • you can perform any rotation but you cannot
  • break bonds.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide7

Stereo-

isomers

Chiral & Achiral

For CH2BrCl:

Rotate the

molecule to

align bonds:

ACHIRAL

Smith. General Organic & Biological Chemistry 2nd Ed.

slide8

Stereo-

isomers

Chiral & Achiral

For CHBrClF:

CHIRAL

Smith. General Organic & Biological Chemistry 2nd Ed.

slide9

Stereo-

isomers

Chiral & Achiral

Identify the chiral centers in…

cholesterol

glucose

cysteine

slide10

Chiral Molecules

Enantiomers

Many drugs are chiral, and often they must interact with a chiral receptor to be effective.

Thalidomide was an anti-nausea drug sold to pregnant in the 60’s as a mixture with its enantiomer. It was found that the enantiomer interacted with the body differently, and caused catastrophic birth defects.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide11

Chiral Molecules

Enantiomers

Ibuprofen is an active anti-inflammatory agent

whose enantiomer is inactive, and it is sold as a

racemic mixture.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide12

Chiral Molecules

Enantiomers

Two different enantiomers can interact differently

with smell receptors in the brain and have

different perceived odors.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide13

Stereo-

isomers

Multiple Chiral Centers

Draw the Enantiomer of the amino acid Glutamate

slide14

Chiral Molecules

Multiple Chiral Centers

Smith. General Organic & Biological Chemistry 2nd Ed.

slide15

Stereo-

isomers

Multiple Chiral Centers

chemwiki.ucdavis.edu

slide16

Chiral Molecules

Fischer Projections

A Fischer Projection takes a 3D tetrahedral shape,

and re-draws it:

Smith. General Organic & Biological Chemistry 2nd Ed.

slide17

Chiral Molecules

Fischer Projections

We can draw the cross and Fischer projection

for both enantiomers of 2-butanol:

Smith. General Organic & Biological Chemistry 2nd Ed.

slide18

Chiral Molecules

Fischer Projections

Fischer Projections with multiple chiral centers:

A&C; A&D; B&C; B&D are all Diastereomers

Smith. General Organic & Biological Chemistry 2nd Ed.

slide19

Chiral Molecules

Fischer Projections

Draw a Fischer Projections of the carbohydrate erythrose’s stereoisomers (all 4).

slide20

Chiral Molecules

Optical Properties

  • Two enantiomers have identical physical properties (melting pt., boiling pt., solubility, etc.)
  • They differ in how they interact with plane-polarized light.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide21

Chiral Molecules

Optical Properties

  • An achiral compound does not change the direction of plane-polarized light and is optically inactive.
  • A chiral compound rotates the plane of polarized light through an angle α and is optically active.

Smith. General Organic & Biological Chemistry 2nd Ed.

slide22

Chiral Molecules

Optical Properties

Assign Priority to the substituents bonded to the chiral carbon 1-highest, 4-lowest, to determine R or S rotation

1.  The higher the atomic number of the immediate substituent atom, the higher the priority.

For example, H–  <  C–  <  N–  <  O–  <  Cl–.

2.  If two substituents have the same immediate substituent atom, evaluate atoms progressively further away from the chiral center until a difference is found.

For example, CH3–  <  C2H5–  <  ClCH2–  <  BrCH2–  <  CH3O–.

3.  If double or triple bonded groups are encountered as substituents, they are treated as an equivalent set of single-bonded atoms.

For example, C2H5–  <  CH2=CH–  <  HC≡C–

https://www2.chemistry.msu.edu

slide23

Chiral Molecules

Optical Properties: R & S

Enantiomers have opposite optical properties: ie, R,R vs S, S rotation of light

One of the chiral centers of diastereomers will rotate light in the same way: ie, R,R vs R,S

chemwiki.ucdavis.edu

slide24

Chiral Molecules

Optical Properties: L & D

L is “left handed”

(equivalent to “S”)

D is “right handed”

(equivalent to “R”)