L.3 Stereochemistry Flashcards Preview

Kaplan Organic Chemistry > L.3 Stereochemistry > Flashcards

Flashcards in L.3 Stereochemistry Deck (17)
Loading flashcards...
1

ISOMERS MAP

2

Physical Properties (6) VS Chemical Properties (2)

Physical Properties

  • melting point
  • boiling point
  • solubility
  • odor
  • color
  • density

Chemical Properties

  • functional group
  • how a molecule reacts with other molecules

3

Definition of ISOMERS

each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties.

 

4

Constitutional

What are Structural Isomers?

  • share only molecular formula
  • have different physical and chemical properties

5

Stereoisomers

Conformational Isomers

 

Different only by a single sigma bond

Newman projections

Cyclic Strain

Chair Conformations

 

6

Newman Projections

(Stereoisomers - Conformational Isomers)

 

 

A Newman projection, useful in alkane stereochemistry, visualizes the conformation of a chemical bond from front to back, with the front atom represented by a dot and the back carbon as a circle.

 

 

 

7

Name and define 3 types of cyclic Conformations Strain

(Stereoisomers - Conformational Isomers)

Angle Strain

stretching or comprising angles from their normal size

Torsional Strain 

eclipsing conformation

Nonbonded Strain (van der Waals repulsion)

interactions between substituents attached to non-adjacent carbons

8

Chair conformations

(Stereoisomers - Conformational Isomers)

Cyclic molecules will adopt nonplanar shapes to minimize their strain

Axial; sticking up or down (create more nonbonded strain)

Equatorial; in the plane of the molecule (largest substituents take this place to minimize strain)

 

9

Stereoisomers

Configurational Isomers

6 Main Concepts

Can only be interchanged by breaking and reforming bonds

Enantiomers

Diastereomers

Optical Activity

Racemic Mixtures

Cis-trans

Meso Compounds

 

10

Diastereomers vs Enantiomers

Diastereomers

  • non-mirror images, they differ at some, but not all, chiral centers, they have different physical and chemical properties
  • Cis-trans; cis-trans isomers are a subtype of diastereomers in which groups differ in position about an immovable bond

(such as a double bond or cycloalkane)

Enantiomers

  • non-superimposable images and thus have opposite stereochemistry at every chiral carbon, have same chemical and physical properties except for rotation of plane-polarized light and reactions in chiral environments.
  • Racemic Mixtures, with equal concentrations of enantiomers, will not be optically active because the two enantiomers rotations cancel each other out.

 

11

What is a Chiral Center?

have four different groups attached to the central carbon

12

What does it mean when a molecule has Optical Activity?

How do we annotate this?

The ability of a molecule to rotate plane-polarized light 

d-    or      +      rotate light to the right

       l-  or       - rotate light to the left

13

Relative vs Absolute Configurations

Relative; gives the stereochemistry of a compound in comparison to another molecule

Absolute; gives stereochemistry of a compound without having to compare to other molecules

  • uses the Cahn-Ingold-Prelog priority rules; in which priority is given by looking at the atoms connected to the chiral carbon or double bonded carbons; highest atomic number wins.

 

14

What are E/Z assignations on an alkene?

An alkene is (Z) if the highest priority substituents are on the same side of the double bond

and (E) if on the opposite.

15

What are R & S designations on a stereocenter?

Stereocenter

is R if it's clockwise

It is S if it's counterclockwise

16

Fisher Diagrams

  • DASHES; into the plane of the paper
  • WEDGES; out of the plane of the paper
  • Switching one pair of substituents inverts the stereochemistry of the chiral center - Switching two pairs retain the stereochemistry
  • Rotating a 90 degrees inverts the stereochemistry - Rotating 180 retains stereochemistry

17

Specific Rotation Equation

alpha = alpha observed / C X l

 

Specific rotation in degrees = observed rotation in degrees / concentration g/ml times path lenght in dm