L.3 Stereochemistry Flashcards Preview

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Flashcards in L.3 Stereochemistry Deck (17)
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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


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.




What are Structural Isomers?

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



Conformational Isomers


Different only by a single sigma bond

Newman projections

Cyclic Strain

Chair Conformations



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.





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


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)




Configurational Isomers

6 Main Concepts

Can only be interchanged by breaking and reforming bonds



Optical Activity

Racemic Mixtures


Meso Compounds



Diastereomers vs Enantiomers


  • 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)


  • 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.



What is a Chiral Center?

have four different groups attached to the central carbon


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


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.



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.


What are R & S designations on a stereocenter?


is R if it's clockwise

It is S if it's counterclockwise


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


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