Org. Chem I               Handout #5 

Copyright © Doug E. Frantz The University of Texas at San Antonio 

STEREOCHEMISTRY 

There are two major classes of isomers you should be intimately familiar with: 

1) Constitutional  isomers  (or  structural  isomers)  –  compounds  with  the  same molecular formula but differ in the way atoms are bonded to each other.  • different IUPAC names  • same or different functional groups  • different physical properties   • different chemical properties 

2) Stereoisomers – compounds with  the  same molecular  formula AND atom connectivity but differ in the 3D spatial arrangement of atoms.  • differ only in the way the atoms are oriented in space • have identical IUPAC names (except for a prefix like cis or trans) • always have the same functional group(s) • differ in configuration (its particular three‐dimensional arrangement) 

EXAMPLES: 

 

 

 

 

   

CHIRALITY  A molecule (or anything for that matter) that is not superimposable on its mirror image is CHIRAL.  A molecule that is superimposable on its mirror image is said to be ACHIRAL.  The most common and simple example is to look at your hands.  Your hands  are mirror  images  of  each other.    But, when  you  try  to  lay  them on  top of  each other (palms up), they do not overlap (non‐superimposable).  What about your whole body?   Think about  it  and  see  if  you  can  rationalize  if  your entire body  is chiral or achiral.     

both have the same molecular formula and the IUPAC names except for the prefix (cis vs. trans).

Thus, they are stereoisomers since the only thing that is different is the spatial arrangement of the ethyl groups.

H3CCH3

CH3

CH3

CH3

CH3

(1R,3S)-1,3-dimethylcyclohexaneChemical Formula: C8H16

Molecular Weight: 112.21

(E)-3,4-dimethylhex-3-eneChemical Formula: C8H16

Molecular Weight: 112.21

both have the same molecular formula but...........very different IUPAC names.

Thus, they are constitutional isomers.

H3CH2C CH2CH3 H3CH2C CH2CH3

trans-1,2-diethylcyclobutanecis-1,2-diethylcyclobutane

Chemical Formula: C8H16

Molecular Weight: 112.21Chemical Formula: C8H16

Molecular Weight: 112.21

Org. Chem I               Handout #5 

Copyright © Doug E. Frantz The University of Texas at San Antonio 

  Let’s look at some molecules……..  

  Practice problems.  Draw the mirror image of each compound and label them as chiral or achiral. 

 

Mirror plane

Mirror images that do not overlap when laid on top of each other

Thus, your hands are chiral

H

H F

ClH

HF

Cl

mirror plane

H

H F

Cl

CH3

H F

ClCH3

HF

Cl

rotate to align the bonds to each atom

rotate to try to align the bonds to each atom

CH3

H Cl

F

You can't do it no matter how you rotate!!These are non-superimposable mirror images. Thus, this molecule is chiral.

These are exactly the same molecules. All of the atoms line up perfectly and can be superimposed on each other. Thus, this molecule is

achiral.

Ex. CH2FCl Ex. CH3CHClF

Br

H Cl

H

CH3

HO Cl

H

H2N H

OH

OH

H

CH3

H3C

H

HO2CCO2H