Org. Chem I Handout #5 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: CH3
CH3
CH3
H 3C
CH3
CH3
(E)-3,4-dimethylhex-3-ene
Chemical Formula: C8H16
Molecular Weight: 112.21
(1R,3S)-1,3-dimethylcyclohexane
Chemical Formula: C8H16
Molecular Weight: 112.21
both have the same molecular formula
but...........
very different IUPAC names.
Thus, they are constitutional isomers.
H3CH2C
CH2CH3
cis-1,2-diethylcyclobutane
Chemical Formula: C8H16
Molecular Weight: 112.21
H3CH2C
CH2CH3
trans-1,2-diethylcyclobutane
Chemical Formula: C8H16
Molecular Weight: 112.21
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.
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. Copyright © Doug E. Frantz The University of Texas at San Antonio Org. Chem I Handout #5 Mirror plane
Mirror images that do not overlap when laid on top of each other
Thus, your hands are chiral
Let’s look at some molecules…….. Ex. CH2FCl
H
H
H
Cl
F
Ex. CH3CHClF
mirror plane
Cl
F
CH3
Cl
H
F
H
Cl
F
CH3
H
rotate to align
the bonds to
each atom
rotate to try to align
the bonds to each
atom
H
H
Cl
F
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.
CH3
F
H
Cl
You can't do it no matter how you rotate!!
These are non-superimposable mirror images. Thus,
this molecule is chiral.
Practice problems. Draw the mirror image of each compound and label them as chiral or achiral. OH
H
Br
CH3
H
CO2H
H
H
OH
HO2C
CH3
H
HO
H2N
Cl
H
Cl
H3C
Copyright © Doug E. Frantz The University of Texas at San Antonio