Re: Why do diasterioisomers have different physical properties?

First, let's define diastereomers as "stereoisomers that ARE NOT mirror 
images of each other,"  and where stereoisomers are molecules that have the 
same bonding sequence but differ in the orientation of their atoms in 
space.  So in addition to diastereomers, stereoisomers include enantiomers, 
which are molecules that are nonsuperimposable mirror images of each other.

	Diastereomers usually include molecules that have a double bond, a 
ring, or two or more chiral carbons, so we can say essentially that 
diastereomers exhibit different physical properties because the atoms of 
any two given diastereomoers are oriented differently in space.  This 
orientation leads to things such as dipole moment or increased surface 
area, which in turn affect things like boiling point and polarity.

	Let's first consider trans-2-butene and cis-2-butene.  If you 
construct a three dimensional model of these two molecules, you will see 
that they have the exact same connectivity (two carbons double bonded to 
each other, and each of these carbons having one methyl group and one 
hydrogen bonded to it), but they are not mirror images.  Because of the 
orientation of the methyl groups in space, we observe a dipole moment from 
the cis molecule but not from the trans molecule.  This dipole moment in 
turn affects the boiling points.  The cis molecule boils at 3.7 *C and the 
trans molecule boils at 0.9 *C.  

	We can also look at trans-1,2-dimethylcyclopentane and 
cis-1,2-dimethylcyclopentane.  The trans molecule has two enantiomers and while 
the cis isomer does not.  We also know that the trans isomer has a boiling 
point of 92 *C, and the cis isomer has a boiling point of 100 *C.  We can 
therefore assume that something like dipole moments and larger surface area 
present in the cis form are not present in the trans form, thus giving the 
cis form a higher boiling point and physical properties different from the 
trans isomer.

	These are just a two examples (if you want to see more, you can 
look at the various diastereomers of glucose, a common sugar, but hundreds 
of other examples in other molecules also exist).  

[Moderator's note: glucose is only one of the eight aldohexose sugars -- each of 
which also have their own enantiomers -- which all have the same connectivity. 
These sugars, which are chemically so different that they were given different 
names during the days in which the structures were not known, differ ONLY in the 
orientation of their atoms in space.]

But in the end, we can 
say that the different properties of diastereomers stem from the fact that 
the atoms are not arranged the same way in space, and the orientation of 
atoms is a major factor in determining physical properties.

Hope this helps,
-Dan