Ethanediol comes in two isomers which differ in the placement of the hydroxy group: 1,1-ethanediol and 1,2-ethanediol (ethylene glycol). Here are the structures of the two chemicals to better show what the difference is:
         OH  H        OH   OH
         |   |        |    |
      HO-C---C--H  H--C----C--H
         |   |        |    |
         H   H        H    H
     1,1-ethanediol  1,2-ethanediol or
                      ethylene glycol
Each of the two chemicals is important in its own way. First, 1,1-ethanediol is a hydrate of ethanal (acetaldehyde) which is produced by adding water to ethanal. The reaction occurs in either acidic or basic media and is reversible. The means that an attempt to isolate the 1,1-ethanediol from the solution leads to the recovery of ethanal and water. This is generally true for most aldehydes as well as ketones. There are a couple of well known exceptions one of which is chloral hydrate (2,2,2-trichloro-1,1-ethanediol), the "knockout drops" or "Mickey Finn" of literature.

Now some may ask, if the hydrate can't be isolated. how do we know it exists? The method of doing this is to use water containing a large amount of oxygen-18 isotope. It is demonstrated by letting the reactants mix for some time that recovering the aldehyde or ketone and processing it through a mass spectrometer. The oxygen-18 isotope is now present in the recovered material.

The other isomer, 1,2-ethanediol, is an important industrial chemical. It is a clear, colorless, syrupy and virtually odorless liquid. It is produced from ethylene oxide, also known as oxirane, by the addition of water using an acid catalyst. Ethylene glycol is used as an engine coolant, antifreeze, and as a deicing agent because of its low melting point. Another important use is as a feedstock for polyester polymers.

Data about 1,2-ethanediol (ethylene glycol):

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