4.3 Glycols and Related Compounds

A glycol is an aliphatic diol, that is, an aliphatic alcohol with two hydroxy groups. Ethylene glycol is the most common example, with the formula HOCH₂CH₂OH. Note that the carbon-carbon bond is single and not double like in ethylene the hydrocarbon. Confusingly, ethylene as a group means -CH₂CH₂-, perhaps because both ethylenes are CH₂CH₂ and they only differ on whether other atoms are attached. Polyethylene is made of repeating -CH₂CH₂- units, and can form spontaneously from ethylene gas when the molecules give up their double bonds in favor of becoming one long chain.

There are also methylene groups, -CH₂-, even though there is no such compound as methylene. Methylene glycol, or methanediol, doesn't keep its shape but freely converts back and forth to formaldehyde and water. But methylene chloride ClCH₂Cl is a common solvent.

Propylene glycol is a mixture of isomers HOCH₂CH₂CH₂OH and HOCH₂CH(OH)CH₃. Again, unlike propylene the hydrocarbon, propylene glycol has only single bonds.

The glycols tend to be watery or syrupy liquids that are miscible with water. They are similar to glycerol (glycerine), a polyol with the formula HOCH₂CH(OH)CH₂OH.

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