Ethanol and methanol are ethane and methane derivatives, respectively, with one hydrogen replaced by an hydroxyl group. Are there molecules based on either where more than one hydrogen atom has been replaced by an hydroxyl group? If so, what are they called?

  • $\begingroup$ This is a geminal diol. They're not that stable, but they're intermediates in lots of carbonyl reactions. $\endgroup$
    – Zhe
    Commented Nov 23, 2016 at 19:14
  • $\begingroup$ "That's no hydroxyl ion, that's my wife!" $\endgroup$ Commented Nov 23, 2016 at 22:00

2 Answers 2


Generally speaking, they're called diols when bearing two hydroxyl groups. If they're on the same C, this is called a geminal diol, if they are on neighboring Cs, they're called vicinal diols. The C on geminal diols has a higher oxidation state than that of an alcohol, i.e. which is why they could be seen as relatives of an aldehydes (or ketones). In a diluted aqueous solution, formaldehyde $\ce{H2C=O}$ actually predominantly exists in its geminal diol form $\ce{H2C(OH)2}$. This is no longer true for 1,1-ethanediol (the hydrated form of acetaldehyde).

The simplest vicinal diol 1,2-ethanediol is better known under its trivial name ethylene glycol (a common ingredient of anti-freeze formulations).

Triols with the hydroxyl groups on the same C (with the oxidation state of carboxylic acids) are generally unstable.

  • $\begingroup$ Interesting -- do you have a reference for the formaldehyde/geminal diol equilibrium? Also, thanks for posting, and welcome to Chem.SE! $\endgroup$
    – hBy2Py
    Commented Nov 23, 2016 at 20:56
  • $\begingroup$ @hBy2Py This looks useful for equilibrium references: pubs.rsc.org/en/content/articlepdf/1963/jr/jr9630005217 $\endgroup$
    – R.M.
    Commented Nov 23, 2016 at 21:04
  • $\begingroup$ It looks it was not ethylene but diethylene glycol in wine. $\endgroup$
    – Mithoron
    Commented Nov 24, 2016 at 2:00
  • $\begingroup$ @Mithoron yes, you are right – I had remembered that incorrectly, I will update the answer $\endgroup$ Commented Nov 29, 2016 at 14:07

Zhe and user1573870 already mentioned that geminal diols are usually not stable (Erlenmeyer rule), and user1573870 has pointed out that these compounds are hydrates of carbonyl compounds.

Typically, loss of water is the preferred mode and leads to the corresponding aldehyde (or ketone).

However, there are some exceptions to the rule: Some compounds actually exist as their hydrates. The most famous examples are

  • Chloral hydrate, the crystalline hydrate of 2,2,2-trichloroacetaldehyde
  • Glyoxylic acid, which crystallises as the hydrate (2,2-dihydroxyacetic acid) from aqueous solutions
  • 1
    $\begingroup$ Interestingly, with silicon, which is right below carbon in the periodic table, it's the other way round: here, it's the double bonded oxygen which is not stable, but the geminal hydroxyl groups. To be fair, in silicon, those have a tendency to condensate to form oligo-/polysiloxanes. $\endgroup$ Commented Dec 2, 2016 at 9:09

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