Organic compounds containing two hydroxy groups on one carbon (gem dihydroxy) are unstable and tend to lose water molecule. However, Boron, sulfur and phosphorus with two OH groups are stable. What is the reason?
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1$\begingroup$ Chloral hydrate is fairly stable. $\endgroup$– Klaus-Dieter WarzechaCommented Feb 27, 2016 at 8:07
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$\begingroup$ yes. Its stability is suspected due to hydrogen bonding. $\endgroup$– adianadiadiCommented Feb 28, 2016 at 4:13
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$\begingroup$ Hydrogen bonds have nothing do do with this. $\endgroup$– MithoronCommented Feb 28, 2016 at 20:00
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1 Answer
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The instability of gem-diols is due the fact that hydogen bonding between the 2 OH- groups increases the repulsion causing a strain on the C-O-H bonds.
In case of compounds of boron, a greater bond angle (due to sp2 hybridization of boron) prevents intramolecular hydrogen bonding.
Compounds of sulphur, phosphorous always contain a double bonded oxygen along with the OH- groups on the central atom. This oxygen atom forms hydrogen bonds with OH- groups preventing hydrogen bonding between the OH- groups, thus preventing any strain. 
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1$\begingroup$ Hydrogen bonding should increase the overall stability of the molecule. Do you think, H-bonding makes them to first come closer and then trigger repulsion that eventually leads to instability? $\endgroup$ Commented Feb 28, 2016 at 4:09
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$\begingroup$ Thats exactly what happens in gem-diols on sp3 hybridized atom. And, you were right about the stability of chloral hydrate. The OH- groups form H-bonds with chlorine (not exactly H-bonds since they are formed with chlorine atoms) preventing intramolecular H-bonding between OH- groups (which is the main reason for instability of other gem-diols) $\endgroup$ Commented Feb 28, 2016 at 13:59
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$\begingroup$ You've got to be kidding me. Instability because of hydrogen bonds... Do you even know what you're talking about? $\endgroup$– MithoronCommented Feb 28, 2016 at 19:57
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$\begingroup$ Only in case of gem-diols. The OH- groups bend towards each other due to H-bonding (unless it can be prevented, as in the examples above). Firstly, it causes a strain on the carbon atom because of decrease in bond angle. Also, you can see that the OH- are bent towards each due to H-bonding which results repulsions among the atoms. So preferably, the oxygen donates a lone pair to the hydrogen of other OH- group and leaves as a water molecule leaving behind a carbonyl group. $\endgroup$ Commented Feb 29, 2016 at 4:36
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1$\begingroup$ Strain is not the main reason (forget that I even said anything about bond angle). It is well known that attraction causes decrease in energy (in other words increases stability) and repulsion causes increase in energy (decreases stability). It is due to this fact that H-bonding is usually associated with stability. However if the attractions cause even greater repulsions, (as in some gem-diols), it results in instability. Thus elimination of water molecule is preferred. $\endgroup$ Commented Feb 29, 2016 at 11:33