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?
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.