Why three hydroxyl groups cannot stay together with a carbon atom but it forms ketone? Is it because of steric?

1 Answer
May 17, 2018

No, it is more because of bond energies.

Explanation:

A gem-diol is part of an equilibrium. For example,

#"RC(OH)"_2"R" ⇌ "RC(=O)R" + "H"_2"O"#

Consider the bonds broken and formed in the acetone equilibrium.

Acetone

We are breaking two #"O-H"# bonds in the diol and forming two #"O-H"# bonds in water, so these changes cancel each other.

We are also breaking a #"C-O"# bond in the diol and forming a #"C=O"# bond in the ketone.

The energy difference is

#ΔH = D"(C-O) -" D"(C=O) = (358 - 499) kJ·mol"^"-1" = "-441 kJ·mol"^"-1"#

The reaction is exothermic, so it is energetically more profitable for the diol to dehydrate.

Thus, the position of equilibrium lies to the right because a #"C=O"# bond is much more stable than a #"C-O"# bond