Dependence of steric hinderance on enol content
How can hydrogen form two bonds?


In the case of an enol, hydrogen does not form two full bonds.  What the drawing below is trying to show with the enol hydrogen is that one bond (the O-H with the solid line) is elongated, partially broken, while the other bond (the O..H with the dotted bond) is also elongated and only partially formed.  It's as if the partially broken OH bond and the partially made OH bond are each "half a bond", so all together the electron sharing is such that there is still just one full bond to hydrogen




  How does the steric repulsion destabilize the enol form?


Look at the bottom part of the drawing where we have inserted an alpha-methyl group into the 1,3-diketone.  Notice the steric interactions indicated by the arrows.  The situation looks similar to that seen with cis-2-butene.  The methyl groups are large enough (larger than a hydrogen) and close enough to one another that their electron clouds "bump" into one another causing what we often call "steric destabilization."  Remember that trans-2-butene is more stable than cis-2-butene because the trans isomer does not have the adverse steric interaction present in the cis isomer.  It is that same type of steric interaction that occurs here with the enol when an alpha-methyl group is present, and just like in cis-2-butene, it destabilizes the enol.  Therefore less of the enol will be present at equilibrium when an alpha methyl group is added to the enol.