Why is the thioester bond weaker than a regular ester bond?
Why is the thioester bond weaker than a regular ester bond?


Sulfur is a much bigger atom than oxygen or carbon. As a result, resonance donation from the sulfur into the carbonyl carbon is limited, and the C-S bond doesn't have as much multiple-bond character as the carbonyl carbon-oxygen bond in an ester. 


  What are the contributing factors to the fact that thioesters are less stable than their oxygen counterparts?


Kinetically or thermodynamically? 

Stable with respect to what? 

If we're talking about thermodynamics, and stability with regard to hydrolysis, then yes, thioesters are generally less stable than esters. 

It is more favorable for water to attack a thioester's carbonyl carbon because it has a greater partial positive charge than a corresponding ester's. This is due to the resonance issue discussed above. A thioester also has a more stable leaving group, as the larger size of sulfur allows negative charge to be distributed better.