What are the contributing factors to the fact that thioesters are less stable than their oxygen counterparts?
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2$\begingroup$ Stable with respect to what? Stability can be kinetic or thermodynamic, and also depends on the reagent you're talking about. For sure there are many examples where you can keep a thioester intact whilst manipulating an ester. $\endgroup$– NotEvans.Commented Dec 27, 2015 at 23:33
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4$\begingroup$ For most applications, though, it is the inability of sulphur to participate well in the resonance across to the carbonyl. Thioesters thus look much more like isolated (more reactive) ketones. $\endgroup$– JanCommented Dec 27, 2015 at 23:42
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1$\begingroup$ +1. No arguments with that explanation. I'm just keen to hammer home the point that stability should always be qualified. Alkenes are seen as being fairly 'stable' in a chemical sense but we all know what methane + a spark is capable of. $\endgroup$– NotEvans.Commented Dec 27, 2015 at 23:45
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1 Answer
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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.
answered Dec 28, 2015 at 5:49