[uglepik]

Does anybody have experience with the stability of ester in water at neutral pH? For how long can they be expected to be stable?

For example. If I dissolve an ester in water and put it in the refrigerator. For how long would you say that I can expect my ester to still be my ester? Days, months, years?

Thanks in advance
Andreas

[discodermolide]

Does anybody have experience with the stability of ester in water at neutral pH? For how long can they be expected to be stable?

For example. If I dissolve an ester in water and put it in the refrigerator. For how long would you say that I can expect my ester to still be my ester? Days, months, years?

Thanks in advance
Andreas

Depends on the ester. Obviously those that react are not stable, e.g. TMS esters. But I would say the esters are stable in water at neutral pH at 4°C indefinitely.

[loginprakash]

May be for ever. ~Because with out acid or basic catalysis, it should not undergo hydrolysis. May also depend on the neighboring groups in the ester.

[uglepik]

The ester in question is no activated in any way and not meant to react. It's a primary alcohol conjugated to a carboxylic acid, both with with at least 2 carbons between themselves and any group that could influence electron density. So according to your answers I expect it to be stable for a good long time :-) Thanks a lot!

Ps: For some reason I always freak out over ester stability. Perhaps because I'm more used to working with physiological environments where ester bonds are often an endangered species.

[milo2112]

As long as the pH remains 7, you are good to go. Watch out for dissolution of CO2 in your sample, this will lower your pH (formation of carbonic acid) and you'll start seeing slow hydrolysis. I suggest using degassed water and using a greased glass stopper. If you are REALLY paranoid, just freeze the whole thing- you'll stop almost all degradation.

[orgopete]

Depends on the ester. Obviously those that react are not stable, e.g. TMS esters. But I would say the esters are stable in water at neutral pH at 4°C indefinitely.

That is what I thought, depends on the ester, but I really had no idea. The question is fraught with ambiguity. For those of us in synthesis, stable forever works. For those looking at trace products, I can imagine an ester going away relatively quickly (by comparison). A two phase mixture of ethyl acetate and water will remain two phases forever. A drop of ethyl acetate in a liter of water may disappear relatively quickly. 

"I'm more used to working with physiological environments where ester bonds are often an endangered species." This completes the ambiguity. Esters are an endangered species (whatever that means, see above). What are physiological environments? In vivo, in vitro, enzymes, buffers, 37°C, etc?

As pointed out, carbon dioxide will lower the pH. Also, any amount of hydrolysis will increase the acidity and the hydrolysis can become autocatalytic, though with only a weak acid.

[fledarmus]

Hydrolysis can certainly occur in water, even at pH 7, because due to self-dissociation of water, there is a concentration of both H⁺ and OH^- in the water. The kinetics of these reactions has been studied in considerable detail and reaction rates have been measured. You might want to do a literature search on hydrolysis rates under neutral conditions of the particular ester you are interested in.

Here is a general overview on the topic.
 http://www.ncsu.edu/chemistry/franzen/public_html/Poland/Poznan08a/Text/Ch2.pdf
Page 129 gives reaction rate constants for hydrolysis of a number of simple esters. With those constants, you should be able to calculate the rates of reaction for various temperatures, and find storage conditions that will reduce the rate of hydrolysis to an acceptable level.

[Babcock_Hall]

That's a useful link.  Thank you.