[miaskows]

O-alkylation of sustituted phenols with Alkyl Halides (the Williamson Reaction) could be carried out in aprotic solvents in presents of base. When NaOH or KOH added the main by products are alcohols and alkenes.
R-Ar-OH + C2H5-Hal+ NaOH= R-Ar-O-C2H5+ NaHal+C2H5OH+C2H4 (with traces of other compounds:products of C-alkylation ,etc)
What composition of reaction's products should be expected if instead of NaOH the reaction mix to be treated with Na2CO3 ? (much more cheap than anhydrous NaOH and not so destructive for sensible chemicals)
CO32- is a weak nucleofil itself and I think the reaction can be written down as follows :  
 R-Ar-OH+C2H5-Hal+Na2CO3=R-Ar-O-C2H5 + C2H5OH+C2H4 +NaHal +NaHCO3 +
C2H5-O-CO-O-C2H5 (diethylcarbonate)
From the literature data follows that dialkyl carbonates are fairly stable and widely used compounds.
What about monoalkylcarbonates and their possible salts? I have found nothing about them. Are they exists?
For example:
C2H5-O-CO-OH or C2H5-O-CO-ONa?
Could HCO3- participate in the reaction as nucleofil?
Any help will be appreciated.

[Dude]

If you are really trying to do a Williamson ether reaction, I suspect that you would want to add the base and the phenol together first to form the phenoxide (usually turns a dark color in an aprotic solvent).  The equilibrium concentration of the phenol varies depending on whether sodium carbonate or sodium hydroxide is used (differing pKa's).  The second reaction will be the same since the reacting nuclephile is the phenoxide.  If you form alkenes, the phenoxide is capable of an E2 reaction (picking off a proton with halide elimination), although the alcohol would seem to be formed only from the hydroxide. I don't suspect that you would form appreciable amounts of organic carbonates even if you added the components together, although I could be wrong.  

[miaskows]

Thanks.Your answer helped me a lot. It is so simple! What i have to do is- to assure maximal phenolate formation (along with HCO3-) and only after that to add substrate.