Alkoxides, base strength

Potassium Alkoxides. The most widely used potassium bases are potassium tert-hu. oAde [865-47-4] (KTB) and potassium / i -amylate [41233-93-6] (KTA). These strong alkoxide bases offer such advantages as base strength (pX = 18), solubiUty (Table 5), regio/stereoselectivity because of bulky alkyl groups, and stabiUty because of the lack of a-protons. On storage, KTB and KTA have long shelf Hves under inert atmosphere (see... 

Triflate, tosylate and mesylate are the anions of strong acids. The weak conjugate bases are poor nucleophiles. Nucleophilicity increases in parallel with the base strength. Thus, amines, alcohols and alkoxides are very good nucleophiles. Base strength is a rough measure of how reactive the nonbonding electron pair is thus, it is not necessary for a nucleophile to be anionic. 

One of the factors influencing orientation is base strength (p/ a). However, in order to evaluate the importance of this factor it is necessary to eliminate problems due to ion pairing. Whereas the extent of ion pairing for common alkoxides in the corresponding alcohols varies greatly, DMSO as solvent causes a levelling of these... 

Of intermediate utility between these two extremes for anionic polymerization is alkoxide ion, which has adequate base strength for initiation of the ring-opening polymerization of epoxides (Eqs. 22.24-22.26). 

The reaction may proceed to complete dechlorination, although replacement of a single chlorine is sufficient to make the reaction products water soluble. The ether formed by the dechlorination may degrade to a phenol form or may remain as the ether, depending on the reaction conditions. The processing is carried out using dimethyl sulfoxide (DMSO) as a solvent, which increases the base strength of the alkoxide. In addition, the DMSO aids in the extraction of the PCDD from the soil. 

So now we can expand our chart of acid and base strengths to include the important classes of alcohols, phenols, and carboxylic acids. They conveniently, and memorably, have piCa values of about 0 for the protonation of alcohols, about 5 for the deprotonation of carboxylic acids, about 10 for the deprotonation of phenols, and about 15 for the deprotonation of alcohols. The equilibria above each piCa shows that at approximately that pH, the two species each form 50% of the mixture. You can see that carboxylic acids are weak acids, alkoxide ions (RO ) are strong bases, and that it will need a strong acid to protonate an alcohol. 

Analysis of the butene fraction obtained in reactions between 2-halogeno-butanes and the alkoxides CHa-CHj ONa and CFs-CHg-ONa at 25 C in dipolar aprotic solvents (DMF and DMSO, to minimize solvation effects) has revealed that in each case change from ethoxide to 2,2,2-trifluoroethoxide results in a decrease in the tendency for but-l-ene to be formed. This demonstrates that base strength and not size is of prime importance in determining orientation in elimination reactions between 2-halogenoalkanes and alkoxides of modest proportions. ... 

Bases like hydroxides or alkoxides are of about the right strength barium hydroxide is often used.1 The small amounts of enolate 7 quickly add to the large excess of ketone 8 to give the anion 9 of the product which regenerates the base by taking a proton from water or the alcohol.2... 

Brook rearrangements may be carried out with either catalytic or stoichiometric base. With catalytic base, the reaction can be considered an equilibrium between 41 and 42. The strength of the Si-0 bond (about 500-520 kJ mol-1) compared with the Si-C bond (about 310-350 kJ mol-1) means that, provided the anion 33 forms reasonably rapidly (some degree of stabilisation is required), Brook rearrangement (alkoxide formation) is favoured over retro-Brook. Organolithiums 33 may be present as intermediates in the catalytic Brook rearrangement, but their reactivity cannot be exploited under these conditions. 

The presence of two, or even three, electron-withdrawing groups on a single carbon atom makes the remaining proton(s) appreciably acidic (pKa 10-15), which means that even mild bases can lead to Complete enolate formation. With bases of the strength of alkoxides or weaker, only the multiply stabilized anions form protons adjacent to just one carbonyl group generally have a p Ca > 20. The most important enolates of this type are those of 1,3-dicarbonyl (or 3-dicarbonyl) compounds. 

Controlled addition of a suitable proton donor or electrophile (reductions) or nucleophile (oxidations) is often useful in determining a reaction mechanism. The strength of a proton donor may vary from perchloric acid through acetic acid and a phenol to an alcohol C acids, such as malonic ester, or N acids, such as urea, may also be used. Used as bases may be pyridine, carboxylate ions, alkoxides, or salts of malonic ester. Sometimes it is of interest to determine whether it is the basic or the nucleophilic properties of the compound that are important. The use of two bases with approximately the same pK values but widely differing in nucleophilicity, such as pyridine and a 2,6-dialkylpyridine, might answer the question. 

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