The Effect of the Solvent on the Reactivity of Sodium and Potassium Phenoxides in Nucleophilic Substitution Reactions. Part I. Polyethylene Glycol Dimethyl Ethers as Solvents.

Abstract

The alkylation of K and Na phenoxides with Bu halides was investigated kinetically using tetrahydrofuran (THF) and the dimethyl ethers of polyethylene glycols of variable chain lengths, as solvents. Dried alkali phenoxides were dissolved in various solvents in stoppered flasks at controlled temp. The reaction was started by adding BuCl. Aliquots were withdrawn at regular intervals and added to 0.02N HCl. The course of the reaction was followed by back titration of the excess HCl with 0.02N NaOH. Initial second-order rate consts. were derived from only the first 5% of reaction, because of the effect of salt formation on the observed rate const. later in the reaction. The rates of reaction increase markedly with increasing chain length of the glycol ethers. In MeO(CH2CH2O)4Me the reaction rate is about 150 * greater than in MeOCH2CH2OMe using equimolar concn. (0.1M) of KOPh and BuCl at 25 Deg. With NaOPh a rate increase of comparable magnitude is found by changing the solvent from MeOCH2CH2OMe to MeO(CH2-CH2O)4Me. MeO(CH2CH2O)6Me as solvent at 60 Deg gives reaction rates .apprx.3 times those in MeO(CH2CH2O)4Me. In THF the rate is 1/8 that in MeOCH2CH2OMe at 25 Deg. The reaction rate depends mainly on the capacity of the solvent to solvate the alkali ions specifically. In MeO(CH2CH2O)nMe, several O atoms in the same mol. cooperate in the solvation of an alkali ion. THF having only one ether O/mol. is therefore inferior in this respect. [on SciFinder (R)]