Rates tetrahydropyran reaction

Polar solvents such as ethers and amines react with organometallic initiators, as well as propagating polystyryl and polydienyl carbanions, to decrease the concentration of active centers (21-23). The rate of reaction with ethers decreases in the order Li > Na > K. For example, dilute solutions of poly(styryl)lithium in tetrahydrofuran (THF) at room temperature decompose at the rate of a few percent each minute. Alkyllithium initiators also react relatively rapidly with ethers the order of reactivity of organolithium compoimds with ethers is tertiary RLi > secondary RLi > primary RLi > phenyllithium > methyllithium > benzyllithium (21). An approximate order of reactivity of ethers toward alkylithium compounds is dimethoxyethane, THF > tetrahydropyran> diethyl ether> diisopropyl ether. Tertiary amines can also react with alkyllithium compoimds. The importance of these reactions can be minimized by working at lower temperatures (eg, <0°C) it is also advisable to use only the minimum amounts of ethers and other Lewis bases required as additives. 

Haworth methylation of methyl /3-D-glucopyranoside and its 4-benzyl and 4-(tetrahydropyran-2-yl) ethers was investigated in connection with partial-methylation studies on cellulose.267 For the unsubstituted glycoside, the ratios of relative rate-constants k2 k3 k4 k were estimated to be 8 2 1 8, and, for the 4-ethers, it was found that ke> k2> k3 best agreements between calculated and experimental yields were found with the assumption that the rate constant for reaction at HO-3 is doubled when HO-2 is substituted. Later methylation studies,268 performed to low degrees of substitution, with analysis by gas-liquid chromatography, gave k2> k4> k3 for the reactivity... 

On treatment with acrylonitrile in 2% aqueous sodium hydroxide at 0°, tetrahydropyran-2-yl /3-D-glucopyranoside gave the 2-, 3-, 4-, and 6-0-(2-cyanoethyl) ethers (together with some diethers) in yields that, on extrapolation to zero reaction, showed3 9 k2 k3 k4 k6 to be in the ratios of 3 1 2 8 these values represent equilibrium, not rate, constants. The tendency for substitution at 0-6 is a consequence of the greater stability of an ether derived from a primary (compared to a secondary) hydroxyl group, as a result of lower steric interactions in the former. 

In the second mechanism, the first and second steps are concerted. In the case of hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran, general acid catalysis was shown470 demonstrating that the substrate is protonated in the rate-determining step (p. 259). Reactions in which a substrate is protonated in the rate-determining step are called A-Se2 reactions.471 However, if protonation of the substrate were all that happens in the slow step, then the proton in the transition state would be expected to lie closer to the weaker base (p. 259). Because the substrate is a much weaker base than water, the proton should be largely transferred. Since the Brpnsted coefficient was found to be 0.5,the proton was actually transferred only... 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

Figure lll-E-6. Arrhenius plot of the rate coefficient for the reaction of OH with tetrahydropyran and oxepane. 

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