ALKYL TRANSFER REACTIONS BETWEEN PROTONATED ALCOHOLS AND ETHERS. GAS-PHASE ALKYLATION OF FORMALDEHYDE

Article abstract of DOI:10.1021/j100342a032

Alkyl-transfer reactions involving protonated alcohols and ethers, of the general type R2OR'⁺ + R''2O -> R""OR'⁺ + R2O, may be classified according to the degree of alkylation, i.e., the total number n of alkyl groups in the system.For systems containing two alkyl groups, the reaction is alkyl transfer between protonated and neutral alcohols, ROH2⁺ + ROH -> R2OH⁺ + H2O, and we measured rate constants for R=Me, Et, i-Pr, and t-Bu.The rate constants are (0.6-1.5) x 1E-10 cm³s^-1 when R is a normal alkane and (6 +/- 1) x 1E-10 when R=i-Pr and t-Bu.The larger rate constants in the latter may be due to a lowered barrier for the initial partial R⁺-OH2 bond dissociation.For reaction systems containing three alkyl groups, i.e., the reactions of protonated ethers R2OH⁺ with alcohols R'OH, the possible channels are alkyl transfer from the alcohol, yielding R2OR'⁺, or alkyl transfer from the ether, yielding ROR'H⁺.Both processes are observed in (CH3)2OH⁺ + C2D5OH which yields both (CH3)2OC2D5H⁺ and CH3OC2D5H⁺.For systems containing four alkyl groups an example is the reaction(CH3)2OH⁺ + (CH3)2O -> (CH3)3O⁺ + CH3OH, which is a slow reaction with k<1E-11 cm³s^-1.Finally, for the highest possible degree of alkylation, n=5, an example is methyl transfer in (CH3)OCD3⁺ + (CH3)2O -> (CH3)2OCH3⁺ + CH3OCD3 which is a very slow reaction, observed only above 500K.The rate constants for alcohols show negligible temperature dependence between 300 and 670 K, but in the most highly alkylated system the rate increases strongly with temperature, and an activation energy of 15 kcal^-1 is observed.The results show that alkyl transfer occurs in systems with all possible degrees of alkylation, but the rates tend to decrease with increasing alkylation.In addition to saturated systems, alkyl transfer is also observed with unsaturated ions or neutrals.Examples are alkyl transfer between unsaturated oxocarbonium ions C2H5O⁺ and methanol and ethanol and between protonated alcohols and CH2O.These reactions have rate constants of (1-4) x 1E-11 cm³s^-1.Depending on the temperature coefficients, the alkylation of formaldehyde may be important in astrochemical synthesis.