The mechanism of thermal eliminations. Part 22. Rate data for pyrolysis of primary, secondary, and tertiary β-hydroxy alkenes, β-hydroxy esters, and β-hydroxy ketones. The dependence of transition-state structure for six-centre eliminations upon compound type
Rates of pyrolysis of but-3-en-1-ol, pent-4-en-2-ol, and 2-methylpent-4-en-2-ol, the corresponding methyl hydroxy esters viz. methyl 3-hydroxypropanoate, methyl 3-hydroxybutanoate, and methyl 3-hydroxy-3-methylbutanoate, and the corresponding methyl hydroxy ketones viz. 4-hydroxybutan-2-one, 4-hydroxypentan-2-one, and 4-hydroxy-4-methylpentan-2-one have been measured between 556.4 and 713.7 K. The relative 1°: 2°: 3° rates at 600 K are 1 : 2.0 : 3.47 for alkenes, (1): 9.3 : 44.6 for the esters, and (1): 7.0 : 21.6 for the ketones (the rates for the primary compounds in the latter two series being less accurate because of significant concurrent dehydration). The order of reactivity is ketones esters > alkenes, the realtive reactivities at 600 K being 338 : 2.21 : 1 for 4-hydroxy-4-methylpentan-2-one, methyl 3-hydroxy-3-methylbutanoate, and 2-methylpent-4-enol, respectively. The reactivity of the ketones compared with the alkenes contrasts with the results for the structurally analogous acetates and vinyl ethers which eliminate with near identical rates and for which methyl substitution on the double bond produces substantially less rate modification. At 600 K 3-methylbut-3-en-1-ol is 8.4 times as reactive as but-3-en-1-ol, and phenyl 3-hydroxypropanoate is much less reactive than methyl 3-hydroxypropanoate. The results provide further evidence that within the spectrum of Ei transition states for six-centre eliminations there are two board mechanistic categories. For reactions with more E1-like transition states, breaking of the Cα–X bond is the most important step, whereas for other reactions including those described here, nucleophilic attack of the double bond upon the β-C–H bond becomes the most important step. The higher nucleophilicity of the CO bond vs. the CCH2 bond accounts for the difference in reactivity of ketones and alkenes, while the inductive effect of the methoxy group upon the nucleophilicity of the double bond appears to account for the lower reactivity of the esters compared to the ketones.