Allyl carbonates, pyrolysis

The pyrolysis products from 9.13 can give essentially pure silicon carbide in 89% yield. Intermediates that contain from 5 to 25% allyl functionalized units give ceramics with progressively larger amounts of carbon beyond the 1 1 Si C ratio. This process has been developed into a manufacturing sequence for the production of reinforced composites for aircraft brakes and high temperature coatings. 

Methanol, also called methyl alcohol and once commonly know as wood alcohol, is a clear, volatile liquid mp, -98°C bp, 65°C). Until the early 1900s, the major commercial source of methanol was the destructive distillation (pyrolysis) of wood, a process that yields a product contaminated with allyl alcohol, acetone, and acetic acid. Now methanol is synthesized by the following reaction of hydrogen gas and carbon monoxide, both readily obtained from natural gas or coal gasification ... 

Studies of the oxidation and pyrolysis of olefins show that the majority of the products are derived from reactions which involve the terminal carbon atom of the substituted allyl radical, R—CH=CH—CH2 (i.e. with the canonical form shown). It has been suggested that this is due to a higher spin density at this carbon atom than at the substituted end ofthe radical (Norrish and Porter, 1963 Bryce and Ruzicka, 1960). Thus the measurement of the spin density distribution in substituted allyl radicals would be of considerable interest. 

A procedure for the preparation of allyl alcohol introduced by B. Tollens (1870) and later improved calls for heating glycerol with three successive portions of formic acid to a temperature of 260° (li days) and recovery of the allyl alcohol from the distillate (46% yield). Glycerol monoformate is formed and suffers pyrolysis to the unsaturated alcohol, carbon dioxide, and water. 

Weakening of the carbon-metal bond in this manner is attributed to delocalization of the free-radical electronic charge. Even weaker carbon-metal bonds are formed for the allyl radical, where a double bond is formed, and the benzyl radical, where the central carbon atom is bonded to a benzene ring. A corollary to the rule described above is that the more stable the radical, the more rapidly it is formed. Thus, allyl groups forms radicals most rapidly and methyl groups form radicals least rapidly. This is reflected in the rates of pyrolysis of various organometallic precursor molecules. 

Berson first showed that bicyclo[3.2.0]hept-2-enes are converted thermally to bicyclo[2.2.1 ]hept-2-enes, however, it was the determination of the stereochemistry of the migrating carbon in this 1,3-shift that captured the attention of organic chemists in the late 1960s.There was at least a 19-fold preference for inversion in the pyrolysis of x<9-7-deuterio- nJo-6-acetoxybicyclo[3.2.0]hept-2-ene at 307°C. This is consistent with an allowed reaction in which the allylic moiety is used in a suprafacial manner (Scheme 8.44). 

Pyrolysis of a-alkyl-jS-hydroxy-phenylsulphoxides (32) can be made to produce allylic alcohols [equation (5)], avoiding elimination towards the hydroxy-group, when performed in refluxing xylene containing suspended sodium carbonate." ... 

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