Some gas kinetic studies of reactive organosilicon intermediates.
thesisposted on 2015-11-19, 08:48 authored by Ian Thomas. Wood
The study of organosilicon reactive intermediates is an area of expanding interest. There are many gaps in our knowledge in this relatively new sphere, in addition to the lack of definitive thermos-chemical data and pyrolysis reaction mechanisms for organosilicon compounds in general. The research towards this thesis was designed to encompass all these topics, and consisted of the investigation, kinetically where possible and where appropriate, of processes which thermally generate organosilicon reactive intermediates, and some reactions in which they participate, using low-pressure pyrolysis (LPP). Some supporting work was done using pulsed stirred-flow, sealed tube, and mercury photosensitization techniques. Siliranes have proved useful as silylene sources, but doubt has existed whether silirenes could exhibit similar thermolytic behaviour. The pyrolysis of 1,1-dimethyl-2,3-bis (trimethylsilyl)-1-silirene was investigated between 420 and 550 K and found to give mainly the rearranged product Me3SiSiMe2C=CSiMe3, probably via a biradical intermediate. No significant silylene extrusion occurred. Considerable controversy has surrounded the decomposition of allyltrimethylsilane. Its pyrolysis was studied between 860 and 960 K and the mechanism found to be strongly pressure dependent. At low pressures (<0.3 Torr) thermolysis proceeded two concurrent unimolecular processes: silicon-allyl bond rupture, and formation of 1,1-dimethyl-1-silaethene plus propene by a retroene mechanism. At higher pressures the bimolecular reaction of trimethylsilyl radical addition to allyltrimethylsilane giving vinyltrimethylsilane and tetramethylsilane became important. These results clear up many of the disputes over this compound's pyrolysis mechanism and enabled several thermochemical quantities to be calculated. Between 710 and 830 K vinyldimethylcarbinoxydimethylsilane pyrolysis gave 1,1-dimethylsilanone plus 2-methylbut-2-ene, the Arrhenius parameters being consistent with a retroene reaction. 1,1,3,3 Tetramethyl-1-vinyldisiloxane required higher temperatures, between 860 and 1075 K, for decomposition, which occurred by a radical mechanism. No significant production of 1,1-dimethylsilanone occurred. Several reactions of 1,1-dimethylsilanone were studied, one kinetically, that of insertion into methyltrimethylsilyl ether, along with the equivalent 1,1-dimethyl-1-silaethene process. The Arrhenius parameters showed that similar four-membered cyclic transition states were formed. Same general comments on reactions involving these species have been made.
Date of award1983-01-01
Awarding institutionUniversity of Leicester