Mechanisms of unimolecular gas-phase .gamma.-hydrogen rearrangements of radical cations. Influence of entropy as opposed to energy

Abstract

SilacyclobutaneImine Benzene (mmol) (mmol) (mmol) ,---------Products (% yield^-------(CHz)&Me2 (21.2) PhCH=NPh (14.5) (59) (MezSiNPh)z (41) P h C H d H z (32) (CHz)3SiMe2 (13.2) Ph2CbNPh (5.20) ( 5 5 ) (MezSiNPhX (30) PhzCbCHz (56) PhzC=CHZ (33)b (CH&SiPhz (6.33) PhzC=NPh (3.03) (62) (PhzSiNPh)z (25)*+ aThese are based on the amounts of material actually isolated (either by preparative glpc or crystallization) and are probably 10-207$ lower than the quantities actually present. All reaction products are known compounds and, except as noted, were completely characterized by their ir and nmr spectra and, where applicable, melting points. * Calculated after correction for a 24% recovery of limiting reagent. Material was impure (mp 335-343", lit.6 mp 355.5"). Further recrystallization did not afford a purer sample.benzophenone (4.73 mmol), and benzene (66.7 mmol) afforded a homogeneous, yellow pyrolysate which, after resolution by preparative glpc afforded benzophenone phenylimine (0.79 mmol), styrene (0.78 mmol), 1,ldiphenylethylene (1.95 mmol), octamethylcyclotetrasiloxane (0.04 mmol), hexamethylcyclotrisiloxane (trace), benzophenone (1.74 mmol), and N-phenylbenzaldimine (12.7 mmol). Glpc analysis of the pyrolysate did not detect any 1,3-diphenyl-2,2,4,4tetramethylcyclodisilazane, (MezSiNPh)2. Isolation of benzophenone phenylimine provides definitive evidence for the formation of MezSi=NPh and its reaction with benzophenone since pyrolysis of equimolar quantities of either benzophenone and N-phenylbenzaldimine or benzophenone and (Me2SiNPh)z did not produce detectable quantities of this material. An explanation which accommodates all of these observations is outlined in Scheme 11. Assuming quantitative formation Scheme I1 --t [Me2Si=CH?I + CH?=CH? 3 G1l " Me,Si PhN=CHPh/ 1ph2C-0 + Ph,C=CH, + [Me2Si=O] PhCH =CH, + 1 [MelSi=NPh] -x+ (Me,SiNPh)? I Ph,C=Pu'Ph + [Me,Si=Ol of Me&=CHz and correcting, on the basis of the isolated yield of 1, 1-diphenylethylene, for the fact that 38.5 % of the MezSi=CH2 produced reacted with benzophenone, then the 0.79 mmol of benzophenone phenylimine isolated represents a 25.4 % yield of trappedMezSi=NPh. This is a minimum value since isolated yields were used and factors such as decomposition of the trapping reagents and products and other side reactions were ignored. The absence of significant side reactions and the relatively high yield of benzophenone phenylimine indicate that reaction schemes more complex than I1 are unlikely.The absence of (Me2SiNPh)z and the comparable quantities of styrene and benzophenone phenylimine observed indicate that benzophenone is a very efficient trapping reagent for silicon-nitrogen doubly bonded intermediates. This is in accord with our observationle that benzophenone is an excellent trapping reagent for intermediates containing silicon-carbon double bonds.Acknowledgment.

Field ionization kinetics. Unimolecular decomposition of the hexanal radical cation over the time range from 10 ps to 10 .mu.s

“…The results (Figure 3a) for these m/e 45 and 57 ions are in good agreement with those published in the preliminary communication. 3 The curves shown in this paper have been normalized,19 whereas those in the preliminary communication were not. Considering hexanal (Figure 1), it is evident that similar comparisons could be made between m/e 44 and any of the product ions with the exception of m/e 45.…”

“…Reactions 1 and 2 and reaction 3 show the more significant pairs of C6H120+• -CHgCHO + CH3CH=CHCH3+. (3) products, and heats of reaction obtained by subtracting the heat of formation of the hexanal ion23 from these combined heats of formation of products.12 The heat of reaction of (3) is 6 kJ moT1. The heats of formation of Holmes et al12 for the products in reaction 2 are based on an estimation that AHf( vinyl alcohol) = -111 ± 8 kJ moT.1 (ref 24).…”

“…Like the majority of organic dyes, methylene blue deviates from the Beer-Lambert law.1 It has been theorized that this deviation is due to the reversible formation of dye polymers, which are held together by dispersion forces originating from the delocalized it electrons of the individual dye molecules.2 In the concentration range from 10"5 to 10"3 M, the extent of aggregation of methylene blue is limited to dimerization, which results in absorbances in the visible region at 660 and 610 nm for the respective monomeric and dimeric species. 3 There has been no kinetic study done on the monomer-dimer equilibrium of methylene blue to elucidate a mechanism for the system or to determine the magnitudes of any associated forward and reverse rate constants.…”

“…With these considerations in mind, we have synthesized a series of hexanals tagged with 2H, and have determined their field ionization kinetics (FIK).2, 3 Gilpin and McLafferty4 first recorded the electron impact (El) mass spectra of a large number of aliphatic aldehydes, and identified major types of decomposition including that which has since come to be known as the McLafferty rearrangement. Two thorough El studies5-7 of specifically 2H-labeled hexanals, heptanals, and nonanals have elucidated the major' reaction pathways by determining which atoms are involved in the various decompositions.…”