Silicon bonding bond energies

Catenation is defined as the self-linking of an element to form chains and rings. Carbon, then, given the above discussion, is the all-time champion catenator, much better than silicon (or sulfur, boron, phosphorus, germanium, and tin, the other elements that show this ability). Why should this be so A comparison of the relevant carbon and silicon bond energies as shown below is helpful ... 

The view has also existed in the past that the carbon-silicon bond should be similar in behaviour to the carbon-carbon bond and would have a similar average bond energy. There is some measure of truth in the assumption about average bond energy but because silicon is more electropositive than carbon the C—Si bond will be polar and its properties will be very dependent on the nature of groups attached to the carbon and silicon groups. For example, the CH3—Si group is particularly resistant to oxidation but H13—Si is not. 

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

With regard to the stabilizing effect of the a-substituent at the silicon, the following gradation can be inferred from results of x-ray structures O > S > C > Cl. This sequence correlates with known Si-X bond energies. 

Reactions (1), (2), and (3) are generally carried out at atmospheric pressure and produce films up to 100 im thick. These reactions, which are used extensively in production, are reversible since the formation of HCl promotes the etching off of impurities during deposition due to the high energy states of silicon bonding at the sites of impurities,... 

C06-0129. Use average bond energies (see Table 6-2) to estimate the net energy change per mole of silicon for the conversion of a silicon chain into an Si—O—Si chain. Repeat this calculation to estimate the net energy change per mole of carbon for the conversion of a carbon chain into a C—O—C chain. 

Consideration was given to effects, such as the acceptor properties of silicon and the silicon-halogen bond energies, which determined whether monomeric structures were retained. A mechanism for the exchange reactions was as formulated in which an intermediate, e.g. (34), was involved. 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

Table 1 Molecular parameters of the diatomic oxides and sulfides of carbon and silicon derived experimentally (force constant f and bond energy BE) and theoretically (bond distance d, charge Q, and Shared Electron Number SEN).

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

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