Carbon-silicon bond energy

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. 

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. 

The transition states in both steps of the reaction are not likely to be far removed in energy or structure from the intermediate, which may be used as a model to rationalize variations in the rates and products of such reactions. If silicon is in a position such that it is to the positive charge in one of the resonance forms, this might be expected to lower the energy and increase the rate, provided the carbon-silicon bond can overlap with the vacant TT-orbital. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

The relative reactivities of organometallics containing group 14 elements are closely associated with the electronegativity and dissociation energy of the M—C bonds of the elements. For example, some organotin compounds such as allylstannanes are easily decomposed to form radical species whereas homolysis of the carbon-silicon bond of allylsilanes is rarely achieved, which reflects the dissociation energy of the respective M—C bonds. 

TetraaUcylsilanes are stable compounds. Therefore, it is necessary to activate the compounds for the conpling reaction. Since dialkylsilacyclobutanes have high strain energy, palladium complexes can easily insert into the carbon-silicon bond oxidatively, and the resultant complexes conple with acyl chlorides to give l-sila-2-oxa-3-cyclohex-ene derivatives (Scheme 14). 

Fluorine occupies a unique position in the periodic table and forms the strongest known single bonds with boron, carbon, silicon, and hydrogen. With carbon the bond energy increases with the degree of fluorination, a feature not found in other halogen atoms. It is also observed that... 

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 ... 

One group of alkenes which react efficiently with electrophilic carbon species are the allylsilanes. The carbon-silicon bond is broken as the reaction proceeds, thus providing a low-energy product.Among the electrophilic carbon species... 

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... 

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. 

---