Carbon and Silicon

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

There are two further processes. Silicon-based ceramics can be fabricated by sintering or by hot-pressing. But a new route, reaetion bonding (Fig. 19.6), is cheaper and gives good precision. If pure silicon powder is heated in nitrogen gas, or a mixture of silicon and carbon powders is sintered together, then the reactions... 

The possibility of the existence of organosilicone compounds was first predicted by Dumas in 1840, and in 1857 Buff and Wohler found the substance now known to be trichlorosilane by passing hydrochloric acid gas over a heated mixture of silicone and carbon. In 1863 Friedel and Crafts prepared tetraethylsilane by reacting zinc diethyl with silicon tetrachloride. 

Sand consists mainly of silicon dioxide. When sand is heated with an excess of coke (carbon), pure silicon and carbon monoxide are produced. 

The name silicone was originally coined because of the resemblance of the empirical formulae of silicones, e.g. R2SiO, with ketones, R2CO. However, because of the different nature of the silicon and carbon atoms, in particular that silicon is too large to form stable double bonds, this resemblance is merely superficial. 

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

The crystal structure data suggest some sp2 character for the silicon and carbon atoms of the double bond, but there is a high degree of ir-... 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

After decades of unsuccessful attempted syntheses, Gusel nikov and Flowers in 1967 reported the first compelling evidence for the existence of silenes, compounds containing a double bond between silicon and carbon. This initiated a renewed interest in the synthesis and behavior of stable silenes,8 disilenes,b iminosilanes,c phosphasilenesd and their heavier homologs. 

Table 3. Atomic orbital population of silicon and carbon in disilane and ethane, respectively.

Fig. 3. Calculated ring strain energies per strained angle in silicon and carbon ring compounds using the homodesmotic reaction. The angle a is half of the deviation of the angle between the straight atomic connection lines from the unstrained angle (109.47 deg).

Figure 1. Comparison between silicon and carbon substituted amides...

Early estimates for the Si NMR chemical shift of silylium ions were based on an empirical correlation between NMR and Si NMR chemical shifts of iso-structural silicon and carbon compounds, and predicted for trimethylsilylium a Si NMR chemical shift within the range 225-275. ... 

As is well known, when hydrocarbons transported in an inert gas such as helium are heated, carbon or pyrolytic graphite will deposit on the walls. In the HTCVD, the ethylene helps make the particles stable and, in doing so, carbon is transported into the chamber together with the silicon. It is a sort of symbiosis in the transport between the silicon and carbon. 

Kharlamov AI, Kirillova NV, Karachevtseva LA, Kharlamova AA (2003) Low-temperature reactions between vaporizing silicon and carbon. Theor Exp Chem 39(6) 374-379... 

Kharlamov Al, KiriUova NV (2002) Gas-phase reactions of formation of sihcon carbide nanofilaments from silicon and carbon powders. Theor Exp Chem 38(l) 59-63... 

In the first step, quartzite is reacted with carbon in the form of coal, coke, or wood chips, to form silicon and carbon monoxide ... 

Fig. 2.16 Fragmentation energies for small silicon and carbon clusters as functions of cluster size, as calculated by Raghavachari. The dashed curve represents the fragmentation energies as a function of cluster size n for the reaction Si — Si i + Si. Note the most stable clusters are Si10, Si6 and Si4. The solid curve indicates the fragmentation energies of small carbon clusters as a function of cluster size n for the reaction C — + C. Odd number clusters are more...

---