Silicon is immediately below carbon in the periodic table and the most obvious similarity is that both elements normally have a valency of four and both form tetrahedral compounds. There are important differences in the chemistry of carbon and silicon—silicon is less important and many books are devoted solely to carbon chemistry but relatively few to silicon chemistry. Carbon forms many stable trigonal and linear compounds containing π bonds; silicon forms few. The most important difference is the strength of the silicon–oxygen σ bond (368 kJ mol⁻¹) and the relative weakness of the silicon–silicon (230 kJ mol⁻¹) bond. Together these values account for the absence, in the oxygen-rich atmosphere of earth, of silicon ana logues of the plethora of structures possible with a carbon skeleton.

Several of the values in the table give insight into the reactivity differences between carbon and silicon. Bonds to electronegative elements are generally stronger with silicon than with carbon; in particular, the silicon–fluorine bond is one of the strongest single bonds known, while bonds to electropositive elements are weaker. Silicon–hydrogen bonds are much weaker than their carbon counterparts and can be cleaved easily. Here are a few organosilicon compounds.

In this section we will mostly discuss compounds with four Si–C bonds. Three of these bonds will usually be the same so we will often have a Me3Si group attached to an organic molecule. We shall discuss reactions in which something interesting happens to the organic molecule as one of the Si–C bonds reacts to give a new Si–F or Si–O bond. We shall also discuss organosilicon compounds as reagents, such as triethylsilane (Et₃SiH), which is a reducing agent whereas Et₃C–H is not.

The carbon–silicon bond is strong enough for the trialkyl silyl group to survive synthetic transformations on the rest of the molecule but weak enough for it to be cleaved specifi cally when we want. In particular, fluoride ion is a poor nucleophile for carbon compounds but attacks silicon very readily. Another important factor is the length of the C–Si bond (1.89 Å)—it is significantly longer than a typical C–C bond (1.54 Å). Silicon has a lower electro negativity (1.8) than carbon (2.5) and therefore C–Si bonds are polarized towards the carbon. This makes the silicon susceptible to attack by nucleophiles. The strength of the C–Si bond means that alkyl silanes are stable but the most useful chemistry arises from carbon substitu ents other than simple alkyl groups.

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مواضيع ذات صلة

Sulfoxonium ylids

Sulfonium ylids

Sulfonium salts as electrophiles

Sulfonium salts

Thioacetals

A sulfoxide-stabilized anion in a synthesis

Sulfur-stabilized anions

Sulfur is a very versatile element

Symmetry in intramolecular crossed Claisen condensations

Intramolecular crossed Claisen ester condensations

Crossed Claisen ester condensations between two different esters

Directed C-acylation of esters