Carbon in the Periodic Table

Carbon is found in the middle of a period and in Group 4 of the periodic table. It has properties that allow it to form shared covalent bonds with other carbon atoms, and also to form covalent bonds with hydrogen, oxygen, nitrogen and some other non-metallic elements (see also Section 2.2.2 on covalent bonding, and the periodic table near the front of the book)  

Carbon is a relatively small atom, it has an atomic number of 6 and an electronic structure of 2 4. It has four outer electrons available to bond to other atoms in order to complete the outer shell up to the full eight. When it combines with, say, hydrogen, carbon has no tendency to form ionic bonds, but forms strong covalent bonds. 

---

Figure 1.1 The position of carbon in the periodic table. Other elements commonly found in organic compounds are shown in the colors typically used to represent them.

Carbon is, of course, unique in the number of hydrides it forms, but the elements in the proximity of carbon in the Periodic Table have a similar, if more restricted, propensity to form hydrides. Silicon, germanium, boron and phosphorus are obvious examples. For hydrides of these elements, and especially for their organic derivatives, the methods of substitutive nomenclature can be applied to obtain suitable names. 

When selecting atom substitutions for new molecule design, chemists usually look only to the right of carbon in the periodic table. The contrarian looks to the left and finds boron—commonly viewed as a metal, but in fact quite nonmetallic in many respects. In his excellent review of boron analogues of biomolecules, Morin showed why working with boron is so attractive <94T12521>. Here are some of the unique potential applications for any new boron compound ... 

Silicon lies below carbon in the periodic table, and it is reasonable to assume that both carbon and silicon are. (//-hybridized in H3CSiH3. The C—Si bond and all of the C—H and Si—H bonds are cr bonds. 

With more than 100 elements besides carbon in the periodic table (Appendix 2), you might fear that the number of H chemical shift correlations is endless. However, except for a few specialized applications, the most important heteroatoms to which hydrogen finds itself bonded are oxygen and nitrogen. But before we discuss these two specific cases, here is a useful generalization As the electronegativity (Table 6.1) of X increases, both the acidity and chemical shift of a hydrogen bonded directly to X increase. 

Levi, P., 1984. Carbon. In The Periodic Table. Random House. 

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 n bonds silicon forms few. The most important difference is the strength of the silicon-oxygen o bond (368 kj mol ) and the relative weakness of the silicon-silicon (230 kjmor ) bond. Together these values account for the absence, in the oxygen-rich atmosphere of earth, of silicon analogues of the plethora of structures possible with a carbon... 

To place these 3c-2e carbon systems in a wider context, it should be noted that 3c-2e bonding is widespread in inorganic chemistry, principally in the chemistry of elements to the left of carbon in the periodic table, which is in the chemistry of boron and the metallic elements in general 740-12,14-17,78... 

Silicon is directly below carbon in the periodic table so, according to the Erlenmeyer s expansion of the isoster-ism concept, carbon and silicon can be considered as true isosters. Sila-substitution (C/Si exchange) of biologically active substances is an approach to search for new druglike candidates with improved pharmacological properties and stronger IP position. The application of this isosterism remains however limited. For reviews on the subject, see Fessenden and Fessenden, Tacke and Zilch, Ricci et and Showell and Mills. ... 

Levi, P. 1984. Carbon. In The Periodic Table. Random House. Melendez-Hevia. E.. and Isidore, A. 1985. The game of the pentose phosphate cycle./. Tkeor. Biol. 1 17 251-263. 

A more elaborate example than those shown above is the anionic compound SiFg2- (Figure 1.2), which adopts a classical octahedral shape that we will meet also in many metal complexes. Silicon lies below carbon in the Periodic Table, and there are some limited similarities in their chemistry. However, the simple valence bond theory and octet rule that... 

Figure 15.1 The position of carbon in the periodic table. Lying at the center of Period 2, carbon has an intermediate electronegativity (EN), and its position at the top of Group 4A(14) means it is relatively small. Other elements common in organic compounds are H, N, O, P, S, and the halogens.

---