Tetrahedral carbon compounds

Figure 3. Molecular orbital diagram of a tetrahedral carbon compound (schematic)...

Figure 5.12. Combined correlation of trigonal carbocations and tetrahedral carbons compounds with their boron analogs (only representative examples are shown see Table 5.6).

Betweeu 1890 and 1893, Werner produced the three most important theoretical papers of his career. His doctoral dissertation (1890, cowritten with his teacher Arthur Hantzsch), a true classic of science writing on the topic of stereochemistry, extended Joseph Achille Le Bel and Jacobus Hen-ricus van t Hoffs concept of the tetrahedral carbon compound (1874) to the nitrogen compound. It explained many puzzling paradoxes of geometrically isomeric, trivalent nitrogen compounds and placed nitrogen compound stereochemistry on a firm theoretical basis. 

In the case of optically active tetrahedral carbon compounds the Sn2 reaction is stereospecific and always leads to an inversion of configuration. With phosphorus compounds, the corresponding reaction is not always stereospecific and does not necessarily lead to the inversion of configuration indicated in (3.81). 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

In addition to compounds with planar, sp2-hybridized carbons, compounds with tetrahedral, sp3-hybridized atoms can also be prochiral. An vp3-hybridizec) atom is said to be a prochirality center if, by changing one of its attached groups, it becomes a chirality center. The —GH2OH carbon atom of ethanol, for instance, is a prochirality center because changing one of its attached -H atoms converts it into a chirality center. 

One consequence of tetrahedral geometry is that an amine with three different substituents on nitrogen is chiral, as we saw in Section 9.12. Unlike chiral carbon compounds, however, chiral amines can t usually be resolved because the two enantiomeric forms rapidly interconvert by a pyramidal inversion, much as an alkyl halide inverts in an Sfg2 reaction. Pyramidal inversion occurs by a momentary rehybridization of the nitrogen atom to planar, sp2 geometry, followed by rehybridization of the planar intermediate to tetrahedral, 5p3 geometry... 

The Tetrahedral Carbon Atom.—We have thus derived the result that an atom in which only s and p eigenfunctions contribute to bond formation and in which the quantization in polar coordinates is broken can form one, two, three, or four equivalent bonds, which are directed toward the corners of a regular tetrahedron (Fig. 4). This calculation provides the quantum mechanical justification of the chemist s tetrahedral carbon atom, present in diamond and all aliphatic carbon compounds, and for the tetrahedral quadrivalent nitrogen atom, the tetrahedral phosphorus atom, as in phosphonium compounds, the tetrahedral boron atom in B2H6 (involving single-electron bonds), and many other such atoms. 

One of the fundamental concepts of structural chemistry is that of molecular asymmetry or chirality. The most typical example is that of a tetrahedral carbon atom with four different substituents, C(abcd), which can produce two different arrangements, which are nonsuperimposable mirror images of one another. Such a carbon atom is usually called asymmetric or chiral. In contrast, when two of the substituents are alike, as in C(abc2), the system is usually termed symmetrical or achiral, except for a special class of compounds... 

Amino acids are characteristic examples of compounds with an asymmetric carbon atom, with the exception of glycine which, since its a-carbon carries two hydrogens, is often said to be without an asymmetric carbon atom. As a typical C(abc2) system, glycine can be used (Schafer et al. 1984G) to illustrate the conformationally dependent chirality of tetrahedral carbon atoms with two substituents of identical constitution. That is, in compounds containing the glycine residue, some conformations usually exist in which the a-carbon is asymmetric and others in which it is not. 

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

In 1874, varft Hoff [26] and Le Bel [27] independently surmised that tetracoordinate carbon is surrounded by substituents in a tetrahedral geometry. This perception marked the very beginning of modern organic chemistry, which is increasingly being determined by stereochemical argumentation. Some time ago, attempts were made to synthesize stable planar tetracoordinate carbon compounds [28—30]. 

Even metals like Cu, Pt, or Pd which form tetrahedral coordination compounds also from asymmetric compounds. In all these cases, therefore, the centre of asymmetry has a tetrahedral configuration just like an asymmetric carbon atom. 

So far we have discussed conformations of a molecule obtained by rotation along sp -sp3 bond i.e., between two tetrahedral carbon atoms. But there are many compounds in which one carbon is in a state of sp2 hybridisation. Examples are substituted alkenes where one carbon atom is tetrahedral and the other trigonal, for example propene ... 

AU the synthesized compounds, regarchess of their probe s size or charge, display microbial activity in P. putida similar to that of the biomimetic analogs lacking the probe. These results provide an indication that the epical site in the tetrahedral carbon-based templates is available for attachment of chemical moieties without hampering iron(in) coordination and receptor recognition. 

Confirmation of this hypothesis is obtained by the X-ray examination of soaps and fatty esters by Shearer, Piper, Grindley and Muller (J.O.S. cxxiil. 2043, 1923) (see also Friedrich, Physikal. Zeit. xiv. 317, 1913, De Broglie, G.R. olxxvi 738,1923, Becker and Jahnke, Z. Phys. Ohem. xcix. 242, 1923), who have shown that two types of chain formation occur in carbon compounds in accordance with our preconceived views on the tetrahedral orientation of the valencies of the carbon atom. The spacing of the planes in these two types is indicated in the following diagram ... 

These results have led us rather far from the apparently straightforward situation indicated by the first set of nuclear quadrupole resonance results, and serve to illustrate how complicated the whole question can become. Whatever the reasons for it however it is clear that the tetrahedral arrangement found in tetracoordinated Carbon compound is much less rigid in the heavier elements of the group. 

Compounds in this dibenzocycloheptene series also manifest antidepressant activity when the trigonal one-carbon bridge is replaced by tetrahedral carbon. Thus, the reaction of hydrocarbon (24-7) with a metal amide in liquid ammonia leads to the corresponding carbanion (29-1). Treatment of that with the ethyl carbamate from A -methyl-3-chloropropylamine (29-2) leads to the alkylation product. The carbamate protecting group is then removed by sequential saponification with a base followed by acidification. This yields the antidepressant agent protriptyline (29-3) [30]. 

The bond angles of the covalently bonded tetrahedral carbon atoms in both the model hydrocarbon compounds and the corresponding polymers are 109 28, and the lengths of the C—H and C—C bonds in hdpe are 0.109 and 0.154 nm, respectively. The C—H and C—C bond energies are about 98 and 80 keal/mol, respectively. 

Alkyl halides (haloalkanes) are a class of compounds where a halogen atom or atoms are attached to a tetrahedral carbon sp ) atom. The functional group is —X, where —X may be —F, —Cl, —Br or —I. Two simple members of this class are methyl chloride (CH3CI) and ethyl chloride (CH3CH2CI). 

Consequently, the bond is fully saturated for A sp = 0 with a bond order of 1, but it is only partially saturated by the time the gap closes for AEap/2 h = 1 (cf eqn (7.92)) when the bond order equals 0.76. This simple second moment model has been extended to include the compound semiconductors. The resultant values of the bond order are given in Table 7.2. We see that the bonds in tetrahedral carbon and silicon are almost fully saturated, but those in zinc selenide and cadmium telluride are only about 75% saturated due partly to the mismatch in the sp orbitals between chemically distinct atoms. 

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