Tetrahedral carbon atoms

The two kinds of covalent bond are not identical, one being a simple covalent bond, a sigma (ct) bond, the other being a stronger (but more reactive) bond called a n bond (p. 56). As in the formation of methane both elements attain noble gas configurations. We can consider the formation of ethene as the linking of two tetrahedral carbon atoms to form the molecule C2H4 represented as ... 

Electrophilic Substitution. The most common mechanism for electrophilic attack at an aromatic system involves the initial attack of an electrophile to give an intermediate containing a tetrahedral carbon atom loss of, usually a proton, from the intermediate, then gives the product ... 

Figure 1.6 A representation ol Van t Hoff s tetrahedral carbon atom. The solid lines are in the plane of the paper, the heavy wedged line comes out of the plane of the page, and the dashed line goes back behind the plane of the page.

Figure 9.1 Tetrahedral carbon atoms and their mirror images. Molecules of the type CH3X and CH2XY are identical to their mirror images, but a molecule of the type CHXYZ is not. A CHXYZ CH3X X l X hK vh H ...

Because carbohydrates usually have numerous chirality centers, it was recognized long ago that a quick method for representing carbohydrate stereochemistry is needed. In 1891, Emil Fischer suggested a method based on the projection of a tetrahedral carbon atom onto a flat surface. These Fischer projections were soon adopted and are now a standard means of representing stereochemistry at chirality centers, particularly in carbohydrate chemistry. 

A tetrahedral carbon atom is represented in a Fischer projection by two crossed lines. The horizontal lines represent bonds coming out of the page, and the vertical lines represent bonds going into the page. 

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. 

Thus we have shown that when s and p orbitals are available and s—p quantization is broken an atom can form four (or fewer) equivalent bonds which are directed towards tetrahedron corners. To the approximation involved in these calculations the strength of a bond is independent of the nature of other bonds. This result gives us at once the justification for the tetrahedral carbon atom and other tetrahedral atoms, such as silicon, germanium, and tin in the diamond-type crystals of the elements and, in general, all atoms in tetrahedral structures. 

The Dependence of Bond Angles on Single Bond-Double Bond Resonance.—In a molecule such as phosgene or 1,1-dichloroethylene the value 125°16 for the angle Cl-C-0 (0) is predicted by the theory of the tetrahedral carbon atom in case that the C-Cl bonds have no double bond character. If the double bond resonates equally among all three positions, giving the Cl-C bond one-third double bond character, we expect from symmetry... 

The results shown in the table provide further evidence of the extraordinary extent to which the tetrahedral carbon atom of van t Hoff and Le Bel determines the structure of organic molecules. 

For a thorough understanding of stereochemistry, it is useful to examine molecular models (like those depicted in Fig. 4.1). However, this is not feasible when writing on paper or a blackboard. In 1891, Emil Fischer greatly served the interests of chemistry by inventing the Fischer projection, a method of representing tetrahedral carbon atoms on paper. By this convention, the model is held so that the two bonds in front of the paper are horizontal and those behind the paper are vertical. 

Oxygen Bonded to Activated Tetrahedral Carbon Atoms... 

Schafer, L., C. Van Alsenoy, and L. Van den Enden. 1984. The Possible Chirality Of Tetrahedral Carbon Atoms with Two Substituents of Identical Constitution. J. Chem. Ed. 61, 945-947. 

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. 

A tetrahedral carbon atom with four different groups attached to it is an... 

Figure 5.4 The tetrahedral carbon atom of 2-butanol that bears four different groups. [By convention such atoms are often designated with an asterisk ( )].

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). 

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 ... 

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