Tetrahedral arrangement

Pure anhydrous aluminium chloride is a white solid at room temperature. It is composed of double molecules in which a chlorine atom attached to one aluminium atom donates a pair of electrons to the neighbouring aluminium atom thus giving each aluminium the electronic configuration of a noble gas. By doing so each aluminium takes up an approximately tetrahedral arrangement (p. 41). It is not surprising that electron pair donors are able to split the dimer to form adducts, and ether, for example, forms the adduct. 

The concept of oxidation states is best applied only to germanium, tin and lead, for the chemistry of carbon and silicon is almost wholly defined in terms of covalency with the carbon and silicon atoms sharing all their four outer quantum level electrons. These are often tetrahedrally arranged around the central atom. There are compounds of carbon in which the valency appears to be less than... 

Ammonia is a colourless gas at room temperature and atmospheric pressure with a characteristic pungent smell. It is easily liquefied either by cooling (b.p. 240 K) or under a pressure of 8-9 atmospheres at ordinary temperature. Some of its physical and many of its chemical properties are best understood in terms of its structure. Like the other group head elements, nitrogen has no d orbitals available for bond formation and it is limited to a maximum of four single bonds. Ammonia has a basic tetrahedral arrangement with a lone pair occupying one position ... 

In the sulphate ion, the four oxygen atoms are tetrahedrally arranged round the sulphur atom, at equal distances hence all the S—O bonds are identical, and their short length suggests that they are double bonds (as in SOj, SO3, and SO3") ... 

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

FIGURE 2 9 Each half filled sp orbital overlaps with a half filled hydrogen Is or bital along a line between them giving a tetrahedral arrangement of four ct bonds Only the major lobe of each sp orbital is shown Each orbital contains a smaller back lobe which has been omitted for clarity... 

Place the equatorial bonds so as to approximate a tetrahedral arrangement of the bonds to each carbon The equatorial bond of each carbon should be parallel to the ring bonds of its two nearest neighbor carbons... 

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

Our discussion to this point has been limited to molecules m which the chirality center IS carbon Atoms other than carbon may also be chirality centers Silicon like carbon has a tetrahedral arrangement of bonds when it bears four substituents A large number of organosilicon compounds m which silicon bears four different groups have been resolved into their enantiomers... 

Stereochemical inversion of the tetrahedral arrangement of bonds to the carbon at which substitution occurs... 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

Fig. 4. Model of the ciystal structure of zeolites X, Y, and the mineral faujasite. At the tight is shown the tetrahedral arrangement of tmncated octahedra surrounding one large cavity. On the left the packing model of zeohte X is shown, containing three types of Na cations.

A somewhat different approach to providing tailored cavities for metal cations was taken by the groups of Cram and Lehn °. Graf and Lehn prepared the spheroidal molecule 21 which has an interesting molecular architecture. The molecule has ten coordination sites within it, six which form an octahedral array and four which are in a tetrahedral arrangement. This remarkable compound is soluble in all solvents from petroleum... 

The optical activity of quartz and certain other materials was first discovered by Jean-Baptiste Biot in 1815 in France, and in 1848 a young chemist in Paris named Louis Pasteur made a related and remarkable discovery. Pasteur noticed that preparations of optically inactive sodium ammonium tartrate contained two visibly different kinds of crystals that were mirror images of each other. Pasteur carefully separated the two types of crystals, dissolved them each in water, and found that each solution was optically active. Even more intriguing, the specific rotations of these two solutions were equal in magnitude and of opposite sign. Because these differences in optical rotation were apparent properties of the dissolved molecules, Pasteur eventually proposed that the molecules themselves were mirror images of each other, just like their respective crystals. Based on this and other related evidence, in 1847 van t Hoff and LeBel proposed the tetrahedral arrangement of valence bonds to carbon. 

Figure 5.4 The molecular structure of basic beryllium acetate showing (a) the regular tetrahedral arrangement of 4 Be about the central oxygen and the octahedral arrangement of the 6 bridging acetate groups, and (b) the detailed dimensions of one of the six non-planar 6-membeted heterocycles. (The Be atoms are 24 pm above and below the plane of the acetate group.) The 2 oxygen atoms in each acetate group are equivalent. The central Be-O distances (166.6 pm) are very close to that in BeO itself (165 pm).

Structure of [An(> -05115)4] showing the tetrahedral arrangement of the four rings around the metal atom. 

In water at room temperature some molecules have more than four nearest neighbors, but at any moment the majority have four neighbors, as in ice. If the molecules tend to have a tetrahedral arrangement like that of ice, the average distance between next-nearest neighbors would be... 

The X-ray measurements at 1.5, 13, and 30° do, in fact, show a large concentration of molecules at about 4.5 angstroms. This is strong evidence in favor of widespread local tetrahedral arrangement in water at room temperature and below. 

Long-range order will, of course, be absent in the liquid. But any small volume of the liquid presumably contains many groups of molecules where a tetrahedral arrangement prevails. The size of each such group... 

Fig. 11-4. Possible tetrahedral arrangements of water molecules wound Li and H ions.

Methane, CH4, has four bonding pairs on the central atom. To be as far apart as possible, the four pairs must take up a tetrahedral arrangement around the C atom. Because the electron arrangement is tetrahedral and an H atom is attached to each bonding pair, we expect the molecule to be tetrahedral (see 1), with bond angles of 109.5°. Thar is the shape found experimentally. 

When we try to apply VB theory to methane we run into difficulties. A carbon atom has the configuration [HeJ2s22pvl2p l,1 with four valence electrons (34). However, two valence electrons are already paired and only the two half-filled 2/ -orbitals appear to be available for bonding. It looks as though a carbon atom should have a valence of 2 and form two perpendicular bonds, but in fact it almost always has a valence of 4 (it is commonly tetravalent ) and in CH4 has a tetrahedral arrangement of bonds. 

When there is more than one central atom in a molecule, we concentrate on each atom in turn and match the hybridization of each atom to the shape at that atom predicted by VSEPR. For example, in ethane, C2H6 (38), the two carbon atoms are both central atoms. According to the VSEPR model, the four electron pairs around each carbon atom take up a tetrahedral arrangement. This arrangement suggests sp hybridization of the carbon atoms, as shown in Fig. 3.14. Each... 

FIGURE 3.14 Each C H bond in methane is formed by the pairing of an electron in a hydrogen U-orbital and an electron in one of the four sp hybrid orbitals of carbon. Therefore, valence-bond theory predicts four equivalent cr-bonds in a tetrahedral arrangement, which is consistent with experimental results. 

AH for this reaction we take to be 3.65 v.e., the energy required to break a C — C bond in aliphatic compounds. The second step of the dissociation consists in a change in the structure of the radicals, involving a change in nuclear configuration from a tetrahedral arrangement of... 

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