Carbon atom, tetrahedral geometry three-dimensionality

Carbon exists in more than 40 known structural forms, or allotropes, several of which are crystalline but most of which are amorphous. Graphite, the most common allotrope of carbon and the most stable under normal conditions, is a crystalline covalent network solid that consists of two-dimensional sheets of fused six-membered rings (Figure 10.26a). Each carbon atom is sp2-hybridized and is connected to three other carbons. The diamond form of elemental carbon is a covalent network solid in which each carbon atom is sp3-hybridized and is bonded with tetrahedral geometry to four other carbons (Figure 10.26b). 

Most of the carbon atoms in sugars have a tetrahedral geometry. Therefore, sugar molecules are not flat, but have a three-dimensional structure. The three-dimensional structure of carbohydrates is commonly depicted using Fischer projections, named in honor of Emile Fischer. Fischer worked out the structures of most of the carbohydrates in the first part of the twentieth century with little... 

In Chapter i you learned the geometry of the bonds around an atom. For example, the four bonds of an -hybridized carbon have a tetrahedral geometry. But what happens when several such carbons are bonded together What is the geometrical relationship between the bonds on different carbons What is the overall shape of the molecule Is more than one shape possible If so, are they different in energy Can they interconvert If so, how fast These and other questions will be answered in this chapter and the next, which discuss the stereochemistry, or three-dimensional structures, of organic molecules. In these chapters you will encounter a new type of isomer stereoisomers. Unlike the constitutional isomers that you have already seen, stereoisomers have the same bonds or connectivity, but the bonds are in a different three-dimensional orientation. 

The existence of only one kind of CH2CI2 molecule means that all four positions surrounding the carbon atom are geometrically equivalent, which requires a tetrahedral coordination geometry. Unfortunately, this fact can only really be made convincing by inspecting a three-dimensional mechanical model of the molecule. 

The three-dimensional representations and the ball-and-stick models for these alkanes indicate the tetrahedral geometry around each carbon atom. In contrast, the Lewis structures are not meant to imply any three-dimensional arrangement. Moreover, in propane and higher molecular weight alkanes, the carbon skeleton can be drawn in a variety of different ways and still represent the same molecule. 

The molecular basis for the left- and right-handedness of distinct crystals of the same chemical substance and the associated differences in optical rotation was developed from the hypothesis of Paterno (1869) and Kekule that the geometry about a carbon atom bound to four ligands is tetrahedral. Based on the concept of tetrahedral geometry, Van t Hoff and LeBel concluded that when four different groups or atoms are bound to a carbon atom, two distinct tetrahedral molecular forms are possible, and these bear a non-superimposable mirror-image relationship to one another (Fig. 3). This hypothesis provided the link between three-dimensional molecular structure and optical activity, and as such represents the foundation of stereoisomerism and stereochemistry. 

We know from Sections 1.7 and 1.8 that an sp -hybridized carbon atom has tetrahedral geometry and that the carbon-carbon bonds in alkanes result from a overlap of carbon sp orbitals. Let s now look into the three-dimensional consequences of such bonding. What are the spatial relationships between the hydrogens on one carbon and the hydrogens on a neighboring carbon We ll see in later chapters that an understanding of these spatial relationships is often crucial for understanding chemical behavior. 

Pure diamond is a crystalline form of only carbon atoms. The crystal lattice is three-dimensional, and all carbon atoms are attached to four other carbon atoms in a perfect tetrahedral geometry. Because the lattice repeats in all three dimensions, there is no easy way to distort the structure, making it a very hard material. The structure of graphite, for comparison, is many stacked-up layers of carbon atoms. These two-dimensional sheets can slide relative to one another, making graphite relatively soft in comparison to diamond. 

For iodide to displace bromide, it must collide with the polarized carbon atom bearing the bromine atom. The 6-1- polarization of that carbon leads to some electrostatic attraction between the electron-rich nucleophile and the electron-deficient carbon. If the iodide ion must collide with the sp carbon atom marked in red (see 9 in Figure 11.3), the angle of approach is important because that carbon is part of a three-dimensional molecule with a tetrahedral geometry. If the electron-rich iodide approaches the 6-1- carbon atom from the... 

When carbon forms four single bonds, they are arranged tetrahedrally around the carbon atom the molecular geometry is tetrahedral (Fig. 21.1). Recall from Chapter 13 that it is not possible to represent this three-dimensional shape accurately in a two-dimensional sketch. Thus the four bonds radiating from each carbon atom in... 

The conformation of a polymer chain is the three dimensional spatial arrangement of the chain as determined by the rotation about backbone bonds. The conformation and configuration of the polymer molecules have a great influence on the properties of the polymer component. It is described by the polarity, flexibility and the regularity of the macromolecule. The helix is a typical ordered conformation type for polymers that contain regular chain microstructure. Typically carbon atoms are tetra-valent, which means that in a saturated organic compound they are surrounded by four substituent in a symmetric tetrahedral geometry. 

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