Why Carbon Is Different

By contrast, there are only about 100,000 known inorganic compounds. 

Wohler s synthesis of urea dispelled the notion that oiganie eompounds were fundamentally different from inorganie compounds—and that they could only be produced by nature. We now know that it is possible to synthesize a wide variety of organie eompounds in the laboratory in fact, many thousands of new organic compounds are produced in researeh laboratories each year. In this chapter, we will consider several types of organic compounds that are important biologically. 

Because of its unique nature, carbon is capable of forming millions of different compounds. Carbon s position in the periodic table (Group 4A, Period 2) gives it the following set of unique characteristics  

The electron configuration of carbon ([He]2s 2p ) effectively prohibits ion foimation. This and carbon s electronegativity, which is intermediate between those of metals and nonmetals, cause carbon to complete its octet by sharing electrons. In nearly all its compounds, carbon forms four covalent bonds, which can be oriented in as many as four different directions  

Boron and nitrogen, carbon s neighbors in Groups 3A and 5A, usually form covalent compounds, too, but B and N form ions more readily than C. 

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Why Carbon Is Different Classes of Organic Compounds Representing Organic Molecules... 

This is a laboratory experiment. Carbon was studied as a half-ceU (section 2.2.3) with a metal lithium electrode as the counter electrode. The difference in voltage between these two electrodes is, naturally, very low. This is one of the reasons why carbon is often used as the negative electrode in hthium-ion secondary batteries instead of metal hthiiun, because it does not decrease the voltage of a cell too greatly. Such a decrease would lower the specific energy (defined in section 2.4.16.). It should be noted that in this half-ceU , carbon is a positive electrode because it is used in conjunction with lithium. 

The fact that carbon has a valency of four, i.e. in the outermost shell surroimding the nucleus of the atom there are fom electrons (see Section 1.1 and Figme 1.1), is the key to why carbon is so versatile. This outermost shell can accommodate eight electrons so each carbon atom has the capacity to share electrons with as many as four different atoms, including other carbon atoms. Thus carbon has the capacity to combine with many different elements to produce a vast array of compoimds. This is one reason why carbon is so important in nature and to society. 

Molecules am act one another. Fiuni that simple fact spring fundamentally important consequences. Rivers, lakes, and oceans exist because water molecules attract one another and form a liquid. Without that liquid, there would be no life. Without forces between molecules, our flesh would drip off our bones and the oceans would be gas. Less dramatically, the forces between molecules govern the physical properties of bulk matter and help to account for the differences in the substances around us. They explain why carbon dioxide is a gas that we exhale, why wood is a solid that we can stand on, and why ice floats on water. At very close range, molecules also repel one another. When pressed together, molecules resist further compression. 

Metallic lead is dark in color and is an electrical conductor. Diamond, the most valuable form of carbon, is transparent and is an electrical insulator. These properties are very different yet both lead and carbon are in Group 14 of the periodic table and have the same valence configuration, s p Why, then, are diamonds transparent insulators, whereas lead is a dark-colored conductor ... 

Clearly, the highlighted carbon is a chiral centre (it has four different groups attached to it). For this reason, the two protons Ha and Hb can never be in the same environment. The fact that there is free rotation around all the single bonds in the molecule is irrelevant. This can best be appreciated by building a model of the molecule. Having done so, look down the molecule from left to right as drawn and rotate the C-0 bonds so that Ha and Hb rotate. It should now be clear why these two protons can never occupy the same space and are therefore not equivalent. 

Constitutional isomerism becomes more complex as the size of the hydrocarbon molecule is increased. For example, there are three constitutional isomers of pentane, C5H12. The number of constimtional isomers increases quite rapidly with an increasing number of carbon atoms. Thus, there are five constimtional isomers of hexane, CeH, nine isomers of heptane, C7H16, 75 isomers of decane, C10H22, and 366,319 isomers of eicosane, C20H42. You can begin to understand why it is possible to make so many different molecules based on carbon. 

To see why this is important, consider a butene (2-butene) in which the double bond is between the two central carbon atoms CH3—CH—CH—CH3. If we think about it for a bit, we can recognize that there are really two of these structures they are stereoisomers that are not enantiomers and not constitutional isomers. Such stereoisomers are termed diastereomers. Diastereomers in this class are also known by the older and largely obsolete term geometrical isomers. They differ in the way that the two methyl groups at the ends of the molecule are disposed with respect to each other. The two possibilities are ... 

But very little is known of the receptor s south end, so to speak, the geometry of the area where the opposite end of the molecule has to fit. Here, with 2-C-17, there is a secondary butyl group, and this contains an asymmetric carbon atom. But now this center of asymmetry is clear across the benzene ring from the nitrogen, and should certainly be in some entirely new part of the receptor site. Why not make this compound with the R and the S forms in this new and unusual location Why not, indeed Why not call them the right-lane and the left lane of the Nimitz Fortunately, both R and S secondary butyl alcohols were easily obtained, and the synthesis given above for the racemic compound was paralleled for each of these isomers, separately. Is there any chemistry that is different with the specific optical isomers from that which has been reported with the racemic There certainly is for the first step, since the butyl alcohols rather than the butyl bromides must be used, and this first step must go by inversion, and it cannot be allowed any racemization (loss of the optical purity of the chiral center). 

The development of the types of skeletons that characterize Tommotian faunas constituted a major evolutionary event. Although skeletons are known to support soft tissue and to facilitate locomotion, such adaptive functions cannot explain why so many different kinds of skeletons developed suddenly in the early part of Tommotian time. It has been suggested that a chemical change within the oceans triggered the production of these skeletons, but this hypothesis does not explain why some skeletons were composed of calcium carbonate and others of calcium phosphate, two compounds with quite different chemical properties. The rapid evolution of various kinds of external skeletons is probably in part attributable to the fact that animals... 

Carbon is the fourth most abundant element in the universe. Its abundance in the Sun is about one-half that of oxygen, butreveals differing ratios to oxygen in other stars and in nebulae. The most abundant isotope of carbon, 12C, is the fourth most abundant nucleus in the universe. The two most abundant, 2H and 4He, are remnants of the Big Bang, whereas l60, the third most abundant, and 12C are created during the evolution of stars. Carbon ranks therefore as one of the great successes of stellar nucleosynthesis. The evolution of stars makes evident why this is so. From the isotopic decomposition of normal carbon one finds that the mass-12 isotope, 12C, is 98.9% of all C isotopes. 

Why the big difference The reason is that the enol ether can be protonated at carbon using the delocalization of the oxygen lone pair in the enol derivative to produce a reactive oxonium ion. 

The adsorption ratios, A = a/ observed thus far for atactic and isotactic systems indicate that self-association can occur in two ways either by expulsion of already-adsorbed molecules (i.e. in the cases that A is <1) or by addition of more adsorbed molecules (i.e. in the cases that A is > 1). Why this is so is not understood. It is curious to note, however, that the solvent in those P-L systems with A > 1 is in every case a cyclic aliphatic liquid, the ring structure of which contains no more than one atom that is not carbon (Nos. 1, 2, 14, and 15 Table 20), whereas none of the solvents in a P-L system with A < 1 is in this category. Because the number of a reported thus far are relatively few (Table 20), owing to the time-consuming procedure and the high technical skill required to obtain data via the protocol described by Guenet, it is not yet possible to adjudicate with certainty whether or not this cyclic vs acyclic differentiation is a real phenomenon or just a fortuitous observation. It does suggest the possibility, however, that the mode of adsorption in the case of cyclic aliphatic molecules may be qualitatively different from that for acyclic molecules. 

It is the aim of this chapter to develop a model for the very broad spectrum of reactivity of fluorine-containing systems towards nucleophiles. Substiment effects of fluorine and fluorocarbon groups on the SnI process were considered earlier, in a more general discussion of carbocations (see Chapter 4, Section VI) effects on the Sn2 process will now be examined. Then the broader principles of displacement of fluorine, as fluoride ion, from carbon in different environments will be discussed to emphasise why, for example, nucleophilic displacement of fluoride ion from perfluoroalkenes occurs extremely rapidly while, in contrast, perfluoroalkanes are characterised by extreme inertness. 

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