Carbon in organic chemistry

The importance of carbon in organic chemistry results from its ability to form carbon-carbon bonds, permitting complex molecules, with the most varied properties, to exist. The importance of silicon in the inorganic world results from a different property of the element —a few coiiipounds are known in which silicon atoms are connected to one another by covalent bonds, but these compounds are relatively unimportant. The characteristic feature of the silicate minerals is the existence of chains and more complex structures (layers, three-dimen sional frameworks) in which the silicon atoms are not bonded directly to one another but are connected by oxygen atoms. 1 he nature of these structures is described briefly in later sections of this chapter. 

The natural abundance of is only 17o> and thus spectra are difficult to observe in unenriched samples. A further disadvantage is that is a less good magnet than a proton. The overall loss of sensitivity compared to H is approximately 6000-fold. However, because of the central importance of carbon in organic chemistry, constant efforts to obtain NMR data have been made over the last 15 years. Recently (1970-1971) Fourier Transform NMR spectroscopy (see Sec. 

The hydrogen atom is a major player among the elements. It is easily the most abundant element in the universe and makes up about 90% of the universe by weight. Hydrogen is involved in most of the everyday compounds that we know of and is particularly important when bonding with carbon in organic chemistry. 

The aliphatic chain is referred to as saturated because all carbon atoms are linked to four other atoms (sp carbons in organic chemistry). When a carbon atom is linked to only three other atoms, one of the chemical bond has to be a double bond to respect the valency of 4, which is characteristic of carbon. In this case, the double bond links two carbons and it is noted C=C. The nomenclature used for saturated fatty acids refers to the number of carbon atoms, and to the lack of double bonds correspondingly, palmitic add, which is the saturated fatty acid with 16 carbons, is noted C16 0 (C16 for the carbon number, 0 for the number of double bond). You can train yourself to write the structure of the following fatty acids C9 0, C14 0, and C18 0. A list of biologically saturated fatty acids can be found in Table 1.2. Note that only the limited piece of information found in the first column is useful to draw the chemical structure of all these lipids. 

Model 4 Oxidation and Reduction of Carbon in Organic Chemistry... 

Breitmaier E and Voelter W 1986 Carbon-13 NMR Spectroscopy High Resolution Methods and Applications in Organic Chemistry (New York VCH)... 

One of the most original and significant ideas in organic chemistry was the suggestion by Hans Meerwein that carbocations (as we now call all the positive ions of carbon compounds) might be intermediates in the course of reactions that start from nonionic reactants and lead to nonionic covalent products. 

One of the cornerstones of the chemistry of carbon compounds (organic chemistry) is Kekule s concept, proposed in 1858, of the tetra-valence of carbon. It was independently proposed in the same year by Couper who, however, got little recognition (vide infra). Kekule realized that carbon can bind at the same time to not more than four other atoms or groups. It can, however, at the same time use one or more of its valences to form bonds to another carbon atom. In this way carbon can form chains or rings, as well as multiple-bonded compounds. 

Reduction (Section 2 19) Gam in the number of electrons as sociated with an atom In organic chemistry reduction of carbon occurs when a bond between carbon and an atom which IS more electronegative than carbon is replaced by a bond to an atom which is less electronegative than carbon Reductive ami nation (Section 22 10) Method for the prepara tion of amines in which an aldehyde or a ketone is treated with ammonia or an amine under conditions of catalytic hy drogenation... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

The most important interatomic bond in polymers, and indeed in organic chemistry, is the covalent bond. This is formed by the sharing of one or more pairs of electrons between two atoms. An example is the bonding of carbon and hydrogen to form methane Figure 5.2). 

The polarity of covalent bonds between carbon and substituents is the basis of important structure-reactivity relationships in organic chemistry. The effects of polar bonds are generally considered to be transmitted in two ways. Successive polarization through bonds is called the inductive fect. It is expected that such an effect would diminish as the number of intervening bonds increases. 

Cycloalkanes are alkanes that contain a ring of three or more carbons. They are frequently encountered in organic chemistry and are characterized by the molecular- formula C H2 . Some examples include ... 

One more hybridization scheme is important in organic chemistry. It is called sp hybridization and applies when carbon is directly bonded to two atoms, as in acetylene. The structure of acetylene is shown in Figure 2.18 along with its bond distances and bond angles. Its most prominent feature is its linear geometr-y. 

Alkenes are hydrocarbons that contain a carbon-carbon double bond. A carbon-carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influenced by the presence of this bond, and the double bond is the site of most of the chemical reactions that alkenes undergo. Some representative alkenes include isobutylene (an industrial chemical), a-pinene (a fragrant liquid obtained from pine trees), and famesene (a naturally occuning alkene with three double bonds). 

In organic chemistry, chirality most often occurs in molecules that contain a carbon that is attached to four different groups. An example is bromochlorofluoromethane (BrClFCH). 

When a Br nsted base functions catalytically by sharing an electron pair with a proton, it is acting as a general base catalyst, but when it shares the electron with an atom other than the proton it is (by definition) acting as a nucleophile. This other atom (electrophilic site) is usually carbon, but in organic chemistry it might also be, for example, phosphorus or silicon, whereas in inorganic chemistry it could be the central metal ion in a coordination complex. Here we consider nucleophilic reactions at unsaturated carbon, primarily at carbonyl carbon. Nucleophilic reactions of carboxylic acid derivatives have been well studied. These acyl transfer reactions can be represented by... 

The addition of the a-carbon of an enolizable aldehyde or ketone 1 to the carbonyl group of a second aldehyde or ketone 2 is called the aldol reaction It is a versatile method for the formation of carbon-carbon bonds, and is frequently used in organic chemistry. The initial reaction product is a /3-hydroxy aldehyde (aldol) or /3-hydroxy ketone (ketol) 3. A subsequent dehydration step can follow, to yield an o ,/3-unsaturated carbonyl compound 4. In that case the entire process is also called aldol condensation. 

The terms primary- secondary, tertiary, and quaternary are routinely used in organic chemistry, and their meanings need to become second nature. For example, it we were to say, "Citric acid is a tertiary alcohol," we would mean that it has an alcohol functional group (-OH) bonded to a carbon atom that is itself bonded to three other carbons. (These other carbons may in turn connect to other functional groups). 

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