Organic chemistry carbon bonding

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. 

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. 

Since compounds of carbon are held together chiefly by covalent bonds, organic chemistry, too, is much concerned with molecular size and shape. To help us in our study, we should make frequent use of molecular models. Figure 1.11 shows methane as represented by three different kinds of models stick-and-ball, framework, and space-filling. These last are made to scale, and reflect accurately not only bond angles but also relative lengths of bonds and sizes of atoms. 

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. 

There is one last important correlation. If a species donates two electrons to an atom such as B or Al, it is called a Lewis base as described in Section 2.5. In some cases, a molecule, a compound, or an ion can donate two electrons to a carbon atom, forming a new bond to that carbon. Because organic chemistry is fundamentally the study of carbon compounds, reactions of carbon take on special significance. For this reason, a special name is given to a species that donates electrons to carbon. It is called a nucleophile. However, a nucleophile is essentially a Lewis base that reacts with a carbon species that functions as a Lewis acid. In most organic chemistry books, a fundamental theme is the reaction of nucleophiles with various carbon species. With an acid-base theme, a nucleophile is simply a modified Lewis base and the carbon with which it reacts is a modified Lewis acid. This relationship will be used throughout the book. 

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. 

B. Giese Radicals in Organic Synthesis Formation of Carbon-Carbon Bonds (Pergamon Press NY) 1986 Bull. Soc. Chirn. Fr. 1990, 127,675 Tetrahedron 1981, 37, 3073 Tetrahedron 1987, 43, 3541 Advances in Free Radical Chemistry 1990, 1, 121. 

Application of 7r-allylpalladium chemistry to organic synthesis has made remarkable progress[l]. As deseribed in Chapter 3, Seetion 3, Tt-allylpalladium complexes react with soft carbon nucleophiles such as maionates, /3-keto esters, and enamines in DMSO to form earbon-carbon bonds[2, 3], The characteristie feature of this reaction is that whereas organometallic reagents are eonsidered to be nucleophilic and react with electrophiles, typieally earbonyl eompounds, Tt-allylpalladium complexes are electrophilie and reaet with nucleophiles such as active methylene compounds, and Pd(0) is formed after the reaction. 

Multiple bonds are very common m organic chemistry Ethylene (C2H4) contains a carbon-carbon double bond m its most stable Lewis structure and each carbon has a completed octet The most stable Lewis structure for acetylene (C2H2) contains a carbon-carbon triple bond Here again the octet rule is satisfied... 

One more hybridization scheme is important m organic chemistry It is called sp hybridization and applies when carbon is directly bonded to two atoms as m acetylene The structure of acetylene is shown m Figure 2 18 along with its bond distances and bond angles Its most prominent feature is its linear geometry... 

Alkenes are hydrocarbons that contain a carbon-carbon double bond A carbon-carbon double bond is both an important structural unit and an important func tional group m 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 pmene (a fragrant liquid obtained from pine trees) md fame sene (a naturally occurring alkene with three double bonds)... 

An ability to form carbon-carbon bonds is fundamental to organic synthesis The addition of Grignard reagents to aldehydes and ketones is one of the most frequently used reactions m synthetic organic chemistry Not only does it permit the extension of carbon chains but because the product is an alcohol a wide variety of subsequent func tional group transformations is possible... 

Aldol condensations are one of the fundamental carbon-carbon bond forming processes of synthetic organic chemistry Furthermore because the products of these aldol con densations contain functional groups capable of subsequent modification access to a host of useful materials is gamed... 

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

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] ... 

Three different types of chemical mechanisms have evolved as attempts to simplify organic atmospheric chemistry surrogate (58,59), lumped (60—63), and carbon bond (64—66). These mechanisms were developed primarily to study the formation of and NO2 in photochemical smog, but can be extended to compute the concentrations of other pollutants, such as those leading to acid deposition (40,42). 

The carbon bond mechanism (64—66), a variation of a lumped mechanism, spHts each organic molecule into functional groups using the assumption that the reactivity of the molecule is dominated by the chemistry of each functional group. 

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). 

At one time it was felt that it would be possible to produce silicon analogues of the multiplicity of carbon compounds which form the basis of organic chemistry. Because of the valency difference and the electropositive nature of the element this has long been known not to be the case. It is not even possible to prepare silanes higher than hexasilane because of the inherent instability of the silicon-silicon bond in the higher silanes. 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

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. 

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