Bonding and Isomerism

Carbon, with four valence electrons, mainly forms covalent bonds. It usually forms four such bonds, and these may be with itself or with other atoms such as hydrogen, oxygen, nitrogen, chlorine, and sulfur. In pure covalent bonds, electrons are shared equally, but in polar covalent bonds, the electrons are displaced toward the more electronegative element. Multiple bonds consist of two or three electron pairs shared between atoms. 

Structural (or constitutional) isomers are compounds with the same molecular formulas but different structural formulas (that is, different arrangements of the atoms in the molecule). Isomerism is especially important in organic chemistry because of the capacity of carbon atoms to be arranged in so many different ways continuous chains, branched chains, and rings. Structural formulas can be written so that every bond is shown, or in various abbreviated forms. For example, the formula for n-pentane (n stands for normal) can be written as  

Some atoms, even in covalent compounds, carry a formal charge, defined as the number of valence electrons in the neutral atom minus the sum of the number of unshared electrons and half the number of shared electrons. Resonance occurs when we can write two or more structures for a molecule or ion with the same arrangement of atoms but different arrangements of the electrons. The correct structure of the molecule or ion is a resonance hybrid of the contributing structures, which are drawn with a double-headed arrow ( h ) between them. Organic chemists use a curved arrow (O) to show the movement of an electron pair. 

A sigma (a) bond is formed between atoms by the overlap of two atomic orbitals along the line that connects the atoms. Carbon uses sp hybridized orbitals to form four such bonds. These bonds are directed from the carbon nucleus toward the corners of a tetrahedron. In methane, for example, the carbon is at the center and the four hydrogens are at the corners of a regular tetrahedron with H-C-H bond angles of 109.5°. 

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Why does sucrose (table sugar) melt at 185 C, while sodium chloride (table salt)—melts at a much higher temperature, 801 °C Why do both of these substances dissolve in water, while olive oil does not Why does the molecule methyl butyrate smell like apples, while the molecule propyl acetate, which contains the same number and kind of atoms, smells like pears To answer questions such as these, you must understand how atoms bond with one another and how molecules interact with one another. Bonding is the key to the structure, physical properties, and chemical behavior of different kinds of matter. 

Perhaps you have already studied bonding and related concepts in a beginning chemistry course. Browse through each section of this chapter to see whether it is familiar, and try to work the problems. If you can work the problems, you can safely skip that section. But if you have difficulty with any of the problems within or at the end of this chapter, study the entire chapter carefully because we will use the ideas developed here throughout the rest of the book. 

Online homework for this ohapter can be assigned in OWL, an online homework assessment tool. 

An atom consists of a small, dense nucleus containing positively charged protons and neutral neutrons and surrounded by negatively charged electrons. The atomic number of an element equals the number of protons in its nucleus its atomic weight is the sum of the number of protons and neutrons in its nucleus. 

Electrons are located in orbitals. Orbitals are grouped in shells. An orbital can hold a maximum of two electrons. 

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This enzyme [EC 3.5.4.16] catalyzes the reaction of GTP with two water molecules to produce formate and 2-amino - 4 - hydroxy - 6 - erythro -1,2,3- trihydroxypropyl) -dihydropteridine triphosphate. The reaction involves hydrolysis of two C-N bonds and isomerization of the pentose unit. The recyclization step may be nonenzy-matic. 

Reduction of 1,2-cpoxy-3-butene over Raney nickol ut 76° gives t-butanol, whereas palladium on charcoal merely causes reduction of tlie double bond and isomerization to a-butyraldehyde7 (Eq. 343). Reduction of 3-chloro-l,2-epoxy-3-buteue over palladium cm calcium... 

Stable bisamino silylenes such as 83 and 85 react with organo substituted isonitriles by insertion into the N-C bond to afford exclusively silanitriles (e.g., 160) which have tetracoordinated silicon atoms <1998POL999, 2000ACR704>. In particular, no silaketenimines, the products of simple addition of the silylenes to the isonitriles, were detected. The reaction between 83 and pivaloyl isonitrile is thought to proceed via insertion of the silylene in the N-C bond and isomerization of the silaisonitrile 161 to the silanitrile 160 (Scheme 20). In the case of silylene 85 the outcome of the reaction depends on the actual conditions. Excess of silylene 85 during the course of the reaction affords product 162, which results from the addition of 2 equiv of silylene 85 to the isonitrile <2001JOM209>. 

Functionalized dienes can be obtained by C-C bond formation between 1,3-dienes and alkenes via oxidative coupling with electron-rich ruthenium catalysts but also via insertion into Ru-H and then Ru-C bonds. For example, Ru(COD)(COT) catalyzed the selective codimerization of 1,3-dienes with acrylic compounds to give 3,5-dienoic acid derivatives [18] (Eq. 13). -coordination of 1,3-diene to a hydridoruthenium leads to a 7r-allylruthenium species to selectively give, after coupling with the C=C bond and isomerization, the functionalized conjugated 1,3-dienes. 

Intramolecular condensation of 197 gave 198, which then aromatized to 119. Alternatively, tautomerization of the allylic amine 195 involving the C-6 carbonyl followed by rotation of the C(5)-C(5a) bond and isomerization gave 197 (Scheme 34) <1998JOC1290>. 

You should already know about chemical bonding and isomerism. 

Saturated hydrocarbons, including branched and unbranched chain alkanes as well as cycloalkanes, react with carbon monoxide in the presence of copper(I) oxide in HSOsF-SbFs to afford ter tiary and secondary carboxylic adds in high yield (eq 20). The reaction proceeds at 0 °C under 1 atm CO. In some cases the reaction involves cleavage of C C bonds and isomerization of the intermediate carbocatlons. 

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