Covalent bonding reactions

STEP 2 Reaction of a nucleophile and an electrophile to form a new covalent bond. Reaction of the 3° carbocation intermediate (a Lewis acid) with iodide ion (a Lewis base) completes the valence shell of carbon and gives the product ... 

STEP 3 Reaction of an electrophile and a nucleophile to form a new covalent bond. Reaction of the resonance-stabilized cation (an electrophile) with methanol (a nucleophile) gives the conjugate acid of the acetal ... 

Reactions of proteins with Hpid hydroperoxides and other oxidised Hpids give rise to different types of Hpoproteins, in which Hpids and proteins are bound by physical bonds (as in natural Hpoproteins), but also by covalent bonds. Reactions with hydroperoxides, free radicals, epoxides and aldehydes lead to the fission of some protein bonds, formation of protein radicals and oligomers, cross links between protein chains and some sensitive functional groups of amino acids are oxidised (see Section 2.5.1.1). 

At the elastomer end, it is reasonable to draw a closer analogy with rubber-to-metal bonding agents. The RLF s latex terpolymer is a diene rubber, and is likely to be vulcanized by any sulfur or accelerators that diffuse over from the carcass rubber. It has further been conjectured that the pendant pyridine rings of the same terpolymer should be attacked by the reactive R-F species. Thus, a total chemical network, but involving two or more kinds of cross-linking or covalent bonding reactions, is plausible. ... 

Alternatively a reaction between a species with a pair of electrons and a species with a vacant orbital to form a covalent bond, heteronuclear molecule See homonuclear molecule. 

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

Obviously sufficient energy is available to break the A1—Cl covalent bonds and to remove three electrons from the aluminium atom. Most of this energy comes from the very high hydration enthalpy of the AP (g) ion (p. 78). Indeed it is the very high hydration energy of the highly charged cation which is responsible for the reaction of other essentially covalent chlorides with water (for example. SnCl ). 

Silicon, unlike carbon, does notiorm a very large number of hydrides. A series of covalently bonded volatile hydrides called silanes analogous to the alkane hydrocarbons is known, with the general formula Si H2 + 2- I uf less than ten members of the series have so far been prepared. Mono- and disilanes are more readily prepared by the reaction of the corresponding silicon chloride with lithium aluminium hydride in ether ... 

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

The carbon atom m bromomethane can accept an electron pair if its covalent bond with bromine breaks with both electrons m that bond becoming an unshared pair of bromide ion Thus bromomethane acts as a Lewis acid m this reaction... 

Heterogeneous reaction (Section 6 1) A reaction involving two or more substances present in different phases Hydro genation of alkenes is a heterogeneous reaction that takes place on the surface of an insoluble metal catalyst Heterolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that both electrons are retained by one of the initially bonded atoms Hexose (Section 25 4) A carbohydrate with six carbon atoms High density lipoprotein (HDL) (Section 26 11) A protein that carries cholesterol from the tissues to the liver where it is metabolized HDL is often called good cholesterol Histones (Section 28 9) Proteins that are associated with DNA in nucleosomes... 

Applications of semi-empirical and ab initio methods frequently follow the course of chemical reactions that break and form covalent bonds. You must take care in choosing a method for these applications. 

Self-Rephca.tingSystems. Recently, molecules have been synthesized that can catalyze covalent bond-making reactions by forming a noncovalently bonded superstmcture, a maneuver that converts an intermolecular reaction into an intramolecular one. In general, in such systems, two... 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

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