Common multiple bond systems

In the previous chapter, we looked at addition reactions to carbon/carbon multiple bonds. In this chapter, we will look at addition reactions to another very common multiple bond system, namely the carbon/oxygen double bond. This system differs quite significantly from the carbon/carbon multiple bond system, primarily because there is an inherent dipole across the bond. In the carbon/carbon double bond system such a dipole only occurs if it is caused by the substituents that are attached to it, while in the carbonyl system it arises inherently due to the differences in the electronegativities of the component parts. 

FIGURE 3.23 Anisotropy caused by the presence of TT electrons in some common multiple-bond systems. 

In their metal complexes, bonding of either species to the metal atom is via a ligand - > metal a donor bond and a metal - >ligand n bond, enabling back donation of electron density to the n orbitals of the C-C multiple bond system to take place. Vinylidene is one of the best 7t-acceptors known, and is exceeded only by S02 and CS in this respect the relationship between phenylvinylidene and other common ligands has been determined (18) from the CO force constants exhibited by a series of Mn(CO)2(q-C5H5) complexes, which increase in this order ... 

Cycloaddition is the most common source of 1,2-oxazetidine skeletons, but sometimes it is also useful to construct 1,3-oxazetidine rings. Indeed, in contrast to the usual reactivity exhibited by mtinchnones, which react with multiple bond systems as azomethine ylides by a [3+2] cycloaddition process, azete 58 was found to react with miinchnone 59, under thermal conditions, to give an isomeric mixture of the oxazabicyclo[2.2.0]hexene (E/Z)-26,... 

A common method of classification for petroleum is the PONA system (PONA is an acronym for paraffins, olefins, naphthenes, and aromatics). Paraffins are straight-chain or branched hydrocarbons in which there are no double or triple bonds between carbon atoms. Olefins are similar to paraffins, but they contain at least one multiple bond in their chemical structure. Naphthenes are saturated hydrocarbons, just like paraffins, but they incorporate a ring of carbon atoms into their chemical structure. Aromatics contain a benzene ring in their structure. 

There is no evidence in any of the gas phase systems for initial multiple bond rupture (i.e., fragmentation reactions). Because of the low reaction temperatures, the alkoxy radical intermediates of the bond fission reactions (or radicals resulting from alkoxy radicals) are mainly involved in radical-radical termination processes ( 0) rather than participating in hydrogen abstraction from the parent peroxide E oi 6-8). Thus it has been commonly believed that the peroxide decompositions were classic examples of free radical non-chain processes. Identification of the rate coefficients and the overall decomposition Arrhenius parameters with the initial peroxide bond fission kinetics were therefore made. However, recent studies indicate that free radical sensitized decompositions of some peroxides do occur, and that the low Arrhenius parameters obtained in many of the early studies (rates measured by simple manometric techniques) were undoubtedly a result of competitive chain processes. The possible importance of free radical reactions in peroxide decompositions is illustrated below with specific regard to the dimethyl peroxide decomposition. 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

There are two approaches to pulse sequence classification depending on the user s occupation. For the chemist who has to solve a structural question or characterize a new compound it is the spectra obtained from the pulse sequence that is of primary importance. The NMR spectroscopist is usually more concerned with the pulse sequence structure and choice of experimental parameters and whether a particular pulse sequence can be improved or modified to solve a specific problem. These two different approaches lead to confusion in pulse sequence nomenclature such that names are often a combination of the purpose of the experiment and the sequence layout. For example the commonly used acronyms HMQC, HSQC and HMBC imply a consistent abbreviation system yet HMQC and HSQC describe the coherence state during the evolution time whilst HMBC denotes an experiment to correlate nuclei using multiple bond heteronuclear scalar coupling. 

Another readily glass-forming system involves metal-metalloid alloys. The most common ones use iron or nickel with phosphorus or boron. Their frequent use results from the catalytic properties of the metals for the widely studied hydrogenation of CO. Alloys prepared with all possible combinations of these four elements in different compositions have been studied. Additionally, some of these alloys, Ni-B, Ni-P, and Fe-Ni-(Cr)-P-B, as well as Pd-Si and Pd-Ge alloys, have been studied in the hydrogenation of hydrocarbons containing multiple bonds. 

This is pretty good All nonhydrogen atoms have octets, and none are charged. Can we find any other reasonable resonance forms There is a common pattern described in the text for systems containing an atom with at least one lone pair attached to one of two atoms connected by a multiple bond. You move the lone pair in" and move a ir bond out ... 

HDA reactions, starting with 1,3-conjugated systems and unsaturated compounds having heteroatom-carbon or heteroatom-heteroatom multiple bond(s), are also accelerated by Lewis acids [158]. Aldehydes are commonly used as dienophiles... 

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