Unsaturated hydrocarbons reactivity

It has been known for more than a century that hydrocarbons containing double bonds are more reactive than their counterparts that do not contain double bonds. Alkenes are, in general, more reactive than alkanes. We call electrons in double bonds 71 electrons and those in the much less reactive C—C or CH bonds Huckel theory, we assume that the chemistry of unsaturated hydrocarbons is so dominated by the chemistry of their double bonds that we may separate the Schroedinger equation yet again, into an equation for potential energy. We now have an equation of the same fomi as Eq. (6-8), but one in which the Hamiltonian for all elections is replaced by the Hamiltonian for Ji electrons only... 

In addition to inorganic radicals, which profoundly modify the properties of a paraflSn hydrocarbon residue, there is a whole series of organic groupings which are distinguished by exceptional reactivity, for example, the ethylene and acetylene groupings, and the phenyl and naphthyl radicals. Thus the characterisation of unsaturated hydrocarbons and their derivatives, e.g., the aromatic compounds, becomes possible. 

Unsaturated Hydrocarbons. Olefins from ethylene through octene have been converted into esters via acid-catalyzed nucleophilic addition. With ethylene and propjiene, only a single ester is produced using acetic acid, ethyl acetate and isopropyl acetate, respectively. With the butylenes, two products are possible j -butyl esters result from 1- and 2-butylenes, whereas tert-huty esters are obtained from isobutjiene. The C5 olefins give rise to three j iC-amyl esters and one /-amyl ester. As the carbon chain is lengthened, the reactivity of the olefin with organic acids increases. 

Alkenes — Also known as olefins, and denoted as C H2 the compounds are unsaturated hydrocarbons with a single carbon-to-carbon double bond per molecule. The alkenes are very similar to the alkanes in boiling point, specific gravity, and other physical characteristics. Like alkanes, alkenes are at most only weakly polar. Alkenes are insoluble in water but quite soluble in nonpolar solvents like benzene. Because alkenes are mostly insoluble liquids that are lighter than water and flammable as well, water is not used to suppress fires involving these materials. Because of the double bond, alkenes are more reactive than alkanes. 

Compounds that contain only hydrogen and carbon are called hydrocarbons. The hydrocarbons that have only single bonds all have similar chemistry and they are called, as a family, the saturated hydrocarbons. If there are carbon-carbon double bonds, the reactivity is much enhanced. Hence hydrocarbons containing one or more double bonds are named as a distinct family, unsaturated hydrocarbons. Both saturated and unsaturated hydrocarbons can occur in chain-like structures or in cyclic structures. Each of these families will be considered. 

Unsaturated hydrocarbons are quite reactive —in contrast to the relatively inert saturated hydrocarbons. This reactivity is associated with the double bond. In the most characteristic reaction, called addition, one of the bonds of the double bond opens and a new atom becomes bonded to each of the carbon atoms. Some of the reagents that will add to the double bond are... 

Thermolysis of m-PtH(CH2CMe3)(cy2PC2H2Pcy2) at 45-80°C yields a bent platinum(0) complex (Figure 3.13) that is intensely reactive to a whole range of unactivated C—H bonds in saturated and unsaturated hydrocarbons. 

Such a molecule, containing alternating single and double bonds, would be expected to be quite reactive. Actually, benzene is quite unreactive, and its chemical properties resemble those of the alkanes much more than those of the unsaturated hydrocarbons. For example, the characteristic reaction of benzene with halogens resembles that of the reaction of the alkanes ... 

The anhydrous salt which had been used for drying unsaturated hydrocarbons exploded on heating to 220° C for reactivation. The need to avoid contact with acidic materials is stressed. (Traces of an alkyl perchlorate may conceivably have been formed from free perchloric acid). 

Metal vapor chemistry showed that the lanthanides had quite an extensive chemistry with unsaturated hydrocarbons. Some of the early surveys of metal vapor reactions with unsaturated hydrocarbons included some lanthanide metals and showed that reactivity was present for these metals (14-18). Subsequent synthetic studies in which the products were isolated and characterized led to some of the most unusual organolanthanide complexes currently known (19-28). 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

The rate constant of R02 addition to the ir-bond of the unsaturated hydrocarbon depends on its structure and, hence, varies widely (see Table 2.10). The problems of reactivity of reactants in such reactions will be discussed in Chapter 6. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

M. N. Paddon-Row, K. D. Jordan, Through-Bond and Through-Space Interactions in Unsaturated Hydrocarbons Their Implications for Chemical Reactivity and Long-Range Electron Transfer , in J. F. Liebman, A Greenberg (Eds.) Modem Models of Bonding and Delocalization, Vol. 6, VCH Publishers, New York 1988, pp. 115-194. 

Heterocycles with conjugated jr-systems have a propensity to react by substitution, similarly to saturated hydrocarbons, rather than by addition, which is characteristic of most unsaturated hydrocarbons. This reflects the strong tendency to return to the initial electronic structure after a reaction. Electrophilic substitutions of heteroaromatic systems are the most common qualitative expression of their aromaticity. However, the presence of one or more electronegative heteroatoms disturbs the symmetry of aromatic rings pyridine-like heteroatoms (=N—, =N+R—, =0+—, and =S+—) decrease the availability of jr-electrons and the tendency toward electrophilic substitution, allowing for addition and/or nucleophilic substitution in yr-deficient heteroatoms , as classified by Albert.63 By contrast, pyrrole-like heteroatoms (—NR—, —O—, and — S—) in the jr-excessive heteroatoms induce the tendency toward electrophilic substitution (see Scheme 19). The quantitative expression of aromaticity in terms of chemical reactivity is difficult and is especially complicated by the interplay of thermodynamic and kinetic factors. Nevertheless, a number of chemical techniques have been applied which are discussed elsewhere.66... 

The reactivity of polyenes is influenced by their substituents, and whether or not the multiple double bonds of the unsaturated hydrocarbon are conjugated or isolated from... 

Covering monometallic (Pd, Sn) and multimetallic (Pd-Sn, Pd-Ag) systems, several examples are presented in this chapter to illustrate the possibility offered by this chemistry to control the particle size distribution and the bimetallic interaction at a molecular level. This work is supported by a multitechnique characterization approachusing SnM6ssbauerspectroscopy,X-rayphotoelectron spectroscopy (XPS), low-energy ion spectroscopy (LEIS), and transmission electron microscopy (TEM). Catalytic performances in hydrogenation of different unsaturated hydrocarbons (phenylacetylene, butadiene) are finally discussed in order to establish structure-reactivity relationships. 

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K jK or KiK IK K depending upon whether the principal surface species are the TT-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78), but not if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. 

The regioselectivity of the nucleophilic additions on allenylidene complexes (C vs Cy) is subtly controlled by the electronic and steric properties both of the substituents on the unsaturated hydrocarbon chain and the ancillary ligands on the metal atom, as well as by the nucleophile employed. In this section we will summarize the nucleophilic reactivity of mononuclear Group 6-9 allenylidenes. 

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