Cyclic hydrocarbons, reactivities

Although the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the n orbitals of both linear and cyclic conjugated polyene systems that it provides are correct as to symmetry and the relative energy of the orbitals. In many reactions where the n system is the primary site of reactivity, these orbitals correctly describe the behavior of the systems. For that reason, the reader should develop a familiarity with the qualitative description of the n orbitals of typical linear polyenes and conjugated cyclic hydrocarbons. These orbitals will be the basis for further discussion in Chapters 9 and 11. 

The nature of dangerous reactions involving organic chemicals depends on the saturated, unsaturated or aromatic structures of a particular compound. Saturated hydrocarbons are hardly reactive, especially when they are linear. Branched or cyclic hydrocarbons (especially polycyclic condensed ones) are more reactive, in particular as with oxidation reactions. With ethylenic or acetylenic unsaturated compounds, the products are endothermic . 

The simplest of the saturated cyclic hydrocarbons, or cycloalkanes, is cyclopropane, C3Hg, the molecules of which are made up of three carbon atoms to each of which two hydrogen atoms are attached. Cyclopropane is somewhat more reactive than the corresponding open-chain alkane, propane, C3Hg. Other cycloalkanes make up a part of ordinary gasoline. 

Cycloheptane is about the largest ring in which the participation of the whole molecule can be expected in catalystic reactions. This molecule exhibits practically all the reactions described for cyclic hydrocarbons. The ring opening of methylcycloheptane is also selective, but total reactivity is much less than that of methylcyclopentane (702) ... 

Relative Reactivities of Cyclic Hydrocarbons with 4-7 Membered Rings" (78)... 

Let us consider the origins of benzene s aromatic stabilization. Another cyclic hydrocarbon, cyclooctatetraene (pronounced cyclo-octa-tetra-ene), certainly looks conjugated according to our criteria, but chemical evidence shows that it is very much more reactive than benzene, and does not undergo the same types of reaction. It does not possess the enhanced aromatic stability characteristic of benzene. 

Fused-ring cyclic hydrocarbons such as naphthalene and anthracene display the enhanced stability and reactivity associated with simple aromatic... 

Yoshimuzi 17>, using this method with different values for m, calculated the electronic distribution produced by substituting a heteroatom for hydrogen. He found that a value of the m parameter such that ma = 0.12 was necessary in order to reproduce the dipole moments of a set of linear paraffins. Fukui et al. 18>, using the positive square root of 0.12, i.e. m— +0.35, were able to correlate the ionization potentials, heats of formation, and bond energies in linear as well as cyclic hydrocarbons and their derivatives. It was also shown that the method permits a coherent interpretation of inductive effects to be made so that a relation exists between some calculated values and the reactivity. 

With the initial synthesis of cyclopropane in 1882 , and the report of its thermal structural isomerization to propene in 1896, this simplest of cyclic hydrocarbons began its extraordinarily fruitful stimulation of fresh insights on fundamental problems in organic chemistry, ranging from basic concepts of ring strain and structural isomerism to questions of thermochemistry and reactivity and of a aromaticity And from the beginning there was controversy, extending a few years before suitably authoritative commentators confirmed the fact that cyclopropane is indeed converted thermally to propene. ... 

Aromaticity is the simplest way to explain the stability of unsaturated cyclic hydrocarbons with (4n + 2) electrons delocalized in the rr-orbitals perpendicular to the ring plane.1 Even though the introduction of the aromaticity concept in chemistry is quite old, its definition is still controversial. It is not surprising to find many attempts to define this term depending on different approaches to describe the electronic structure. In view of these problems of subjectivity, it is remarkable that aromaticity is useful to rationalize and understand the structure and reactivity of many organic molecules. As a result, the concept of aromaticity is truly a cornerstone in organic chemistry. In 1971, Wade proposed a similar concept to describe delocalized cr-bonding in closed-shell boron deltahedra.2-4 However, stability based on aromaticity had not been confirmed for any metallic moiety until Li et al. published their seminal paper entitled Observation of all-metal aromatic molecules, 5... 

However, as far as we know, DFT reactivity and stability descriptors, such as the absolute or global hardness (17), have not been employed to describe or assess the aromaticity and its trends in these all-metal aromatic clusters. This is important to be done since hardness has been an excellent descriptor of traditional aromaticy in organic cyclic hydrocarbons,33 and one may wonder whether this descriptor is useful to reproduce this extended concept of all-metal aromaticity. ... 

Table 18.1 Cyclic hydrocarbons as organic liquid carriers data on weight and volume hydrogen capacity, reactivity, toxicity, liquidity and economic availability for hydrogen delivery infrastructure... 

Redistribution of the electron density in perfluorinated heterocycles leads to the signihcant increase in the basicity of the fluorine substituent, resulting in an unusual reactivity toward strong electrophiles. This effect makes possible reactions, which are virtually unknown for cyclic hydrocarbons, such as reactions of perfluorinated ethers or amines with strong Lewis (AICI3, SbFs, SO3) or protic acids. Short summaries on these reactions can be found in two reviews... 

Unsaturated aliphatic and cyclic hydrocarbons have numerous biological sources. Ethylene, H2C = CH2, the simplest of the series, is emitted by green plants in substantial quantities. It has hormonal activity and has been implicated in the control of many physiological processes in plants. The natural tropospheric concentration of ethylene is very low due to its high reactivity with ozone, -OH, and other atmospheric oxidants (Robinson and Robbins, 1968), but in polluted atmospheres its concentrations can be much higher. It is a product of combustion of wood, coal, oil, natural gas, and petroleum. Elevated levels of ethylene, such as may occur in homes where coal gas is used for cooking, can be very deleterious to plants. 

The above reaction incorporating D is obviously restricted to those polycyclic hydrocarbons that react in the Co2(CO)g catalytic system but not all poly-cyclics are reactive e.g., phenanthrene reacts only very slowly under these conditions (65). Quite a few hydrocarbons have been investigated. When they react they generally seem to give only one product (62) making it possible to have specific positions labelled on specific hydrocarbons. This is obviously not a general method for preparing deuterated aromatic compounds. 

The relatively higher reactivity of the smaller cyclic hydrocarbons can be justified on the basis of their increased strain energy (Chapter 4). 

Ozonolyses of various dienes containing an enol carbonate afford products in which the isolated double bonds rather than the enol ones are cleaved. Rate measurements for ozonolysis of a variety of cyclohexene derivatives indicate that the enol carbonate function has a very low reactivity towards ozone. NMR and computational data indicate that the enol carbonate is not particularly electron deficient in its double bond. It is therefore suggested that the retardation is instead caused by ozone association with the carbonate group. The steric and electronic effects of alkyl and aryl substituents on the rate of ozonation of cyclic hydrocarbons have been studied. ... 

Addition of the strong CH bond of cyclopropane (106 Kcal/mole) is favoured over insertion into the relatively weak C—C bond this result rules out hydrogen abstraction reaction of radicals which favour CH bond with low bond energies. In fact, cyclopropane is the cyclic hydrocarbon with the highest reactivity towards CH insertion (Figure 1). 

In subsequent efforts, we succeeded in introducing significantly improved variants of the Minisci reaction. Pyridine-N-oxide proved to be reactive enough to undergo radical allqrlation with cyclic hydrocarbons even in the absence of an activator (Scheme 1.20). ... 

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

---