Hydrocarbon structures stability-reactivity

The intramolecular 1,2-H shifts of alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example the ku values of Me2CHCCl, PhCHMeCCl, and EtCCl exceed 108 s 1 in hydrocarbon solvents at 25°C (Table 4).60 86 87 However, due to the stabilizing effect of the oxa spectator substituent, acetoxycarbenes react at much reduced rates relative to their chlorocarbene analogues,90,91 thus providing kinetically accessible results for a wide array of bystander-substituted alkylacetoxycarbenes.81 92... 

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... 

Ever since the discovery of "d " alkylidene complexes, we have been looking for complexes that contain a heteroatom directly bound to the alkylidene a carbon atom, a situation that appears to be necessary to ensure the stability of many "low oxidation state" carbene complexes. We have now prepared several examples. So far we can say that such species do not appear to have structures or reactivities that clearly set them apart from their hydrocarbon analogs, with the notable exception of possible bonding of the heteroatom to the metal in an anti rotamer of the Re=CHSPh complex. It is interesting to note that we have not yet found any interpretable reactions of complexes of the type... 

Hiickel s rale of aromaticity states that monocyclic conjugated hydrocarbons with 4m + 2 Jt-electrons are aromatic, whereas systems with 4m ti-electrons are antiaromatic. Consequently, the number of t-electrons is crucial to determine stability, structure, and reactivity of aromatic and antiaromatic systems. In 1954, Doering and... 

Polyisobutene is non-crystalline when unstretched and is therefore soluble at room temperature in hydrocarbons and halogenated hydrocarbons. The material is resistant to most acids, alkalis and aqueous solutions, as would be expected from its saturated hydrocarbon structure and absence of tertiary hydrogen atoms. The lack of tertiary hydrogen atoms renders polyisobutene more resistant to oxidation than polypropylene also, the less numerous and partially shielded methylene groups in polyisobutene are less reactive than those in polyethylene. However, polyisobutene is rather susceptible to thermal degradation since chain scission is favoured by the greater stability of the resultant tertiary free radical ... 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic. Cycloheptatrienyl cation, although aromatic, is still a carbocation and reasonably reactive toward nucleophiles. Its special stability does not imply a rock-like passivity, but rather a much greater ease of formation than expected on the basis of the Lewis structure drawn for it. A number of observations indicate that cycloheptatrienyl cation is far-more stable than most other car bocations. To emphasize its aromatic nature, chemists often write the structure of cycloheptatrienyl cation in the Robinson circle-in-a-ring style. 

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