Carbonium ions definition

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

The chemical behavior of ions, ion pairs, and polarizable molecules partakes of the same indistinctness as the definitions of these species. Any attempt to make a complete catalog of the reactions of ions will almost certainly include borderline reactions whose intermediates are in fact ion-pairs or even covalent molecules. For many purposes the identification of a reaction as carbonium ion-like, or what the Germans would call Krypto-ionenreaktion, is as useful as the certain knowledge that the intermediate is actually a carbonium ion. Many of the ionic reaction mechanisms in the literature do not represent actual free ions and were not so intended by their authors. The ionic representation is often merely a convenient simplification if it is an oversimplification it is one that is easily rectified when the pertinent data become available. The value of such approximate mechanisms is that... 

It is appropriate to open a discussion of carbonium ions with the evidence from conductivity work since it is this property that is most closely related to the definition of an ion. Other methods for detecting or measuring the concentration of ions are valuable largely to the extent that their results can be related to the results of conductivity experiments. 

A carbocationic species in which there is at least one pentavalent carbon atom (e.g., CHs ). 2. Traditional name for chemical species that are now referred to as carbenium ions. Considerable confusion exists in the literature with this term for carbocations. The -onium suffix usually refers to a higher covalency when compared to the neutral atom thus, CH5+ would be a true carbonium ion (in terms of the first definition). Additional ambiguity results when the term ethyl carbonium ion is used to describe both CH3CH2 and to R—CH2CH2. For these reasons, the terms carbocation or carbenium ion are now preferred. 

The first successful polymerizations were obtained in solution, with cationic initiators, under conditions typical of ring-opening polymerization. Bredereck and Hutten polymerized the perbenzyl ether and peracetate of levoglucosan, using, as initiators, various carbonium ions formed in situ from organic halides and silver perchlorates. The products were apparently not stereoregular, but were definitely poly-... 

Dowden (27) considers the active centers for carbonium ion formation to be associated with surface cation vacancies. A proton, derived from water contained in the catalyst, is attracted to the anions surrounding the vacancy. A hydrocarbon molecule is assumed to be held by polarization forces above this lattice defect and the proton will be distributed between the hydrocarbon and the anions, forming a carbonium ion of a definite lifetime. 

Zeolites are crystalline aluminosilicates that have exhibited catalytic activities ranging from one to four orders of magnitude greater than amorphous aluminosilicates for reactions involving carbonium ion mechanisms such as catalytic cracking (144). As a result extensive efforts have been undertaken to understand the nature of the catalytic sites that are responsible for the observed high activity. The crystalline nature of zeolites permits more definite characterization of the catalyst than is possible for amorphous acidic supports such as alumina and silica-alumina. Spectral techniques, in conjunction with structural information derived from X-ray diffraction studies, have led to at least a partial understanding of the nature of the acidic sites in the zeolite framework. 

There is quite a body of evidence to indicate the possibility of acid catalysis and carbonium ion formation. The experiments with nuclear-substituted benzyl alcohols referred to previously (p. 396f) definitely demonstrate that the effect of the substituent, x, on the rate of the reaction ... 

The scope of this review is a detailed survey of reactions proceeding through vinyl cations and an attempt of a systematic definition of the properties of these intermediates with reference to those of saturated carbonium ions. Although attention will be particularly devoted to linear cations, bridged unsaturated species will be considered as alternative structures of vinyl cations rather than as a distinct type of reactive intermediates. The 77--complex terminology (Dewar, 1949) widely abused in the past decades to indicate especially cyclic cations and recently reassessed by Banthorpe (1970) will be generally avoided. The most recent Btudies not covered by published reviews on the subject (Rappoport, 1969 Richey and Richey, 1970 Richey, 1970 Hanack, 1970) are discussed in greater detail than others and data are collected in pertinent Tables. 

It might be supposed that stereochemical evidence would be rather definite in distinguishing from Sn2 reaction paths. In /SVl a stable carbonium ion is formed as an intermediate following ionization [Eq. (XVI.1.2)]. If the carbon center is originally asymmetric and the starting compound, RX, is optically active, then the product, R Y, would be racemic. This turns out to be true in many cases. However, when the atom adjacent... 

Unfortunately these results are not definitive since they are also consistent with heterolytic splitting of the palladium(II)-carbon bond to give an ethyl carbonium ion, which in the absence of a nucleophile... 

The characteristic feature of a carbonium ion is, by definition, the electron-deficient carbon and the attendant positive charge. The relative stability of a carbonium ion is determined chiefly by how well it accommodates that charge. 

The amount of energy required to remove an electron from a molecule or atom is called the ionization potential. (It is really the ionization energy.) The ionization potential of a free radical is, by definition, the A// for the conversion of the radical into a carbonium ion ... 

The hydroxide anion may act as a base when it reacts with a proton to form a water molecule. This is a typical acid/base reaction under the Bronsted-Lowry definition. However, it may also act as a nucleophile when it is attracted towards a positive carbon centre. There is, however, an important difference between the positive centre of the proton and that of the carbonium ion. The difference lies in their relative charge densities has the proton or the carbonium ion the greater charge density ... 

On a silica-alumina catalyst poisoned by the exchange of the protonic sites for Na+ the 420 band is eliminated, but the 330 m/x. one remains. This definitely disproved the assignment of the latter to the carbonium ion 92). It is more likely that just this band does belong to the charge-transfer optical transition sought for. In fact it is situated in the same spectral range where charge-transfer bands of a variety of intermolecular and 7r-complexes are known to lie. 

However, two compounds which are potentially within this definition have been prepared and their spectra obtained (80 and 81, see Table 16). The 7-norbomadienyl cation, 80 220,227), definitely prefers not to avail itself of bicycloaromatic stabilization of the bridge carbonium ion by "both double bonds, as the expected C211 symmetry is ruled out by the n.m.r. spectrum which in fact resembles that of the 7-norbomenyl cation, 71. Thus, bishomoconjugation only is observed, and the barrier to bridge flipping is estimated to be >19.6 kcal/mole 228). 

I+. For the definition of carbocations, including differentiation of tetra- and pentacoordinated carbonium ions frcm trivalent carbenium ions, see Olah, G.A., J. Amer. Chem. Soc. (1972)... 

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