Covalent precursors

To assess, at least qualitatively, how much of the observed shift in the triphenylcarbonium ion is due to the change of hybridization from to sp and how much to the effect of the positive charge, a comparison of the chemical shifts of the triphenyl-C -carbonium and trimethyl-C -carbonium ions with their parent sp -hybridized covalent precursors and with some C -compounds having p -hybridization is useful. Data of Table 9, indicate that the C -shifts of ap -hybridized compounds (olefins and aromatic hydrocarbons), at least in the molecules studied (Lauterbur, 1957,1962), are very similar and fairly independent of the nature of the molecules. 

The H NMR spectra also indicate that the methylene protons in the isobu-toxy group are equivalent in the carbenium ion [11], in contrast to their magnetic inequivalence in the covalent precursor. That is, addition of Lewis acid to the alkyl chloride leads to rapid ionization as shown by one broad signal at 3.6 ppm for the methylene group. This resolves into a doublet if ionization is sufficiently fast. Two AB doublets are observed if exchange is slow. Note that the methylene carbon atom and carbocationic center are connected through an oxygen atom in the Newman projection shown in Eq. (5). 

The overall consumption rate of the covalent precursor (ks) is determined by HPLC and/or titration measurements this correlates with monomer consumption in propagation. The rate of racemization of optically active 1-phenylethyl chloride (ka) is determined by polarimetric measurements, Racemization is usually faster than solvolysis, confirming that activation is reversible and that internal return may occur before the carbenium ion reacts with an external nucleophile, Racemization requires not only that the C—Cl bond of the covalent precursor is broken, but that the lifetime of the ion pair is long enough for the flat carbenium ion to rotate, such that both sides of the carbenium ion are completely equivalent as shown in Eq. 18. 

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

In physical organic chemistry, kinetics has been used successfully to distinguish between Sn 1 and Sn2 mechanisms. When ion formation is the rate-determining step, the reaction rate does not depend on the concentration of other reagents such as solvent or monomer. Second-order kinetics does not necessarily mean that a direct bimolecular reaction (e.g., Sn2) takes place between the dormant species, D, and monomer, M. If the covalent precursor, D, is in dynamic equilibrium with the carbenium ion, C, and only the latter reacts with M to give the product P, then the overall kinetics depends on the ratio of the rate constants k i and k2 ... 

The icHiic chain carriers produced in the initiation process are composed of edes pos ssing different degrees of association in equilibrium among fliemselves. If one considers that covalent precursors or products are also present in tiiese equilibria, wdiether or not they participate in the propagation, the number of intermediates to be reckoned with is further increased. Two types of equivalent sequences can be written, according to the nature of the anion, B representing the counterion derived frcan a Br nsted acid or MtX +i that were formed from a Lewis acid ... 

It IS well known that adamantane or its 1-halo derivatives, for example 50, can be easily transformed into a stable tertiary 1-adamantyl cation 26. As was already mentioned vide supra), this cation owes its increased stability to the unique stabilization effects involving the participation of the remote centers in the charge delocalization. The pattern of NMR spectra may be used as a sensitive probe to the charge delocalization effects. Thus for the series of aliphatic tertiary carbenium ions, the presence of the positive charge induces a downfield shift of the H NMR signals (relative to those of the parent covalent precursor) at the adjacent centers. The magnitude of this effect decreases in the order S> y In the case of 26, a substantial downfield shift of -protons is also observed, but this effect is pronounced even stronger for the y-protons... 

Initiatioh with Stable Carbenium or Onium Ions and with Their Covalent Precursors... 

Initiation with Stable Carbenium or Onium Ions and with Their Covalent Precursors 3.2.5 l.B-EHoxolan-Z ylium (Dioxoletiium) Salts... 

Stable Carbenion and Onium Ions and Their Covalent Precursors. The most representative initiators from this class are the following ... 

The transition state structure for the covalent precursor/cage pair inter-conversion Figure 1) is thus determined by the... 

Figure 1 is quite simple but, to our knowledge, no determination of the individual values of the activation parameters for the k(, and kd processes have previously been available. One of the primary purposes of the present work is to discuss an analysis that yields such values. These activation parameters, in turn, are used to illustrate the curvatures that exist in Eyring or Arrhenius treatments of the temperature dependences of the observed rate constants for free radical recombination, trapping and formation by thermolysis of a covalent precursor in solution. 

Because of their stability and ease of generation, triarylmethyl cations have been the subject of numerous quantitative studies aimed at determining the effects of structure on carbonium ion stability. Most of these studies have utilized ultraviolet spectroscopy as the probe and have taken advantage of the difference in electronic spectra between the carbonium ion and a covalent precursor, usually the corresponding triarylcarbinol. This permits determination of the equilibrium constant for the... 

There are rather few methods within this category, as only low-boiling-point covalent precursors can be used. 

One typical example of the occurrence of halogen braiding in a transient species is the bromination of alkenes its textbook description involves a non-covalent precursor complex 10, in which the Br-Br - alkene angle is linear (Scheme 4). Subsequently, the Br-Br bond is broken, and a bromonium intermediate 11 is created. The latter process could also be described as a debromination of the strong halogen-bond donor dibromine by the alkene, or as an X-philic [69] SN2-type reaction at bromine. 

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