Carbenium stability

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The meso carbon atom should present a carbenium structure with a low TT electron density in the ground state, in the excited state this carbon possesses the carbeniate structure (C ) with a high tt electron density (119). An electron-donating group in such a position should stabilize the ground state and rise the excited state to the highest level hypsochromic shift results as a whole. 

Aliphatic primary amines also undergo the diazotization reaction in weakly acidic solution however the resulting aliphatic diazonium ions are generally unstable, and easily decompose into nitrogen and highly reactive carbenium ions. The arenediazonium ions are stabilized by resonance with the aromatic ring ... 

The reaction is strictly intramolecular the migrating group R is never completely released from the substrate. The driving force is the formation of the more stable rearranged carbenium ion 4, that is stabilized by the hydroxy substituent. The... 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

The stability of carbocations depends on the nature of alkyl groups attached to the positive charge. The relative stability of carbenium ions is as follows [2] with tertiary ions being the most stable ... 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

The Stability of Si-H Bonds under Carbenium Ion Polymerization Conditions... 

Conclusive evidence for the stability of Si-H bond toward the benzyl carbenium ion was obtained by quantitative IR measurements. 

Having demonstrated that the Si-H bond is quite stable in the presence of Me3AI coinitiator and benzyl carbenium ion, further model experiments have been carried out to explore the stability of the Si-H bond in the presence of propagating carbenium ions. 

The IR calibration line shown in Fig. 36 may not be valid with the products formed in these series of experiments since in this case the densities and formula weight of the products may not be assumed the same as that of 7, which was used to construct the calibration line. Thus only H1 NMR spectroscopy has been employed to explore the stability of Si-H bonds in the presence of carbenium ions simulating propagating carbenium ions of isobutylene and styrene. 

Thus, in the systems under consideration, MeX may form haionium ions with growing carbenium ions. Since the stability of haionium ions depends on the polarizability of ttie halogen38 —I > —Br > —Cl, Mel should form the most stable haionium ions, le., have most pronounced poisoning effect, followed by MeBr and MeCl. Indeed, Mel may even compete for the carbocation with highly nucleophilic counterions. 

The description of reactive intermediates, which are short-lived species, is the main field of application of quantum chemical model calculations, due to the fact that the intermediates are difficult to observe and characterize. For example, the influence of structure on the stability of various carbenium ions — which have been used as models of the cationic chain end — and the delocalization of the positive charge were treated on this basis. 

All alkyl ions tested demonstrate a comparable behaviour independent of the sign of their charges. The decrease of the reaction enthalpies AH (11) with the change from the methyl to the ethyl cation (AAH (ll) = 165 kJ mol-1) and from the ethyl to the but-2-enyl cation (AAH°(11) = 117 kJ mol-1) corresponds to the increase of stability of these carbenium ions, which are expressed by the difference of their heats of formation (AAH f = —118 and AAHj = —42 kJ mol-1 90)) and of their hydride ion affinity (AHIA = 176 and 126 kJ mol-1 91)), respectively. 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

This graduation of stability is also expressed by the charges of the carbenium ion centre qc+ as well as the level of the LUMO (Fig. 7) The more stable the cation, the smaller its charge, and the higher the LUMO. 

Here the alcoholic hydroxyl is first protonated and then eliminated as water. The allylcarbenium ion (2) is initially stabilized by elimination of the proton at C-14. Then the ether link is opened after protonation of the ring oxygen with the formation of carbenium ion (3), whereby the neighboring C-C bond of the piperidine ring is cleaved with aromatization of the C ring. The carbenium ion (4) formed is stabilized by elimination of a proton and ring closure to apomorphine (5). 

Although more recent quantum chemical calculations involving long-range and environmental effects inside zeolite crystals are resulting in similar stabilities for carbenium... 

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