Carbonium ions reactions

Carbonylation, or the Koch reaction, can be represented by the same equation as for hydrocarboxylation. The catalyst is H2SO4. A mixture of C-19 dicarboxyhc acids results due to extensive isomerization of the double bond. Methyl-branched isomers are formed by rearrangement of the intermediate carbonium ions. Reaction of oleic acid with carbon monoxide at 4.6 MPa (45 atm) using 97% sulfuric acid gives an 83% yield of the C-19 dicarboxyhc acid (82). Further optimization of the reaction has been reported along with physical data of the various C-19 dibasic acids produced. The mixture of C-19 acids was found to contain approximately 25% secondary carboxyl and 75% tertiary carboxyl groups. As expected, the tertiary carboxyl was found to be very difficult to esterify (80,83). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

Although carbonylation of the 2-norbomyl ion at or below room temperature leads to exclusive formation of the 2-ea o-norbomyloxo-carbonium ion, reactions at higher temperatures have shown that the 2-cwdo-norbornyloxocarbonium ion is just as stable as the exo-isomer (Hogeveen and Roobeek, 1969). This means that at low temperatures the carbonylation is kineticaUy controlled, and at high temperatures thermodynatnically controlled. The detailed free-enthalpy diagram in... 

A certain dualism is observable in carbonium ion-carbanion chemistry, a dualism rather like that of lines and points in projective geometry. The reader may recall that interchanging the words "line and "point in a theorem of projective geometry converts it into a statement that is also a theorem, sometimes the same one. For most carbonium ion reactions a corresponding carbanion reaction is known. The dualism can be used as a method for the invention of new, or at least unobserved, carbanion reactions. The carbanionic reaction corresponding to the carbonium ion rearrangement is of course the internal nucleophilic... 

One of the most interesting parts of this publication is a report by G. Salomon of Delft on a Symposium on Carbonium Ion Reactions held at Leiden in March 1952, just a few days before our symposium. It is clear that this marks, as nearly as can be, the general... 

Boronat, M. and Corma, A. (2008) Are carbenium and carbonium ions reaction intermediates in zeolite-catalyzed reactions Sci. Dir. Appl. Catal. A,... 

Peter Hervey Given was bom in 1918. He was educated at Oxford University, receiving a B.A. in Chemistry in St. Peter s Hall, Oxford, and the M.A. and D.Phil. in the Dyson Perris Laboratory under the direction of Professors D. LI. Hammick and Sir Robert Robinson (who was the Nobel laureate in chemistry for 1947). Given s thesis research dealt with carbonium ion reactions of aromatic hydrocarbons on cracking catalysts (1-. ... 

Non-Kolbe reactions are often favoured by skeletal reaiTangements which generate a more stable carbonium ion. Reaction of the cyclic ketal 22 is driven by formation of a carbonium ion stabilised by the oxygen substituent [114]. Reactions of nor-bomanecarboxylic acids are driven by the norbomane carbonium ion rearrangement [115, 116], Oxidation of adamant-1-ylacetic acid in methanol affords 1-methoxyhomoadamantane via a skeletal rearrangement [117],... 

Trifan, D. S., and R. Bacskai The role of iron in carbonium ion reactions of ferrocene derivatives. Tetrahedron Letters [London] 1960/13, 1. 

It is subsequently interesting to enquire the degree of additivity of effects in electrophilic substitution and carbonium ion reactions of substituted five-membered rings, using aj, for 4-substituents and at for 5-substituents. As reasoned previously, the p values for such correlations compared with those for the equivalent reactions in benzene indicate the status of the o+ values for the heteroatoms. If the p values are the same... 

Dephosphorylation can also occur from the radical a to the phosphate group (62, in Scheme 12). Oxidation of this radical (62) leads to a carbonium ion (reaction 125) which, in its subsequent reactions (126 and 127), eliminates the phosphate group, affording ribo-pentodial-dose (63). Oxidation by other radicals in this system is not very effective, and is more readily brought about by Fe3+ ions [compare G(63) in NaO and N20/Fe3+ see Table V]. 

Substituted-5-methoxyhydantoins at 80°C with acids in the presence of the dienes affords satisfactory yields of Diels-Alder adduct. In certain cases products of carbonium ion reactions between the hydantoin and diene were also observed (69TL2631). One explanation for this behavior could be the free existence of (101) generated from (102). 

These results can be interpreted as being due to the reversible formation of the 3-substituted dehydrohydantoins by way of the 5-carbonium ions. Reaction of the dehydrohydantoins or the ions with dienes is slower than reaction with methanol but is irreversible. This system is useful for the preparation of certain cycloadducts of dehydrohydantoins. Its limitations result from the presence of methanol and acid, which preclude observation or isolation of the dehydrohydantoin and the utilization of most heteroatom-substituted dienes. 

The principle of the Lewis acid catalyzed rearrangements of hydrocarbons is well documented 4,81. Lewis acids react with a promotor deliberately added or present as an impurity in the reaction mixture to form carbonium ions which initiate intermolecular hydride transfers involving the hydrocarbon. These hydride transfers appear to be fairly unselective processes. While the expected tertiary > secondary > primary selectivity order is observed, the differences are significantly reduced relative to typical carbonium ion reactions. Possibly this is due to a hydride transfer mechanism which involves a pentaco-ordinate carbon transition state in which charge development on carbon would be minimized 38dh... 

The difficulty associated with the preparation of the starting materials for these carbonium ion reactions decreases their general utility for the preparation of protoadamantyl derivatives. A recently reported isomerization of the 1-adamantyloxy radical conveniently overcomes this problem. Thermolysis of 1-adamantanol hypoiodite, prepared in situ in dry benzene, gives endo-3-iodomethylbicyclo [3.3.1 ] nonane-7-one, which, when treated with base, gives 4-protoadamantanone (Eq. (22)) in an overall yield from 1-adamantanol of approximately 30 % 79,79af... 

Side-chain carbonium ion reactions Solvolysis 6. Of 1 -arylethyl-p-nitrobenzoates -6.0 1.0 x 10s 2.9 x 102 b... 

A direct comparison between 1-methylindole and 1-methylpyrrole is possible only for an a-carbonium ion reaction, the solvolysis of... 

Thus it appears that carbonium ion reactions of the Friedel-Crafts type can only proceed in ternary or three component systems, i.e., in the presence of a suitable third component commonly called the cocatalyst. Consequently, it must be assumed that either the Bronsted acid or the Lewis acid forms a primary complex with the substrate — otherwise carbonium ion reactions would become termolecular which is extremely unlikely (58). Apparently the active ion pair is formed by either a bimolec-ular reaction between "primary complexes and the third component (a, b and c) or by a unimolecular rearrangement from a ternary complex... 

Evidently, however, the use of strong acids or stable dual-acid anions is only one of the necessary requirements to initiate carbonium ion reactions in general and cationic polymerizations in particular. A specific substrate (Lewis base) which is able to accept the proton (or carbonium ion) and can be converted into a new conjugated Lewis acid is equally important. This newly formed electrophile in conjunction with the deprotonated dual-acid will be the reactive species. 

Unfortunately, from the point of view of the physical organic chemist, the mass-spectrometric approach suffers from certain intrinsic limitations. In the first place, the range of pressures accessible to the investigator is severely limited, and most of the available data refer to experiments carried out at pressures well below one torr. In the second place, the mass spectrometer detects only charged species, and the neutral molecules, which represent the final products of the carbonium-ion reactions and are of prime concern to the physical organic chemist, cannot be determined at all. Finally, since the structure of the ionic species, that are analysed exclusively according to their m/e ratio, cannot be directly deduced from mass spectra, it is difficult to discriminate isomeric ions, and to study the isomerization reactions of the carbonium ions, which play such an important role in their solution chemistry. 

Agudo et al. (1) found that the rate of oxidation of hexanes at 200°-350°C is significantly enhanced by NaX. Replacement of Na ions by Ca or Mn ions in NaX decreased activity for the oxidation of all hexane isomers, and the sequence of activity was NaX > MnX > CaX, while the cracking activity varied in the reverse order CaX > MnX > NaX. It was proposed that nonheterogeneous processes are initiated at the zeolite surface, and that the superior oxidation activity of NaX is attributed to its greater ability to initiate reactions by H-atom abstraction and to sustain free-radical reactions, whereas CaX and MnX favor carbonium-ion reactions. 

To date, crystalline zeolite catalysts have been most effective in catalyzing carbonium ion reactions such as catalytic cracking and hydrocracking. Other carbonium ion reactions such as alkylation and isomerization also are catalyzed by certain forms of zeolites. I expect to see these applications expand— provided suitable catalyst compositions are developed to allow economically viable processes. Although X- and Y-type faujasite can be used in catalytic cracking and hydrocracking, the Y-type is preferred for paraffin-olefin alkylation. Y-type faujasite is suitable for use in hydroisomerization catalysts, but synthetic mordenite is also a promising material. 

Side reactions at the azidohydrin stage 5 have not been unequivocally identified but the subsequently formed iminodiazonium ion 6 can fragment to nitriles and species derived from a carbonium ion , reaction (34), and such a mode of reacdon accounts for most of the... 

Let us now bring our list of carbonium ion reactions up to date. A carbonium ion may ... 

As with other carbonium ion reactions we have studied, it is the formation of the carbonium ion (step 2) that is the more difficult step once formed, the carbonium ion rapidly loses a hydrogen ion (step 3) to form the products. (We shall see proof of this in Sec. 11.16.)... 

For closely related reactions, a difference in rate of formation of carbonium ions is largely determined by a difference in that is, by a difference in stability of transition states. As with other carbonium ion reactions we have studied, factors that stabilize the ion by dispersing the positive charge should for the same reason stabilize the incipient carbonium ion of the transition state. Here again we expect the more stable carbonium ion to be formed more rapidly. We shall therefore concentrate on the relative stabilities of the carbonium ions. 

Also judging from the mecK ism, we might expect Friedel-Crafts alkylation to be accompanied by the kind of rearrangement that is characteristic of carbonium ion reactions (Sec. 5.22). This expectation, too, is correct. As the following examples show, alkylbenzenes containing rearranged alkyl groups are not only formed but are sometimes the sole products. In each case, we see that the... 

If the SnI reaction involves intermediate carbonium ions, we might expect it to show one of the characteristic features of carbonium ion reactions rearrange mem. In an Sn2 reaction, on the other hand, the halide ion does not leave until the nucleophilic reagent has become attached there is no free intermediate particle and hence we would expect no rearrangement. These expectations are correct. 

Problem 22.4 Racemization in certain free-radical and carbonium ion reactions has been attributed (Secs. 7.10 and 14.13) to loss of configuration in a )at intermediate. Account for the fact that the formation of alkyl carbanions, R —which are believed to be pyramidal—also lead to racemization. 

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