Carbonium ions carbonyl

Hydroxypyrroles. Pyrroles with nitrogen-substituted side chains containing hydroxyl groups are best prepared by the Paal-Knorr cyclization. Pyrroles with hydroxyl groups on carbon side chains can be made by reduction of the appropriate carbonyl compound with hydrides, by Grignard synthesis, or by iasertion of ethylene oxide or formaldehyde. For example, pyrrole plus formaldehyde gives 2-hydroxymethylpyrrole [27472-36-2] (24). The hydroxymethylpyrroles do not act as normal primary alcohols because of resonance stabilization of carbonium ions formed by loss of water. 

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). 

The two main reasons for studying the reversible reaction (3) were (a) to complete the picture of the Koch reaction in terms of quantitative information and (b) to set up a scale of reactivity towards a neutral nucleophile for carbonium ions of different structure. The first item is important from a practical point of view because there are reactions competing with the carbonylation step (3), which can be divided into intramolecular and intermolecular processes. Rearrangement of the intermediate alkylcarbonium ion, e.g. 

In previous studies (Hogeveen, 1970) the reactivity of long-lived carbonium ions towards molecular hydrogen has been investigated and interesting differences between secondary and tertiary alkyl cations have been observed. Tliis reaction is too slow, however, to be extended to other types of carbonium ions. The reactivity of carbonium ions towards carbon monoxide is much higher (about six powers of ten) than towards molecular hydrogen, which enabled us to determine not only the rate of reaction (3) for some tertiary and secondary alkyl cations, but also the rate of carbonylation of more stabilized carbonium ions. 

As mentioned in the Introduction, rearrangements of the intermediate alkyl cation in the Koch synthesis may compete with the carbonylation. Under the kinetically controlled conditions prevailing in the Koch synthesis of carboxylic acids, the rearrangements occur only from a less stable to a more stable carbonium ion, e.g. from a secondary to a tertiary ion. The reverse rearrangements—from a more stable to a less stable... 

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... 

For the determination of stabilizations of carbonium ions the equilibrium constants of carbonylation-decarbonylation have been used in previous Sections. For the ions discussed in this Seetion, however, the rate constants of decarbonylation are not known and, therefore, the rate constants of carbonylation will be used as a criterion for such stabilizations. This kinetic criterion is a useful indicator if there are no significant steric factors in the carbonylation step and if this step is indeed rate-determining in the overall process (Hogeveen and Gaasbeek, 1970). The following rate constants in Table 2 are of particular importance. 

The study of the carbonylation of carbonium ions, as summarized in this Review, has afforded valuable information on a number of problems. 

First, the rates of carbonylation of secondary and tertiary alkyl carbonium ions can now be compared quantitatively with the known rates of competing intramolecular rearrangements of these ions. The product distribution in the Koch synthesis of carboxylic acids depends, amongst other things, on these relative rates. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

The enone function undergoes very fast intermolecular radical addition reactions and will trap the intermediate from oxidation of the carboxylate group. This step leads to a radical centre adjacent to the electron withdrawing carbonyl function of (he original enone and this centre is not readily oxidised to the carbonium ion. Dimerization to a diketone is thus the final stage in the reaction [108]. 

The first step corresponds to a normal carbonyl addition, as is also observed with aliphatic aldehydes but here the equilibrium does not lie so far to the right. A strongly acidic medium is necessary for the next stage, the formation of a carbonium ion. For example, it is found that PH3 only reacts with acetone when the solution is more than 8-molar in hydrochloric acid. 

It seems safe to say that coordination will generally decrease the reactivity of donor atoms in the first row of the periodic table through steric effects. With some reactions the extent of this steric hindrance may be small. Ammonia can be transformed into chloramines when coordinated (34), and aromatic acid chlorides coordinated to A1C13 or TiCl4 may be esterified even when the functional group is a hindered one, as in mesitylene carbonyl chloride (47). These last reactions may proceed through a very reactive carbonium ion, whose existence is rendered possible by the polarization of the ligand which it suffers as a result of coordination. 

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