Carbenium ions esters

Another potential mechanistic complication is capture of the intermediate carbenium ion by the conjugate base of the acid. When CF3C02H is used as the acid, this would lead to trifluoroacetate esters. Kursanov et al. showed that, under the reaction conditions for ionic hydrogenations, trifluoroacetate esters can be converted to the hydrocarbon product (Eq. (3)). 

The intermediacy of the trifluoroacetate ester does not undermine the efficacy of the overall hydrogenation reaction, since the ionizing solvent CF3C02H converts the ester back to the carbenium ion under the reaction conditions, resulting in its ultimate conversion to the hydrogenation product. 

The alkylation reaction is initiated by the activation of the alkene. With liquid acids, the alkene forms the corresponding ester. This reaction follows Markovnikov s rule, so that the acid is added to the most highly substituted carbon atom. With H2S04, mono- and di-alkyl sulfates are produced, and with HF alkyl fluorides are produced. Triflic acid (CF3S020H) behaves in the same way and forms alkyl triflates (24). These esters are stable at low temperatures and low acid/hydrocarbon ratios. With a large excess of acid, the esters may also be stabilized in the form of free carbenium ions and anions (Reaction (1)). 

The esters differ from each other in stability. To decompose the isopropyl ester, higher temperatures and higher acid strengths are needed than for decomposition of the s-butyl ester. It is claimed that the resulting carbenium ions are stabilized by solvation through the acid (25-27). Branched alkenes do not form esters. It is believed that they are easily protonated and polymerized (28). 

Once the tertiary cations have been formed, they can undergo electrophilic addition to alkene molecules (Reaction (4)). The addition is exothermic and contributes most of all the reaction steps to the overall heat of reaction. It has been proposed (24) that instead of the alkenes, the corresponding esters are added to the carbenium ions, restoring the acid in this way (Reaction (5)). The products of both potential steps are the same. 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

Some explanation is required why my title involves the adjective cationoid instead of the traditional cationic . As most of those familiar with the subject have known for some years, I use this term to include both the cationic polymerisations by carbenium ions and also those polymerisations in which an alkene is inserted into the strongly polarised covalent bond of an ester, the cationoid insertions I have seen no convincing reason for changing my well known opinion that these are different types of reactions, and that a clear distinction between them is heuristically useful. 

Here we consider only the inactivation 2(b) of a carbenium ion by neutralisation. In the present context neutralisation can occur in two ways. The first and simplest is a combination of the carbenium ion with the anion to form an ester, i.e., a compound in which the two previously charged species become linked by a covalent bond ... 

However, there are also many systems in which the evidence indicates that the propagating species cannot be a carbenium ion. Such reactions have been termed pseudo-cationic and in these polymerisations the propagating species is believed to be an ester. The most thoroughly investigated systems comprise aromatic monomers (styrene, acenaphthylene [11]) and protonic acids (HC104) or iodine [11] as initiators. The simplest representation of the propagation is as the addition of the ester (stabilised by four styrene molecules) across the double-bond of the monomer [12] ... 

If the monomer molecules are fairly firmly attached to the growing end, as are the four styrene molecules which stabilise the propagating polystyrylperchlorate ester, then propagation remains of first order with respect to monomer the depletion of the monomer causes the ester to ionise near the end of the reaction, and the resulting carbenium ion then consumes the residual monomer rapidly [21, 22]. 

A further prejudice that inhibited the acceptance of the F-Cat mechanism was the belief that esters, admitted to exist, but called dormant species , were in equilibrium with conventional propagating carbenium ions. This belief persisted despite the fact that no... 

There has been considerable argument over the mechanism of the polymerisation of styrene by perchloric acid in halogenated solvents in the temperature range ca. 30 °C to ca. -20 °C [1-11]. By spectroscopic and conductance measurements, Gandini and Plesch [1] could not detect any carbenium ions during the polymerisation this and other evidence led them to conclude that the dominant propagating species is the ester, polystyryl perchlorate (I), which is stabilised by an excess of styrene. 

We conclude that the polymerisation of styrene by perchloric acid in methylene dichloride at 0 °C involves at least two types of independent propagating species. Rate studies and conductivity measurements, in conjunction with an independent study of molecular weight distributions [26, 27], indicate that these species include the perchlorate ester and the free polystyryl carbenium ion and that the term pseudocationic is a very appropriate description of what is a distinctive form of organic reaction. 

We wish to emphasise that the formation of esters (E) from alkenes (M) and acids (HA), the catalysis of the reactions of E by HA or MtXn, and the activation of E, such as organic chlorides, by the co-ordination of a Lewis acid, such as A1C13, are all very familiar chapters in conventional organic chemistry. It follows that the pseudo-cationic theory is nothing more than a generalisation of conventional organic-chemical ideas and a revival of some pre-Whitmore interpretations which had become occulted by the usefulness and novelty of the carbenium ion concept. 

Explanation The ester polystyryl perchlorate is stabilised by M, but it decomposes slowly to Pn4. In the moderately pure system the [Pn+] are consumed by impurities, mainly water, and only when depletion of M leads to fast decomposition of E are enough Pn+ formed to give colour and conductivity. In the very pure system the scavenging of water, etc., by the ions is completed before all the M has been consumed, so that the Pn+ formed thereafter contribute to the rate. At the end of a typical polymerisation of this type the [Pn+] is ca. 10"7 mol l"1. If [H20] > [HClO4]0, the k1 is unaffected because the rate of reaction of E with H20 in CH2C12 is much smaller than the rate of polymerisation, but the Pn+ and/or the HC104 are hydrated so that no colour or conductivity appears. The visible and conducting ions are not polystyryl carbenium ions, but a cocktail of others in which the substituted indanyl ion is the most abundant [28]. 

Explanation The increase of both Y and DP with time implies a growing species of long life, which is characteristic of esters but not of carbenium ions, and an absence of transfer reactions. The suppression of all kinds of transfer reactions seems to require some very special features in the anionoid moiety of the ester which are not yet understood fully but which, by hypothesis, cannot affect the reaction pattern of an isolated, unpaired cation. The fact that living polymerisations can occur in toluene is a convincing demonstration that these reactions cannot involve carbenium ions, because growing cations alkylate toluene [33-37], a process which produces low DPs, independent of Y. 

Explanation During the polymerisation two types of growing chain-ends coexisted, which are not rapidly interconvertible. These are the ester and the carbenium ion. The E forms the low polymer, the carbenium ions the high polymer. 

The second version of ions-at-any-price is that the active species are the normal carbenium ions, but that they are present in very low concentrations and are in equilibrium with the ester which is regarded as inactive and inactivatable [29, 48]. 

The Commentator s arguments can be answered summarily by scrutinising their basis, which is, in his own words, minute amounts of reactive carbenium ions being in equilibrium with covalent esters . The Commentator has produced no evidence of any kind for the existence of an equilibrium between ions and esters in the systems of interest here. 

To 2.2.8 the Commentator s second sentence is not true. In an aromatic system p-X-C6-H4-CH=CH2, any changes in X which make the double bond more reactive than it is with X=H, will have the same effect on the reactivity of any derived covalent species, such as an ester, but it will have the opposite effect on the reactivity of the derived carbenium ion p-X-C6H4-C+HCH3. This is one of the most cogent arguments against the Commentator s views. 

Pseudo-cationic polymerisations are reactions in which an alkene is inserted between the positive carbon atom and the negative heteroatom of a polar bond at the growing end of a polymer chain, without the formation of a carbenium ion they do not differ essentially from the well-known additions of esters to alkenes. The theory of these reactions was devised by Gandini and Plesch [2] and has been brought up to date by Plesch [3]. 

In some of their publications Higashimura s group, and others using the same terminology, are close to our view when they write about the modifiers reducing the reactivity of the carbocation . However, since in our view there is no carbenium ion to be stabilised, we see these donors as reducing the polarity of the ester bond and the reactivity of the 0-protons, and they obstruct physically the transfer of a P-proton to the monomer or to any other base. 

FIGURE 3.8 Deprotection of carboxyl groups by acid-catalyzed hydrolysis (A) of amides and (B) of esters. Protonation generates a relatively stable carbenium ion that usually requires heat to fragment it. 

---