General Mechanisms

In general, there are two immiscible phases in reaction mixture, viz. an aqueous phase which contains a salt (a base or nucleophile) and the other an organic phase containing the substrate which is expected to react with the salt. When a phase transfer catalyst (usually contains a lipophilic cation) is added to the reaction mixture, the lipophilic cation (which has solubility in both aqueous and organic phases), exchanges anions with the excess of anions in the salt solution. 

Once nucleophile or base comes in nonpolar (organic) media, the displacement or deprotonation can take place with the product formation. The overall mechanism can be represented as follows  

In principle, a suitable PTC could transfer ions, free radicals and molecules 

The rate of phase transfer catalyzed reaction depends on the following  

When [X ] and [Nu ] are constant in the aqueous phase by keeping the aqueous phase saturated with M+X and M+Nu , the Q+Nu and Q+X in organic phase will be in constant proportion. 

With the general mechanism of olefin metathesis established by experimental work, early theoretical studies focused on the details of several of the steps outlined above. Ligand exchange to form the initial olefin complex could occur by either an associative or dissociative mechanism. Experimental evidence from Grubbs and coworkers [2] pointed to a dissociative process. The structure of the active olefin complex was also a matter of uncertainty, as both bottom-bound (trans to L) and side-bound (cis to L) complexes have been reported (Chapter 8). Finally, the detailed structure and reactivity of the metallacyclobutane have been the focus of several theoretical investigations, as this intermediate was not initially experimentally observed (Chapter 8). 

In this chapter, some important and fundamental mechanisms for ROP are discussed and illustrated by showing some selected relevant examples. In addition, some advanced designs of more specific ROP processes are briefly introduced. 

Handbook of Ring-Opening Polymerization. Edited by P. Dubois, 0. Coulembier, and J.-M. Raquez Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim iSBN 978-3-527-31953-4 

In transalkylation, one of the alkyl groups is transferred from one alkylaromatic molecule to another aromatic molecule. The mechanism of transalkylation was studied extensively in Friedel-Crafts chemistry. Though the reaction conditions are quite different from those of Friedel - Crafts catalysts, it seems quite probable that an essentially same mechanism is operative also in transalkylation with solid-acid catalysts. Thus, Kaeding et al. proposed the following mechanism for disproportionation of toluene over zeolites.  

The protonation of an alkylaromatic molecule occurs at its ipso position (eq. 1). This weakens the carbon — methyl bond and initiates transfer to a second aromatic molecule (eqs. 2 and 3). Transfer of a proton back to the zeolite from the protonated xylene gives the xylene product and regenerates the acid site in the catalyst (eq. 4). 

Systematic study of Lewis acids derived from aluminum in the de-O-benzylation of compounds bearing multiple alkyl ethers gave an insight into the mechanism. The proposed mechanism rationalizes also the regioselectivity observed in the de-O-alkylation reaction. 

Deprotection of primary silyl ethers can also be achieved using DIBAL-H [64]. 

The proposed mechanism is similar, with the necessary chelation of an aluminum derivative between two oxygen atoms. This deprotection happens chemoselectively on primary silyl ethers if secondary silyl ethers are present on the substrate. 

---

In contrast to its effect upon the general mechanism of nitration by the nitronium ion, nitrous acid catalyses the nitration of phenol, aniline, and related compounds. Some of these compounds are oxidised under the conditions of reaction and the consequent formation of more nitrous acids leads to autocatalysis. 

The kinetics of nitration of anisole in solutions of nitric acid in acetic acid were complicated, for both autocatalysis and autoretardation could be observed under suitable conditions. However, it was concluded from these results that two mechanisms of nitration were operating, namely the general mechanism involving the nitronium ion and the reaction catalysed by nitrous acid. It was not possible to isolate these mechanisms completely, although by varying the conditions either could be made dominant. 

The evidence outlined strongly suggests that nitration via nitrosation accompanies the general mechanism of nitration in these media in the reactions of very reactive compounds.i Proof that phenol, even in solutions prepared from pure nitric acid, underwent nitration by a special mechanism came from examining rates of reaction of phenol and mesi-tylene under zeroth-order conditions. The variation in the initial rates with the concentration of aromatic (fig. 5.2) shows that mesitylene (o-2-0 4 mol 1 ) reacts at the zeroth-order rate, whereas phenol is nitrated considerably faster by a process which is first order in the concentration of aromatic. It is noteworthy that in these solutions the concentration of nitrous acid was below the level of detection (< c. 5 X mol... 

The general mechanism of the rearrangement of aryl and diaryl-thiazoles seems to exclude the zwitterion route. Instead it takes place through bending of thiazoles bonds (98.213). Moreover, tricyclic sul-fonium cation intermediates, after irradiation of deuterated phenyl-thiazoles, have been suggested by several workers (98). 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

The scope of electrophilic aromatic substitution is quite large both the aromatic com pound and the electrophilic reagent are capable of wide variation Indeed it is this breadth of scope that makes electrophilic aromatic substitution so important Elec trophilic aromatic substitution is the method by which substituted derivatives of benzene are prepared We can gam a feeling for these reactions by examining a few typical exam pies m which benzene is the substrate These examples are listed m Table 12 1 and each will be discussed m more detail m Sections 12 3 through 12 7 First however let us look at the general mechanism of electrophilic aromatic substitution... 

Recall from Chapter 6 the general mechanism for electrophilic addition to alkenes... 

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

Now that we ve outlined the general mechanism for electrophilic aromatic substitution we need only identify the specific electrophile m the nitration of benzene to have a fairly clear idea of how the reaction occurs... 

Figure 12 3 adapts the general mechanism of electrophilic aromatic substitution to the nitration of benzene The first step is rate determining m it benzene reacts with nitro mum ion to give the cyclohexadienyl cation intermediate In the second step the aro maticity of the ring is restored by loss of a proton from the cyclohexadienyl cation... 

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

On the basis of the general mechanism for acid catalyzed ester hydrolysis shown in Figure 20 4 write an analogous sequence of steps for the spe cific case of ethyl benzoate hydrolysis... 

On the basis of the general mechanism for basic ester hydro ... 

Basing your answers on the general mechanism for the first stage of acid catalyzed acetal hydrolysis... 

Hydrazine cleaves amide bonds to form acylhydrazides according to the general mechanism of nucleophilic acyl substitution discussed in Chapter 20... 

Film Rupture. Another general mechanism by which foams evolve is the coalescence of neighboring bubbles via film mpture. This occurs if the nature of the surface-active components is such that the repulsive interactions and Marangoni flows are not sufficient to keep neighboring bubbles apart. Bubble coalescence can become more frequent as the foam drains and there is less Hquid to separate neighbors. Long-Hved foams can be easHy... 

Mechanism. The general mechanism of effective sizing involves the foUowing sequential steps. (/) Efficient retention of the sizing agent in the... 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

Antibiotics have a wide diversity of chemical stmctures and range ia molecular weight from neat 100 to over 13,000. Most of the antibiotics fall iato broad stmcture families. Because of the wide diversity and complexity of chemical stmctures, a chemical classification scheme for all antibiotics has been difficult. The most comprehensive scheme may be found ia reference 12. Another method of classifyiag antibiotics is by mechanism of action (5). However, the modes of action of many antibiotics are stiU unknown and some have mixed modes of action. Usually within a stmcture family, the general mechanism of action is the same. For example, of the 3-lactams having antibacterial activity, all appear to inhibit bacterial cell wall biosynthesis. 

The hybridic nature of the Si—H bond is utili2ed to generate C—H bonds by ionic hydrogenation according to the foUowiag general mechanism, ia which a hydride is transferred to a carbocation. 

Aromatic Compounds. The accepted general mechanism (38—40,51) for the reaction of an aromatic compound with sulfur trioxide involves an activated intermediate as shown in equation 1. 

Mechanisms for Formation and Hydrolysis of Finishes. The general mechanism for acid-cataly2ed formation and hydrolysis of /V-methy1o1 cellulose cross-links has been shown to pass through a catbonium ion intermediate as in equations 4 and 5 (41) ... 

Ethylene Dichloride Pyrolysis to Vinyl Chloride. Thermal pyrolysis or cracking of EDC to vinyl chloride and HCl occurs as a homogenous, first-order, free-radical chain reaction. The accepted general mechanism involves the four steps shown in equations 10—13 ... 

The organic photochromic systems that have been studied are numerous and it is helpful to classify them into a few categories by way of the general mechanism of the photochromic reaction in each category. 

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

Impact torque. A function of system looseness or backlash. Generally, mechanical-joint flexible couplings have inherent backlash. 

The three-dimensional structure of HLA-B27 at 2.1 A resolution suggests a general mechanism for tight peptide binding to MHC. Cell 70 1035-1048, 1992. 

Several limiting generalized mechanisms can be described for polar additions ... 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

---