The unimolecular mechanism

In principle, a planar carbonium ion should lose a beta proton from either side of the plane with equal facility and consequently there should be no stereoselectivity in El reactions. This simple interpretation can be distorted, however, in solvents which do not favour ionic dissociation, or if neighbouring group participation in the ionisation is afforded by a beta hydrogen or another group. 

The ratio of substitution to elimination products in the solvolysis in 80% aqueous ethanol of a series of r-butyl and r-amyl substrates is little influenced by a change of leaving group from Cl to Br, 1 and A similar in- 

A change to a less basic solvent can influence the stereochemistry of unimolecular elimination. In nitrobenzene or acetic acid, erythro-3-deutcro-2-butyl tosylate yields deuterium-free m-2-butene and monodeuterated /ra 5-2-butene, suggestive of a jy/i-elimination (104). In more basic solvents, such as water, acetamide and 80% aqueous ethanol, the labelling of olefins is reversed and a/i/i-elimination is followed. The change in stereochemistry is attributed to the relative basicities of the tosylate anion and solvent and hence the tendencies of these species to accept a beta proton from the carbonium ion (105). 

A similar explanation has been suggested to account for the variation in the product distribution from the decomposition of the 2-phenyl-2-butyl cation generated from a variety of substrates-. In solvolytic elimination reactions of l,l,4.4-tetramethylcyclodecyl-6- or -7-tosylates jy/i-elimination appears to predominate in both acidic and basic solvents, an ion pair mechanism being suggested to account for the observed results . It is obviously unsound to generalise when considering stereochemistry of El reactions. 

If planar carbonium ions were the intermediates in El reactions in the cyclohexyl series, menthyl and neomenthyl compounds should give the same product ratios. However, the olefin distribution is quite different in the two El processes and the stereospecificity is less marked than in the E2 reactions of these substrates (Table 15). Whereas the concerted eliminations always show anti stereospecificity, the unimolecular eliminations only exhibit this preference when a tertiary beta hydrogen is trans to the ionising group (e.g. neomenthyl series). Possibly in this case the tertiary hydrogen aids ionisation by forming a type of non-classical bridged intermediate, viz. 

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While monomolecular collision-free predissociation excludes the preparation process from explicit consideration, themial imimolecular reactions involve collisional excitation as part of the unimolecular mechanism. The simple mechanism for a themial chemical reaction may be fomially decomposed into tliree (possibly reversible) steps (with rovibronically excited (CH NC) ) ... 

The unimolecular mechanism involves formation of a protonated cyclopropane ring first, which avoids the formahon of a primary carbenium ion until after skeletal rearrangement has taken place. Such reachon intermediates were first... 

Activation parameters have been measured for these reactions, and are also in agreement with the unimolecular mechanism, since the entropies of activation are all positive. For lsO- exchange from solvent water into mesitoic acid, under the conditions described above, Bunton et al.49 found a AS of... 

Note, however, that the comparison is with measurements for the benzoyl halides in a more aqueous medium than the acetyl halides and that this would favour a greater contribution from the unimolecular mechanism in the benzoyl halides. 

Another important factor determining the bimoleeular course of simple homogeneous gaseous decompositions is that the unimolecular mechanism would result, in many instances, in the production of free atoms, thus... 

In the bimolecular reaction (eqn. 3.2-47), bond-breaking and bond-forming take place simultaneously and therefore a negative activation volume would be expected. In the unimolecular mechanism, bond-breaking during the first, rate-determining slow reaction (eqn. 

Nucleophilic catalysis in the unimolecular mechanism is straightforward, since there is but one reactant that can bring the nucleophile into the transition state. If, however, a bimolecular substitution is found to be subject to nucleophilic catalysis and if it is concluded that, say, one molecule of the nucleophile, B, is involved in the transition state, then two main possibilities exist. The nucleophile may be brought into the transition state either by the organometallic substrate as in process (22) or by the electrophile as in process (23), viz. 

The acid-catalyzed ring openings of ethylenimine and 2-ethyl-ethylenimine have also been characterized as A-2 reactions (Earley et al., 1958). The entropies of activation are —9-4 and —10.0 e.u., respectively. However, AS for the reaction of 2,2-dimethylethylenimine is —1.9 e.u., suggesting incursion of the unimolecular mechanism, a conclusion which is supported on other grounds (Earley et al., 1958). The volume changes of activation have also been measured (Earley et al., 1958). These results, if interpreted in terms of Whalley s (1959) criterion, would indicate the A-l mechanism for ethylenimine and the A-2 for the ethyl and dimethyl derivatives, a conclusion which seems unacceptable. 

For several weeks these two dichlorides decomposed at the same rate by the unimolecular mechanism. But one day, without warning, the ethylene dichloride started to decompose much faster than the 1,1-dichloroethane, such that I could obtain the same conversion at 100 °C lower temperature. After reflection, I realized that the ethylene dichloride used had been recovered from the dry ice trap and then redistilled before use. Normally the 1,1- and 1,2-dichloroethanes were purified by careful fractional distillation. Clearly, my recovered sample contained a catalyst, or lacked an inhibitor. The latter seemed more probable, so I treated the 1,2-dichloroethane with chromic acid or potassium permanganate, shaking overnight. After redistillation, the purified dichloride still gave variable results, some days decomposing very fast and some not. The simply distilled material always had a constant rate of decomposition. The rate for 1,1-dichloroethane was always constant and independent of comparable chemical treatment. 

Finally, I identified another factor, a variable air leak. When I eliminated this, both dichloroethanes decomposed slowly by the unimolecular mechanism. When I let in a controlled flow of air (or chlorine), the 1,2-dichloroethane (in contrast to the 1,1-isomer) now decomposed rapidly at a much lower temperature. I had discovered my first new reaction the radical chain decomposition of the dichloride as in Scheme 2. 

Dichloroethane 3 always decomposes by the unimolecular mechanism even when oxygen, chlorine, or other radical generators are added. This is because radical attack on 3 gives the derived radical 9 which cannot carry the chain. 

All this was later put on a sound basis as a result of more precise measurements of rate constants and of activation energies. However, it did not require precise measurements to predict which chlorinated hydrocarbons would decompose by a radical chain mechanism and which by the unimolecular mechanism. Clearly, if the chlorinated hydrocarbon, or the product from the pyrolysis of the chlorinated hydrocarbon reacted with chlorine atoms to break the chain then the chain mechanism would not exist. Such chlorinated hydrocarbons would decompose by the unimolecular mechanism. Mono-chlorinated derivatives of propane, butane, cyclohexane, etc. would afford propylene, butenes, cyclohexene, etc. All these olefins are inhibitors of chlorine radical chain reactions because of the attack of chlorine atoms at their allylic positions to give the corresponding stabilized allylic radicals which do not carry the chain. 

The unimolecular mechanism is unusual for carbonyl substitution reactions. Those in the last chapter as well as the carbonyl addition reactions in Chapter 6 all had nucleophilic addition to the carbonyl group as the rate-determining step. An example would be the formation of an ester from an anhydride instead of from an acid chloride. 

Chain Mechanisms. The fact that first-order kinetics are observed for a gas-phase reaction does not prove that the unimolecular mechanism described above must be involved. Indeed very many organic decompositions that experimentally are first order have complicated free-radical chain mechanisms. 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

Thus alcohols, like halides, undergo substitution by both S l and Sn2 mechanisms but alcohols lean more toward the unimolecular mechanism. We encountered the same situation in elimination (Sec. 16.3), and the explanation here is essentially the same we cannot have a strong nucleophile—a strong base—present in the acidic medium required for prolonation of the alcohol. 

Other information obtained from considerations of entropies of activation at the same temperature is discussed later in this section. It is however worth noting that such comparisons amply confirm earlier views (Gold and Jefferson, 1953 Brown and Hudson, 1953 Erva et al., 1959 Schaleger and Long, 1963) that, in the absence of complicating features, bimolecular neutral and acid-catalysed solvolysis shows more negative values of AS than reaction by the unimolecular mechanism at 50° this difference amounts to 8-14 caldeg (see Tables 4-6). 

In the unimolecular mechanism, i.e. the SE1 mechanism, the electrophilic substitution takes place in two steps that parallel those that occur in the SN1 mechanism. Accordingly, write down the two steps for a general electrophilic substitution reaction between RX and E+, and indicate which is the slow step. 

It will be seen later that the observed effects of substituents require a modification of this picture of the unimolecular mechanism. 

It has been seen (p. 92) that, on energetic grounds, a radical non-chain process may be excluded except in very special cases, and so no further consideration need be given to this mechanism. This leaves the decision to be made between the radical chain and the unimolecular mechanisms. There is, at the present time, no criterion which is both necessary and sufficient to prove that a given reaction is proceeding by a unimolecular mechanism. Necessary conditions for a unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) absence of induction periods, (d) lack of effect of inhibitors, and (e) an Arrhenius A factor of the order of 1013 sec-1. An additional useful test, though neither a necessary nor a sufficient condition, is the absence of stimulation of the reaction in the presence of atoms or radicals. Finally, the effects of structural alterations on the rates of those related reactions that are claimed to be unimolecular should be capable of interpretation within the framework of current chemical theory. 

Transition structures of CH4 elimination in MMes have been investigated in detail with ab initio see Ab Initio Calculationi) quantum mechanics calculations. The most favorable transition structure for the elimination is close to a trigonal-bipyramidal geometry. A dimeric mechanism through intermolecular hydrogen abstraction is found to be much lower in activation free energy than the unimolecular mechanism. The dimeric transition structure is mainly stabilized by an M-CH2-M bridge. ... 

The unimolecular mechanisms Sj l and El involve the formation of carbocation intermediates. Rearrangements are possible when carbocations are intermediates in a reaction. Thus reactions occurring by the Sj l and El mechanisms are most likely to have a rearranged carbon skeleton. 

The product distribution changes shown in Table VII can only be understood in the context of the unimolecular mechanism shown in Equations 77-84. However, because the nascent CF3 F from Reaction 78 has been incompletely stabilized at 132 atm, this evidence is not sufficient to disprove the occurrence of the F-for-2F primary process (Reaction 76). 

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