Elimination-Addition Route

The difference will also be reflected in the activation parameters. In most cases the activation energy is higher for the elimination-addition route, but examples are known where the differences are small. 

Several of the above criteria are usually used together in order to ascertain the reaction mechanism. The examples below will demonstrate their use, as well as the close relationship between the elimination-addition and the addition-elimination routes. 

The rule of trans addition of thio nucleophiles to acetylenes (Truce and Simms, 1956 Truce et al., 1960, 1961 Truce, 1961) and other criteria had been used by Truce and coworkers in their systematic study of the substitution routes of the chloroethylenes with p-toluenethiolate ion. 

The monosubstitution of vinyl bromide (Truce et al., 1956b) probably occurs via elimination-addition owing to the low reactivity of the halo-olefin. Acetylene is the only product formed with alkoxide ions or aniline. 

Although 1,1-dichloroethylene (136) can undergo (raws-elimination, its hydrogens are less acidic than those of the 1,2-isomer, which e.g., is able to form a mercury derivative of (132) with K2HgI4 while (136) cannot. 

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The operation of the addition-elimination route and its details are inferred from the use of several criteria which will be discussed in the order below (a) The element effect, i.e. comparison of the substitution rates of compounds which differ only in the leaving group, extended also to study of the competition of two leaving groups attached to the same or to different carbon atoms, of the same molecule, (b) The stereochemistry of the substitution, (c) The reactivity of various systems as a function of the structural parameters. The hydrogen-exchange criterion will be discussed in connection with the elimination-addition route. 

Since the bond cleavage is not important in the addition-elimination route, the element effect could be used for differentiation between it and the elimination-addition route, provided that a closely related model to the system studied is available. Such models for calibration of the element effect for the a-arylsulphonyl-/8-haIoethylenes (11) and the a-arylsulphonyl-j3-halopropenes (12) are the a-methyl analogues (13) for which the a,/3-elimination-addition route is impossible. Table 1 shows that for either cis- or trans- 13), the Br/ ci ratios are near unity for highly... 

If the above analysis is correct, pathway (90) -> (93) is forbidden and concurrent isomerization of the starting material with clean retention in the substitution is impossible. Since retention is sometimes obtained from a combination of the addition-elimination and the elimination-addition routes, the appearance of concurrent isomerization would point to this route for product formation rather than to addition-elimination... 

Gudkova, 1962). This is unexpected on the basis of steric interactions and was ascribed to the operation of the elimination-addition route. 

The effect of /3-methyl groups was investigated only for the cis-a-arylsulphonyl-j3-bromo- or /3-chloro-ethylenes. The 2-4- to 3-9-fold rate decrease with MeO- ion, as well as the 1-3- to 3-2-fold decrease for /3-bromo-a-p-nitrobenzenesulphonylethylene-di-n-butylamine and cyclo-hexylamine reactions, may point to a contribution of the elimination-addition route with these nucleophiles. When the elimination becomes more difficult, either for thep-methyl derivative or with a chlorine leaving group, a j8-methyl group decreases the substitution rate 20- to 84-fold with the same amines. With PhS- ion, for which other substitution routes are less probable, the rate retardation is higher (322- to 3100-fold). 

When a proton is available for expulsion in the vicinity of the leaving group, both may be eliminated. Consecutive addition of the nucleophile Nu and hydrogen to the elimination product finally yields a vinylic substitution product. These elimination-addition routes are the... 

The nature of the attacked system. Unactivated systems, such as those having only the leaving group as a vinylic substituent, usually react via the elimination-addition route. With the increase in the activation, competition with other routes may become important. 

The nature of the nucleophile. The elimination-addition route requires a strong base which is capable of abstracting the proton. Competition between attack at carbon and at hydrogen will occur if hydrogen and carbon basicities of the nucleophile are both high. The base and the nucleophile are not necessarily the same. A strong base present in the reaction mixture, e.g. RO, may be responsible for the elimination, while a weaker one, which is a better nucleophile, e.g. RS, may add preferentially to the acetylene. The elimination-addition route... 

Isolation and study of the behaviour of the intermediate. Although a substituted acetylene is the intermediate in the elimination-addition route, its isolation is dependent on the relative rates of its formation and destruction by the nucleophilic addition. Acetylenes can sometimes be isolated as the main reaction products, but in other cases, they have only been detected spectroscopically, or they may be trapped if they are very reactive. Detection of acetylene does not always prove that it is a reaction intermediate. Since generalization regarding the stereochemistry of the nucleophilic addition to acetylenes may be misleading, the independent behaviour (stereochemistry of addition, rate of disappearance) of the alleged intermediate acetylene should be studied. In favourable cases, these data enable quantitative dissection of the substitution process into its addition-elimination and elimination-addition components. 

Only (157b), (157c) and (160a) could be formed via an exclusive addition-elimination route, and this route, followed by rearrangement, should be less favourable than the a,/3-elimination-addition route which quantitatively explains the product ratios. 

Since both the stereochemical results and the exchange experiments are inconclusive regarding competition of the two reaction routes for the cis isomer, the dissection of the reaction into the contributions of the two routes requires the isolation or the estimation of the intermediate. Under suitable conditions, p-toluenesulphonylacetylene is the main product from bromo-(ll), while it is only detected by infrared spectroscopy during the reaction of chloro-(ll). Dissection of the overall substitution rate constant (kt) into contributions from elimination-addition ( ellm) and addition-elimination ( 8ub) is possible when the rate of the alkoxide-catalysed addition of alcohol to the intermediate acetylene (fcadd) and the concentrations of the latter during the reaction are known. Such analysis for cis-(ll) and (12) (Table 10) shows that at 0° the contributions of the two processes to the overall rate are nearly equal, but the importance of the elimination-addition route increases with the temperature, kum/fenih = 3 0 and 1-4 for (11) and (12) respectively at 25° (DiNunno... 

The corresponding halocrotonate esters react with EtO- mostly via the j8,y-elimination-addition route. However, the isolation of a 2-butyne... 

In the addition-elimination routes, either via a carbanionic intermediate (I) or via a neutral adduct (II), the anionic nucleophile Nu or the neutral nucleophile NuH attacks the /3-carbon with the expulsion of X. In the a,/8-route (IV), the /9,/3-route (VI) and the /8, y- elimination-addition routes (VII), HX is eliminated in the initial step, and the nucleophile and hydrogen are then added to the intermediates. Substitution occurs also by heterolytic C—X bond cleavage in an SN1 process (X). Initial prototropy followed by substitution can also give vinylic substitution products (XII, XIV), as well as two consecutive Sn2 reactions (XV) where the leaving group leaves from an allylic position. 

A nucleophile with high carbon-basicity is necessary in this route, but if it is too basic, competition by elimination-addition routes will take place owing to attack on hydrogen rather than on carbon. 

Reports of nucleophilic vinylic photosubstitution reactions, which occur via the S l mechanism, are conspicuously scarce. One such example is the cobalt carbonyl catalysed photostimulated carbonylation of vinylic halides339. By this method 1-bromo and 1-chlorocyclohexene are converted into 1-cyclohexenecarboxylic acid in 98 and 97% yield, respectively. In a prototype vinylic S l reaction, of / -bromostyrene with the enolate anion CH2COCMe3, an ionic elimination-addition route seems to be followed along with the S l route340. 

The elimination-addition route [Eq. (36)] constitutes a rare but important dissociative mechanism for displacement at silicon (150). Along the reaction coordinate one must assume the formation of unsaturated silicon species, which are now of interest to great number of organosilicon chemists (151). 

A methyl group attached to the double bond should decrease the rate of the nucleophilic attack by increasing the electron density and the steric interactions at the double bond. In addition, an a-methyl group should decrease the overall rate even for highly basic nucleophiles, by blocking the elimination-addition route. Substituent effects in a-aryl-sulphonyl-j8-haloethylenes bear out this prediction. The rate retardation by an a-methyl group, more pronounced for the less reactive... 

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