Eliminations vs. substitution

The factors that influence elimination v. substitution are discussed subsequently (p. 260). Evidence for the involvement of C—H bond fission in the rate-limiting step—as a concerted pathway requires— is provided by the observation of a primary kinetic isotope effect (cf. p. 46) when H is replaced by D on the ft-carbon. 

Eliminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes (e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from -substituted ketones are very facile. The dehydration of /5-hydroxy ketones has been covered in section V. 

Data assembled by Parker (201 demonstrate these effects for bimolecular reactions involving sulfur nucleophiles and haloaliphatic substrates. As an illustration for the case of Reactions 4, the S 2 displacement of iodide from CH3I by SCN at 25°C is accelerated relative to its rate in water tty 0.2 log units in methanol, by 1.1 log units in 10% aqueous dimethyl sulfoxide (v/v), and tty approximately 2.4 log units in dimethyl formamide (DMF). Furthermore, the rates of bimolecular elimination and substitution of cyclohexyl bromide in the presence of thiophenolate at 25°C both increase by 2.7 log units when the solvent is changed from ethanol to dimethylfonnamide (20). 

In both imidazoles and their /V-substituted derivatives, halogenation occurs preferentially in the 4(5)-positions there is a slight preference for 5-substitution in 1-substituted substrates. Although the 2-position is much less reactive, it is difficult to prevent substitution at that site. Indeed, polyhalogenation is so facile that it is seldom feasible to make monohalogenated imidazoles directly. Both sodium hypochlorite and NCS convert imidazole into its 4,5-dichloro derivative contaminated by the 2,4,5-trichloro product. Even very mild conditions are unlikely to promote monochlorination, and bromination and iodination arc similar. Mechanisms can vary, however, from substrate to substrate. It is likely that C-2 halogenations are the result of addition-elimination [1]. 

As the reflux ratio approaches infinity (total reflux), Equation 3.21 requires that 11 = 21- The other equations are the same as in the case of zero reflux ratio. The problem equations are solved by simple eliminations and substitutions, giving the following results X = 0.50, V, = 0.75, X, = 0.25, /ji = 0.50. The K-values of propane are calculated as before ... 

The transformation of one 1,2,4-triazine ring into another 1,2,4-triazine has been reported. Reaction of /V-substituted cyanamides with 1,2,4-triazines affords, via exclusive addition of the nitrile group across C5—N2 of the 1,2,4-triazine nucleus, the nonisolable bicyclic intermediate 16, from which trifluoroacetonitrile is eliminated to yield 1,3,5-triazines 17 as the main products. Elimination of methyl thiocyanate is a side reaction affording 1,2,4-triazines 18.2U2... 

More recently it has been shown that (V-substituted cyanamides undergo cycloaddition exclusively to C5/N2 of the acceptor-substituted 1,2,4-triazine ring 12. Elimination of trifluoroace-tonitrile from the intermediate bicycle 13 leads to the corresponding 1,3,5-triazines 14 in 31 to 45% yield.20... 

Eliminating V/12 from this equation by using Eq. (13.7.17b) gives V/u in terms of V/is. Substitution of the resulting expressions into Eq. (13.7.8) (with E = 0) then gives... 

Then the potential and energy dependence are eliminated by substituting the relativistic kinetic energy Ep - mc for the difference - V in the arguments off and/ . Furthermore, one has to adjust the Hamiltonian to render it hermitean and to normalize the solutions [69,102]. The performance of the SR variant of the RESC method was demonstrated for the Ag and Au atoms, their hydrides and chlorides [69,102]. 

If Ar is used as the expansion gas the v = 0 and 1 vibrational populations are little changed by collisions in the jet, so internal cooling is not complete in this case. On the other hand, the nonequilibrium population of higher vibrational levels is often useful to the spectroscopist. In addition, we have found that the CN population in V > 0 can be eliminated by substituting He as the expansion gas, although with He the rotational cooling is not as efficient as with Ar. 

Q2. Hence, in order to solve the column, two of the variables must be specified. The distribution coefficients may be supplied as additional data or may be calculated from appropriate correlations. The column equations may be manipulated in different ways, depending on which of the variables are specified and which are to be solved. For instance, a chain of eliminations and substitutions may be carried out to develop an expression for L,/V the ratio of reflux to vapor from the top tray, or reflux ratio ... 

Use these three equations to eliminate V

[Pg.168]

If you try to solve these n equations for all of the elements of the v veetor (vi...Vn), you ean eliminate one variable using one equation, a seeond variable using a seeond equation, ete., beeause the equations are linear. For example you eould solve for vi using the first equation and then substitute for vi in the seeond equation as you solve for V2, ete. Then when you eome to the nth equation, you would have n-1 of the variables expressed in terms of the one remaining variable, Vn. 

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