Rate of HC1 elimination

Several investigations77,88,90 indicated that steric factors do not determine the velocity of dehydrochlorination in branched alkyl chlorides, and the paramount factor which determines the rate of HC1 elimination is electronic in nature. 

The higher rate of HC1 elimination of 4-chloro-l-phenyl-1-butanone with respect to 5-chloro-2-pentanone by a factor of 9.9 confirmed the participation of the C=0 group. The 7r-electron delocalization of the benzene ring increases the basicity and nucleophilicity of the carbonylic oxygen and led to greater stabilization of the transition state through assistance to the C—Cl bond cleavage in the transition state (equation 80). 

The data presented in Table 26 indicate that the resonance effect of the u-CHsO and p-CH30 increase the rate augmentation by the phenyl ring. However, the m-CH30 does not interact resonatively with the reaction center, as shown by the elimination rate of HC1 which is similar to that of the unsubstituted phenylethyl chloride. 

The rate of the addition reaction is determined by a phase boundary process, the contracting area expression [eqn. (7), n = 2] is obeyed and E = 37 kJ mole-1. The value of E for decomposition of the frarcs-chloro isomer (right-hand side of above equation) is 51 kJ mole-1, which is less than that (130 kJ mole-1) for HC1 elimination from the cis-chloro isomer. 

Thus of the geometrical isomers of hexachlorocyclohexane, C6H6C16, one is found to undergo elimination of HC1 at a rate slower, by a factor of 7-24 x 103, than any of the others it is found to be the one (35) that cannot assume the above trans-diaxial conformation. 

The above reaction scheme could not explain the marked effect of water on the reaction rate. Another drawback of the above reaction scheme is that if each monomer addition were followed by the shift of a hydride ion and isomerisation, the two charges should always remain in the vicinity and it should allow the elimination of HC1 or the addition of chloride ion to proceed easily otherwise the reaction scheme will not be feasible. 

This Rh(I)-based mechanism does not properly explain the pronounced rate acceleration observed on addition of HC1. Rather, we suggest that the well-known propensity of Rh(I) to oxidatively add HC1 evokes a different, Rh(III)-based mechanism. The elementary steps along such a pathway (/3-hydride insertion then C-C reductive elimination) differ substantially from those depicted in Scheme 4. Despite this uncertainty, our current mechanistic understanding of the non-H+ mediated reaction constitutes a satisfactory model because it has led to the successful refinement and expansion of our initial findings. 

We studied the P-elimination and replacement reactions using our pyridoxamine analogs [43]. Initially, we examined the relative rates for the P-elimination reactions between chloropyruvic acid and small pyridoxamine analogs carrying a basic side chain (Table 2.2). In the dimethylamino series the HC1 elimination rates were fastest with the shorter chain of 4, as expected for a process that requires only proton removal from the pyridoxamine 4 -CH2 group. The contrast with the data for transamination... 

A generalized rate expression for the reaction of allylic chloride (RC1) with metal stabilizers (MX2) includes terms for the unimolecular elimination of HC1 from RC1 and for the bimolecular reaction of RC1 with all metal containing species. 

A comparison of two single-pulse shock-tube-technique experiments applied to the HC1 elimination of ethyl chloride and -propyl chloride in the temperature range of 960-1100 K was made by Evans and coworkers48. The observed rate coefficients were compared with those of previous works. In this investigation it was believed that the activation energy Ea of 242.6 kJ mol-1 for CH3CH2C1 - CH2=CH2 + HC1 is more appropriate than the most commonly reported E values of 234.2-236.8 kJ mol-1. 

A possible alternative decomposition as described in equation 25 was not observed. A four-membered cyclic transition state and an Arrhenius factor similar to that of the HC1 elimination from chlorocyclobutane was assumed for the RRKM calculations. The experimental unimolecular rate coefficients are consistent with the Arrhenius equation log kx... 

The rates of pyrolysis of o-, m- and z -methylphenylethyl chloride have been compared with that of ethyl chloride (Table 15). Even though the phenyl group participated in the HC1 elimination, electron release of the methyl group at the three isomeric position of the aromatic ring was found to be, within the experimental errors, ineffective on the rates when compared to the unsubstituted phenylethyl chloride. Consequently, the effect of the CH3 substituent was too small for the reinforcement of the phenyl assistance as reflected by the pyrolysis rate. 

The formation of CH2=CH2, as the main product, gave a first-order rate coefficient which was invariable to initial pressure and surface conditions. Yet, the formation of CH3CH2C1 was variable and increased in a packed vessel. Consequently, the production of ethyl chloride is heterogeneous. A polar six-membered cyclic transition state was suggested for the elimination of HC1 or C1COOH, a mechanism already advanced by Clinch and Hudson191. 

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