Nucleophilic displacement in the gas phase

Nucleophilic Displacement in the Gas Phase as a Function of Temperature, Translational Energy, and Solvation Number... 

Some quite interesting conclusions can be drawn from these values. The major conclusion is that many ions that are usually thought to be good nucleophiles are poor by this definition. For example, RO- reacts readily because its reactions are usually very exothermic (especially with CH3Br). As in solution, RO- cannot undergo reversible displacement in the gas phase. Thus, the inertness of ethers to displacement reactions is a consequence of... 

A variety of studies on nucleophilic displacement reactions have been carried out in the gas phase, utilizing pulsed ion cyclotron resonance (ICR) spectroscopy. Many of these reactions occur with conveniently measurable efficiencies... 

The dynamics and mechanism of nucleophilic displacements involving ions in the gas phase have been reviewed. The article covers aspects of kinetics (especially the... 

A rel = 1.2 x 10 6) (Ingold, 1957). Neopentyl halides are known in fact to be extremely stable towards nucleophilic displacement. There are few examples in the gas phase that allow for a full comparison since alkyl halides with P-hydrogens may be prone to undergo elimination reactions simultaneously with displacement. For example, the reaction of F with C2H5C1 proceeds with a rate constant krel = 0.86, compared to that of CH3C1 (Jose, 1976), whereas (CH3)3CCH2C1 reacts with k[cl = 0.61 (Olmstead and Brauman,... 

The observation in the gas phase of positive ion-molecule reactions which can be interpreted as a nucleophilic displacement was first reported by Holtz el al. (1970). A typical example is the tertiary ion-molecule reaction in a mixture of HCl and CH3F. Protonation of CH3F as a result of a primary ion-molecule reaction is followed by reaction (51). 

Comisarow (1977) has shown that, in the gas phase, methoxide ions react readily with methyl trifluoroacetate and methyl benzoate by an SN2 mechanism, while no reaction is observed as a result of nucleophilic displacement at the carbonyl centre. As for the case above, the SN2 reaction is highly exothermic, while the same is not true for the equivalent of reaction (64b). There is at present no satisfactory explanation of why (64a) apparently proceeds very slowly in the gas phase. 

The trends in the gas-phase reaction (67) follow closely the correlations encountered for the similar reaction in protic solvents (Jencks and Carriuolo, 1960). This observation, plus the fact that in the case of esters SN2 reactions become competitive with attack at the carbonyl, has been rationalized by several authors (Asubiojo and Brauman, 1979 Comisarow, 1977 Takashima and Riveros, 1978) on the basis of the poor solvation expected for the Sn2 transition state due to charge delocalization. Thus, SN2 reactions are expected to display much larger differences in the gas phase than in solution, when compared with nucleophilic displacement at carbonyl centres. This is reflected in a larger sensitivity of the activation parameters. 

The corresponding reaction in esters (76), an alcoholysis, is surprisingly slow in the gas phase and has not been observed by icr techniques even though both conditions for a nucleophilic displacement are satisfied as shown in (77) and (78). There is at present no satisfactory explanation of why this type of reaction is slow in the gas phase. 

Figure 11.6 Menschutkin reaction of ammonia and chloromelhane. In the gas phase nucleophilic displacement fails to take place, while in water solvation of the anions allows the reaction to proceed...

Although the attack of nucleophiles upon aromatic rings in the gas phase was reported more than a decade ago, not many publications on this topic have appeared since then. Thus, it was shown in earlier work that alkoxide ions react with fluorobenzene to give F and with hexafluoro-benzene to give pentafluorophenoxide anions (Briscese and Riveros, 1975), that (M + N02) adducts can be formed from reaction of N02 with < -, m-and /7-dinitrobenzenes (Bowie and Stapleton, 1977) and that (M — H + 0) ions are generated in reactions of Ot ions with benzene, naphthalene, pyridine, alkylbenzenes, methylpyridines and fluorotoluenes by displacement of a ring hydrogen atom (Bruins et al., 1978). 

A typical example of such reactions is the exothermic Sn2 nucleophilic displacement reaction Cl -I- CH3—Br Cl—CH3 - - Br . Table 5-2 provides a comparison of Arrhenius activation energies and specific rate constants for this Finkelstein reaction in both the gas phase and solution. The new techniques described above cf. Sections 4.2.2 and 5.1) have made it possible to determine the rate constant of this ion-molecule reaction in the absence of any solvent molecules in the gas phase. The result is surprising on going from a protic solvent to a non-HBD solvent and then further to the gas phase, the ratio of the rate constants is approximately 1 10 10 The activation energy of this Sn2 reaction in water is about ten times larger than in the gas phase. The suppression of the Sn2 rate constant in aqueous solution by up to 15 orders of magnitude demonstrates the vital role of the solvent. 

It has been demonstrated that only a small number of solvent molecules are needed to bridge most of the gap between the enthalpy diagram for nucleophilic displacement reactions in the gas phase and that in solution [475, 477, 485-488]. 

Alkoxysilanes, R3SiOR, in contrast to dialkyl ethers, also undergo nucleophilic displacement reactions in the gas phase (reaction 143)156b. 

Nucleophilic displacement reactions X + CH3Y — CH3 X + Y were studied in the gas phase [X = 0H (H20)n or CH30 (CH30H)n Y = Cl or Br] under conditions of variable temperature (200-500 K) in a flow reactor and of variable translational energy in a beam apparatus. In both cases, the solvation number of the ionic reactant was varied 0 competition between nucleophilic displacement and proton transfer the use of solvate as a stereochemical marker to probe mechanism, and the comparison between the gas phase and the solution of the reaction thermodynamics and kinetics. 

The choice of system deserves a few comments. Hydroxide ion and methyl halides were chosen for the present study. Nucleophilic displacement reactions have already been studied extensively in the gas phase (for reviews, see references 1 and 9-13) and current techniques are limited to reactions where only one of the two reactants is charged—substrate or nucleophile. Reactions with a negatively charged nucleophile offer an attractive choice because they have been investigated so extensively in solution. Methyl halides are the substrates of choice because elimination is not a possible pathway (4). Hydroxyl is a convenient nucleophile because its large hydration energies (14) minimize decomposition of the hydrated ions during their preparation and reaction. 

The thermodynamics are very different in the gas phase (Figure 4). Proton transfer is much less endothermic (AH° = +6 kcal/mol, see under Kinetics) nucleophilic displacement is much more exothermic (AH° = — 56 kcal/mol) (J), and the measured rate constant demonstrates that the barrier must be low. 

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