Added protic solvent effect

Effect of Added Protic Solvent In aqueous dioxane or acetonitrile (40% water by volume) 4>d was > 0.2, compared with values < 0.1 in the pure aprotic solvent (Table I). Most of this additional yield was quenched in the presence of 0.1 M sorbic acid, which suggests that the additional degradation occurs from the triplet state. Thus, addition of a protic co-solvent has apparently caused the triplet state to become reactive. 

As Figure 1 shows, in dioxane d increased as the mole fraction of added protic solvent, either water, methanol or ethanol, was increased over the range 0.10 - 0.60. Trace amounts of a protic solvent had no effect on < >d. Over the range of concentrations where < >d is increasing with the amount of added protic solvent, water, methanol and ethanol have similar effects however the maximum value of < >d occurs for addition of ethanol. Photolysis of I in 40% D20/dioxane gave < >d = 0.22, which is indistinguishable from the value obtained in 40% water/dioxane. 

There is also direct IR spectroscopic evidence that addition of a basic co-solvent to aqueous solutions reduces the percentage of non-associated free OH groups which are necessary for hydrogen-bonding to Sn reactants [585], When basic co-solvents are added, they scavenge the free OH groups in water, thus lessening the specific protic solvent effect [585]. 

Similar results are found for the above-mentioned Menschutkin reaction (c). The added pro tic solvent can also combine with the main solvent. Such solvent/solvent association leads to a diminution of the specific inhibitory and catalytic effect of protic solvents on the Sn2 reaction (c) [584], The basicity of the main solvent determines the extent of deactivation of the protic solvent through H-bond association this is analogous to Eq. (5-107). 

There is another somewhat puzzling differential effect relating to attack at the sites Ca vs. X in process (2). As mentioned above, ArBr/Arc appears to increase as the solvent becomes more protic. This is evident, for example, in the reactions of (EtO)3P or PhjP with haloalkynes in alcohol-THF mixtures > . It would appear that species 41 (Scheme 6) is not as v/ell solvated as species 40 in a protic solvent. It should be noted, however, that Simpson and Burt have studied the variation of products in similar systems, e.g. (EtOIsP—PhC=CCl, and found a maximum in [PhC CH] and a minimum in PhC=CPO(OEt)2 between 0 and 5m added ethanol . 

The reductive elimination step is effected by adding an excess of 5,65% Na(Hg) (2.5 mg atom of Na g" ) to an efficiently stirred solution of the sulfone in 3 1 THF-MeOH at -20 °C. A protic solvent is essential for the reaction and a detailed studyrevealed that MeOH was superior to EtOH or Pr OH at low... 

Besides the specific solvent effects such as catalysis and solvation by a protic solvent, the effects related to the properties of a solvent regarded as a continuous dielectric can be important along the reaction path. Until recently, the problem of quantification of such phenomena within the framework of the quantum-meehanical theory received relatively little attention. An early attempt to simulate solvent as a continuous medium in the ab initio geometry optimization was described by Tomasi et al. [139, 163]. They used an electrostatic interaction potential, added as a perturbation to the Hamiltonian of the reactive system, to account for the interaction between... 

Several less common monomers have been polymerized anionically. Ree and Minoura" effected the conversion of aminoacetonitrile to NHCH2C(NH) n— by treatment with sodium s-butoxide in dioxan hydrolysis yielded the polypeptide—[NHCHaCO] —. Aminopropionitrile behaved similarly. High polymer has been formed from 8-nitrostyrene using alkoxide ion as initiator in protic solvents. Block copolymer is formed from polyisoprenylpotassium in benzene on adding 1-nitropropene. Vinyl ferrocene has been shown to undergo homo- and co-polymerization by butyl-lithium or lithium metal, but the utility of these synthetic procedures is severely limited by side reactions. The poly-... 

The condensation of the silanols to siloxanes in non-protic organic solvents proceeds to completion. Lasocki has observed that the rate of silanol condensation is affected by small amounts of water added to the non-protic (dioxane) solvent [68]. The maximum effect is observed at approximately 0.75 M water. The impact of water on the rate may be due to solvation of ionic intermediates, its effect on the activity of the acids or bases, and its effect on the polarity of the solvent. 

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