Solvolyses of a carbonate and a benzhydryl chloride inside micelles. Evidence for a porous cluster micelle

Abstract

415 mum 2.67)5 and the isomerization of 2-aryl-3-methyl-2-butene (2.34),20 both in acetic acid.Whether the maximum theoretical C-H isotope effect is reckoned to be 6.2 (zero-point energy only), 10 (allowing for bending vibrations), or higher still if the tunnel effect is included,21 these are small isotope effects and are therefore indicative of an asymmetric transition state,22 relatively close either to the carbonium ion or to the product. Since the former is of much higher energy, consideration of the Hammond postulate% suggests that the transition state for slow proton transfer will be in fact carbonium ion like. ConclusionThis reaction, the dehydration of 1,l'-diadamantylmethylcarbinol in aqueous acetic acid, is unusual in that variation of the solvent composition causes the deuterium KIE to vary over a wide range. This phenomenon is attributable to the fact that one of the components of the reaction medium, water, causes the intermediate carbonium ion to revert to starting material, while the major component, acetic acid, is apparently inert. A very simple kinetic scheme leads to an equation which expresses the overall KIE in terms of the water concentration. The data Two water-insoluble compounds, bis(4-nitrophenyl) carbonate and p-chlorobenzhydryl chloride, solvolyze in micelles at rapid rates indicative of highly aqueous binding sites. This is used as evidence against the classical Hartley micelle and in support of a "porous cluster" model bearing water-filled regions where guests bind hydrophobically. The conclusions from the kinetic data agree with those from a variety of other approaches used in this laboratory including I3C NMR, gas solubility, ORD, and molecular models.Reactivity inside a micelle generally differs from that in bulk water. This fact has been widely used to probe the 0022-3263/81/1946-0415$01.00/0 nature of micellar enviromnents.lJ Of course, it is seldom easy to interpret small catalytic or inhibitory effects with

Hydration studies on some polyhydroxy non-electrolytes and non-ionic surfactants

“…Menger (28) has proposed the porous cluster model which portrays the micelle as a loose cluster of surfactant molecules with water-filled cavities. A few experimental evidence supports water penetration (29)(30)(31)(32)(33). Cabane (34) has presented a critical study of different micellar models.…”

Proximity effect on the general base catalysed hydrolysis of amide linkage: The role of cationic surfactant, CTABr

“…Our result is consistent with the earlier observations that the decreased availability of water in the micellar pseudo phase as also its lower polarity than bulk water, slow down the water addition reactions. 19(a), 21 Now turning to the value of k W a reasonable assessment of the reactivity of the IP can be made as follows. A substantial catalytic effect of the ion-pair(k WIP /k W > 14 ± 3) in the aqueous pseudo-phase is envisaged.…”

Ambientes micelares em química analítica

“…Os efeitos que as micelas exercem nas reações químicas tem sido estudados durante as últimas décadas, resultando no desenvolvimento de equações teóricas correspondentes aos processos químicos que ocorrem em ambientes micelares [25][26][27][28][29][30][31] . Os sistemas micelares podem modificar a velocidade das reações e surpreende portanto, que estas características foram pouco exploradas para melhorar os métodos cinéticos de aná-lises.…”

“…A formação de associações de colóides pode também ocorrer em vários solventes não-polares; neste caso, os agregados dos tensoativos são denominados "micelas reversas" ou "micelas invertidas" 14,22 . Os mecanismos que envolvem esta modificação no meio reacional são complexos, e não totalmente estabelecidos, sendo que várias teorias descrevem o que pode ocorrer durante este processo [25][26][27][28][29][30][31] .…”