Mechanical stability bromides

Effects of added n-alkyl triethyl ammonium bromides upon the mechanical stability of natural rubber latex (9)... 

Some interesting results have recently become available for the effects of a range of n-alkyl triethyl ammonium bromides upon the mechanical stability of natural rubber latex. The number of carbon atoms in the alkyl group varied from 6 to 18. Figure 6 summarises the results. It is usually believed that the addition of cationic surfactants to an anionic latex such as natural rubber latex invariably leads to a reduction in colloid stability, the effect being attributed to adsorption of the cations with consequent partial neutralisation of the particle charge and reduction of the counterion cloud surrounding the particles. 

Whilst the results summarised in Figure 6 show that this expectation is often realised, they also show that small additions of an n-alkyl triethyl ammonium bromide whose alkyl group contains between approximately 8 and 12 carbon atoms can bring about a small increase in mechanical stability. In such cases, the increase in stability cannot possibly be attributed to enhance-... 

Villanova et al. reported CNCs-reinforced (ethylacrylate (EA), methyl methacrylate (MMA), and butyl methacrylate (BMA)) mixture matrix-based pharmaceutical acrylic beads via suspension polymerization and direct compression. The results showed narrow size distribution, good flow properties and mechanical stability under compression [223]. Jackson et al. studied the controlled release effect of CNCs modified with cationic surfactant cetyl trimethylammonium bromide (CTAB) on molecules of drug. The results showed increment in zeta potential from -55 to 0 mV in a concentration dependent manner and can bind significant quantities of nanosized molecules of hydrophobic anticancer drug, exhibiting controlled release profile over a 2-day period [224]. 

Conductivities of pure ionic liquids varied depending on the anions (3.98 for [bmim][Tf2N ], 3.55 for [bmim][BF4 ] and 0.25 for [bmim][Br ] x 10 S cm at room temperature). Ionic conductivities of all synthesized PEs were in the range 10" -10 S cm at room temperature. The conductivity decreases with increasing polymer content in the three families, being typical behaviour of other polymer electrolytes composed of ionic liquids and polymers. At high ionic liquid concentrations, the ionic conductivity reached values near to the values of the ionic liquids although the mechanical stability is compromised. The lowest ionic conductivity of polymer electrolytes is the one with bromide anion (family 3). ... 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

The mechanism may involve a nucleophilic attack of the double-bond of the phosphole (17) on the phosphorus atom of phosphorus tribromide to provide intermediates that are stabilized by the loss of proton, pseudorotation, and finally the departure of a bromide anion [48, 49],... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Now, just the same sort of rationalization can be applied to the radical addition, in that the more favourable secondary radical is predominantly produced. This, in turn, leads to addition of HBr in what is the anti-Markovnikov orientation. The apparent difference is because the electrophile in the ionic mechanism is a proton, and bromide then quenches the resultant cation. In the radical reaction, the attacking species is a bromine atom, and a hydrogen atom is then used to quench the radical. This is effectively a reverse sequence for the addition process but, nevertheless, the stability of the intermediate carbocation or radical is the defining feature. The terminologies Markovnikov or anti-Markovnikov orientation may be confusing and difficult to remember consider the mechanism and it all makes sense. 

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