Depicting mechanism

FIGURE 6.27 Acidolysis removes a protector by one or both of the depicted mechanisms. Pg = protector, Y = residue, Nu = nucleophile. Nu1 and Nu2 may be identical or different. C02 is liberated when Pg = alkoxycarbonyl. 

Below the glass transition temperature (Tg ca 100 °C) the depicted mechanism is assumed to be the main response of polyacrylonitrile fibers to stress. It may account for an elongation of the fiber of up to W%. As it may be expected, the extension is almost entirely reversible, if the stress is removed, and the kinetic conditions are provided permitting the macromolecules to restore the state of lowest energy (e. g. by steam treatment above Tg). 

The depicted mechanism is of course much more complex than the simple Michaelis-Menten scheme, but it can be simplified by employing the steady state approximation. This may be done in a relatively simple manner by applying either the King-Altman method or the less well-known Christiansen formalism. Applying the King-Altman method to the catalytic cycle given in Scheme 4.3 leads to a rate equation that is equal to the substi-tuted-enzyme mechanism, the detailed derivation of which was debated by Cornish-Bowden. ... 

We use single-barbed arrows to depict mechanisms involving single electron movements (see Section 3.1A). 

As expected from the depicted mechanism, early attempts to control the stereoselectivity of the MBH reaction was focused on the application of chiral amines (Fig. 4.48). Thus, using high pressure conditions (5 kbar) to accelerate the reaction and a C -symmetric DABCO derivative 245 (15 mol%), product 241a (R =Me, R sq-NO CgH ), was obtained in 45% yield and 47% ee (1 mol% hydroquinone, THF, 30°C) [318]. When used with pyrrolizidine derivative 246 (10 mol%, acetonitrile, 0°C) improved results (17-93% yield, 39-72% ee) were obtained in reactions between methyl or ethyl vinyl ketone (237a R =Me and 237b R =Et) and aromatic aldehydes. The presence of NaBF as co-catalyst was required to achieve these results, due to the coordination of aldehyde and hydroxy group of the catalyst to the alkali metal, which fixed the orientation for the attack of the nucleophile to the electrophile in the transition state [319]. 

Properties and Reactions. The desulphurization of a variety of cyclic disulphides to the corresponding sulphides on treatment with aminophos-phines followed second-order kinetics, and was attended by inversion of configuration at one of the carbon atoms adjacent to the disulphide group. For example, m-3,6-di(methoxycarbonyl)-l,2-dithian (130) gave rm f-2,5-di(methoxycarbonyl)thiolan (131), most probably by the depicted mechanism, whilst the /ra j-isomer of (130) gave the c/j-isomer of (131). 

It is worth noting the slightly different second curly arrow on the diagram. This shows that the electrons involved in the C-X bond move from between those two atoms to a position on the more electronegative X atom. This shows the departure of the X atom as an X ion. You will be expected to use curly arrows accurately in your IB examination papers. This system of indicating the movement of electrons is useful in depicting mechanisms such as S l and Sn2 (see Chapter 20). 

During the formation of carboxylic acid like lA, there will be shuttling of metabolites within the intracellular compartments, having the capability to utilize the enzymes of the respective compartments. Jaklitsch et al. (1991) reported that the CadA, which is e key enzyme for the biosynthesis of lA, is located in cytosol. Otiier important enzymes, such as citrate synthase and aconitase, are found in the mitochondria, but some residual level of these enzymes are also found in the cytosolic fraction. The depicted mechanism is that the ds-aconitate is transported to cytosol assisted by the malate-citrate antiporter. The biosynthetic pathway of LA in the citric acid cycle is illustrated in Fig. 10.5. 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

If the diffusion coefficient of species A is less tlian tliat of B (D < D ) tlie propagating front will be planar. However, if is sufficiently greater than tire planar front will become unstable to transverse perturbations and chaotic front motion will ensue. To understand tire origin of tire mechanism of tire planar front destabilization consider tire following suppose tire interface is slightly non-planar. We would like to know if tire dynamics will tend to eliminate this non-planarity or accentuate it. LetZ)g The situation is depicted schematically in figure... 

Place 200 g. (172 -5 ml.) of redistilled furfural (1) in a 1 litre beaker, provided with a mechanical stirrer and surrounded by an ice bath. Start the stirrer and, when the temperature has fallen to 5-8°, add a solution of 50 g. of sodium hydroxide in 100 ml. of water from a separatory funnel at such a rate that the temperature of the reaction mixture does not rise above 20° (20-25 minutes) continue the stirring for a further 1 hour. Much sodium furoate separates during the reaction. Allow to cool to room temperature, and add just enough water to dissolve the precipitate (about 65 ml.). Extract the solution at least five times with 60 ml. portions of ether in order to remove the furfuryl alcohol the best results are obtained by the use of the continuous extraction apparatus (charged with 350 ml. of ether) depicted in Fig. //, 44, 2. Keep the aqueous layer. Dry the ethereal extract with a httle anhydrous... 

The mechanism by which Lewis-acids can be expected to affect the rate of the Diels-Alder reaction in water is depicted in Scheme 2.6. The first step in the cycle comprises rapid and reversible coordination of the Lewis-acid to the dienophile, leading to a complex in which the dienophile is activated for reaction with the diene. After the irreversible Diels-Alder reaction, the product has to dissociate from the Lewis-acid in order to make the catalyst available for another cycle. The overall... 

Wnte a sequence of steps depicting the mechanisms of each of the following reactions... 

Partial but not complete loss of optical activity m S l reactions probably results from the carbocation not being completely free when it is attacked by the nucleophile Ionization of the alkyl halide gives a carbocation-hahde ion pair as depicted m Figure 8 8 The halide ion shields one side of the carbocation and the nucleophile captures the carbocation faster from the opposite side More product of inverted configuration is formed than product of retained configuration In spite of the observation that the products of S l reactions are only partially racemic the fact that these reactions are not stereospecific is more consistent with a carbocation intermediate than a concerted bimolecular mechanism... 

FIGURE 12 7 The mechanism of Friedel-Crafts alkylation The molecular model depicts the cyclohexadienyl cation intermediate formed in step 1... 

In practice, the loss term AF is usually not deterrnined by detailed examination of the flow field. Instead, the momentum and mass balances are employed to determine the pressure and velocity changes these are substituted into the mechanical energy equation and AFis deterrnined by difference. Eor the sudden expansion of a turbulent fluid depicted in Eigure 21b, which deflvers no work to the surroundings, appHcation of equations 49, 60, and 68 yields... 

AH distortions of the nematic phase may be decomposed into three basic curvatures of the director, as depicted in Figure 6. Liquid crystals are unusual fluids in that such elastic curvatures may be sustained. Molecules of a tme Hquid would immediately reorient to flow out of an imposed mechanical shear. The force constants characterizing these distortions are very weak, making the material exceedingly sensitive and easy to perturb. 

The effect of temperature on PSF tensile stress—strain behavior is depicted in Figure 4. The resin continues to exhibit useful mechanical properties at temperatures up to 160°C under prolonged or repeated thermal exposure. PES and PPSF extend this temperature limit to about 180°C. The dependence of flexural moduli on temperature for polysulfones is shown in Figure 5 with comparison to other engineering thermoplastics. 

The mechanism of the Diels-Alder reaction is not as simple as usually depicted. This may, in part, explain some of the problems encountered when this reaction has been appHed on an industrial scale. A number of different theories have been proposed for this reaction (50). 

Element. The process of fabricating lead—acid battery elements as depicted in Figure 4 involves numerous chemical and electrochemical reactions and several mechanical assembly operations. AH of the processes involved must be carefully controlled to ensure the quaUty and reUabiUty of the product. 

The water removal mechanism is adsorption, which is the mechanism for ad Class 4 drying agents. The capacity of such materials is often shown in the form of adsorption isotherms as depicted in Figures 9a and 9b. The initial adsorption mechanism at low concentrations of water is beheved to occur by monolayer coverage of water on the adsorption sites. As more water is adsorbed, successive layers are added until condensation or capidary action takes place at water saturation levels greater than about 70% relative humidity. At saturation, ad the pores are fided and the total amount of water adsorbed, expressed as a Hquid, represents the pore volume of the adsorbent. 

Kinetic data on acetate displacement from C-3 using a number of sulfur and nitrogen nucleophiles in aqueous solution at near neutral pH demonstrate that the reaction proceeds by an 5 1 mechanism (B-72MI51004). The intermediate in this reaction is depicted as a dipolar allylic carbonium ion (9) with significant charge delocalization. Of particular significance in this regard is the observation that the free carboxylate at C-4 is required since... 

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