Steric approach control


Steric Approach Control  [c.94]

More bulky nucleophiles usually approach the cyclohexanone carbonyl from the equatorial direction. This is called steric approach control and is the result of van der Waals type repulsions. Larger nucleophiles encounter the 3,5-axial hydrogens on the axial approach trajectory.  [c.174]

Generally, ions do not strike any of the rods because, as they approach one rod, the weakest part of the field shifts because the RF potentials on the two pairs of rods are 180° out of phase. This shifting field causes an ion to swing around and start heading toward one rod of another pair and so on. The motion is illustrated in Figure 49.5b for a quadrupole. The action of the inhomogeneous field is to constrain an ion within the rod assembly, which behaves like a potential well, forcing an ion toward the central axis of the transmission guide.  [c.380]

As stated earUer, the main idea behind supercritical drying is to eliminate the Hquid-vapor interface inside a pore, thereby removing the accompanying capillary pressure which acts to coUapse a gel network. The value of this approach is demonstrated by the fact that aerogels do have higher porosities, higher specific surfaces areas, and lower apparent densities than xerogels, materials that ate prepared by evaporative drying. However, it is incorrect to think that a gel remains static during supercritical drying. Rather, supercritical drying should be considered as part of the aging process, during which events such as condensation, dissolution, and reprecipitation can occur. The extent to which a gel undergoes aging during supercritical drying depends on the stmcture of the initial gel network. Eor example, it has been shown that a higher drying temperature changes the particle stmcture of base-catalyzed siUca aerogels but not that of acid-catalyzed ones (29). It is also known that gels that have uniform-sized pores can withstand the capillary forces during drying better because of a more uniform stress distribution. Such gels can be prepared by a careful manipulation of sol—gel parameters such as pH and solvent or by the use of so-called drying control chemical additives (DCCA) (30). Clearly, an understanding of the interrelationship between preparative and drying parameters is important in controlling the properties of aerogels.  [c.3]

Melatonin thus could represent a new approach to the physiological control of stress and stress-related infectious diseases (48).  [c.431]

Corrosion control requires a change in either the metal or the environment. The first approach, changing the metal, is expensive. Also, highly alloyed materials, which are resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking.  [c.268]

Steric and stereoelectronic effects control the direction of approach of an electrophile to the enolate. Electrophiles approach from the least hindered side of the enolate. Numerous examples of such effects have been observed. In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a slight  [c.438]

In Chapter 1 the general mechanical properties of plastics were introduced. In order to facilitate comparisons with the behaviour of other classes of materials the approach taken was to refer to standard methods of data presentation, such as stress-strain graphs, etc. However, it is important to note that when one becomes involved in engineering design with plastics, such graphs are of limited value. The reason is that they are the results of relatively shortterm tests and so their use is restricted to quality control and, perhaps, the initial sorting of materials in terms of stiffness, strength etc. Designs based on, say, the modulus obtained from a short-term test would not predict accurately the long-term behaviour of plastics because they are viscoelastic materials. This viscoelasticity means that quantities such as modulus, strength, ductility and coefficient of friction are sensitive to straining rate, elapsed time, loading history, temperature, etc. It will also be shown later that the manufacturing method used for the plastic product can create changes in the structure of the material which have a pronounced effect on properties. The behaviour of the moulded product may therefore be different from the behaviour of a moulded test-piece of the same material.  [c.41]

Cetane improvers were first investigated in the U.S. as part of a demonstration project to retrofit DDC 6V-71 two stroke engines to methanol for three transit buses operating in Jacksonville, Florida. This project started out using hardware conversion where the engine was modified to control air flow and a glow plug was added to the system. Although this was basically the same approach as used in the dedicated 6V-92TA methanol engine, the 6V-71 employed mechanical injectors as opposed to electronic injectors. These mechanical injectors were failing even with 1% castor oil [8001-79-4] added to the methanol. Jacksonville thus decided to use a nitrate based cetane improver containing a lubrication additive and a corrosion inhibitor and the project proved the viabiUty of cetane-improved methanol.  [c.433]

The dual-fired heater shown in Figure 19 illustrates some of the practical situations that can arise. This heater fires both fuel oil and fuel gas. Depending on the overall plant fuel balance during a particular time period, one of the fuels is baseloaded and the other is modulated to control the heater s outlet temperature. Depending on which fuel is used as the temperature controller s manipulated variable, the process gain, that is, the ratio of the percentage of change in outlet temperature for a percentage of change in the manipulated variable setpoint, can be drastically different. The process dead time and lag times are likely to be about the same, however. In addition to the changes in gain owing to different fuels as the manipulated variable, the process dynamics also change significantly if the heater is mn at significantly different feed rates. For example, if the feed rate is reduced 50%, the process gain doubles and the effective dead time also increases. Although the temperature controller may be performing well when fuel oil is the manipulated variable or when it is being mn at 100% feed rate, its performance deteriorates when the manipulated variable is changed to fuel gas or if the feed rate is reduced to 50%, unless the controller is retuned. One approach to addressing this situation is to tune the controller initially so as to strike a compromise between its performance in each situation or over a large range of flows. It is not always possible, however, to reach a compromise that is acceptable for both process and control objectives. For example, such a compromise is not likely to be acceptable for the fired heater shown (Fig. 19) if its outlet was the feed to a reactor that required close control of its inlet temperature, ie, the heater outlet temperature.  [c.75]

A more general approach to quantitying the fracture criterion for a cracked body was first developed by Griffith [50] more than thirty-five years before the introduction of linear-elastic fracture mechanics. The Griffith criterion is based on conservation of energy and is illustrated in Fig. 4 for an infinite plate of unit thickness with a central transverse crack of length 2a. The plate is subjected to a remotely applied stress and is fixed at its ends. The elastic energy in this stressed body is represented by the area under the load versus displacement diagram. As the crack extends over an increment, da, some elastic energy is released and is  [c.497]

Acyclic chains, or their heterocyclic equivalents, which contain numerous stereocenters present a challenge both to synthetic methodology and to synthetic planning. Commonly, substrate steric bias is insufficient to control stereochemistry of many of the available methods for skeletal assembly. As a result, it may not be possible to assess with confidence the clearable or non-clearable status of individual stereocenters. Reliance on intrinsically diastereo- and enantioselective transforms must be heavy. As described in Section 4.1 the stereoselectivity of such transforms rests on the synthetic use of chiral controller groups and reagents as well as on mechanism- or coordination-based stereoselectivity. Alternatively, the identification of a set of nCL stereocenters with a potential chiral starting material may provide another approach to stereosimplification. Some of the most useful general guides for the reduction of stereochemical complexity in acyclic systems are listed below.  [c.56]

The effects of additions in causing the free corrosion potential to move in relation to the potential range for cracking would appear to have considerable practical significance. Thus, the addition of relatively small amounts of nitrates to concentrated NaOH solution, an approach that is sometimes used in treating boiler feed waters in an attempt to avoid caustic cracking, causes the free corrosion potential to become significantly more positive than the cracking potentials in alkali, and failure is no longer observed. However, the same addition will not prevent cracking if the potential is maintained, by potentiostatic control, within the cracking range, so that nitrates should be regarded as unsafe inhibitors of caustic cracking. Na2S04 should also be placed in this category even though the maintenance of a Na2S04/Na0H ratio in boiler feed water in excess of 2.5 is still widely practised as a means of preventing caustic cracking in boilers. In some circumstances additions to environments may promote cracking by causing the free corrosion potential to move into the cracking range. Thus, the free corrosion potential of some mild steels is likely to lie at the boundary of the cracking range in hydroxides so that stress corrosion does not occur. The addition of a small quantity of lead oxide causes the free corrosion potential to move into the cracking range and failure then occurs readily at the free corrosion potential, i.e. without external potentiostatic control. Similarly, the well established effect of oxygen in high temperature pure water in promoting cracking in nuclear reactor pressure vessel steel is probably the result of the oxygen raising the potential into the cracking range .  [c.1190]


See pages that mention the term Steric approach control : [c.67]    [c.801]   
Organic reactions in steroid chemistry (1972) -- [ c.67 ]