Academic literature on the topic 'Polar aprotic solvent'

Polar solvents are those liquids whose relative permittivity is sufficiently high that electrolytes can be dissolved in them. The best-known example of such a liquid is water. The oxygen end of this simple molecule is electron-rich and can stabilize cations. The hydrogen atoms are electron-poor and thus are involved in the solvation of anions. The structure of pure water is very much influenced by the hydrogen bonding between the negative end of the molecular dipole at oxygen and a hydrogen atom on an adjacent molecule. The special properties of water as a solvent for electrolytes are the central reason for its importance in living systems. There are many other solvents which can be classified as polar. Some of them, such as the alcohols, have the same polar group as the water molecule, namely, the hydroxyl group –OH. These solvents are also involved in hydrogen bonding, and are generally classified as protic. Other examples of protic solvents are simple amides such as formamide and acetamide. In these systems, the protic group is –NH2, the hydrogen atom being involved in hydrogen bonding with the oxygen atom in the carbonyl group on an adjacent molecule. There are other polar solvents which are not protic. These involve liquids with large dipole moments. Some examples are acetonitrile, propylene carbonate, and dimethylsulfoxide. In each case, the solvent molecule possesses an electronegative group which is rich in electrons. The opposite end of the molecule is electron deficient but does not have acidic hydrogen atoms which can participate in hydrogen bonding. This class of solvents is called aprotic. In this chapter, the properties of polar solvents are discussed, especially as they relate to the formation of electrolyte solutions. Polar solvents are arbitrarily defined here as those liquids with a relative permittivity greater than 15. Solvents with zero dipole moment and a relative permittivity close to unity are non-polar. These include benzene, carbon tetrachloride, and cyclohexane. Solvents with relative permittivities between 3 and 5 are weakly polar, and those with values between 5 and 15 are moderately polar. The latter systems are not considered in the discussion in this chapter.

Abstract The most frequently used displacement method of a hydroxyl group begins with its conversion to sulfonate. Sulfonates of the secondary hydroxyl groups of sugars are not very reactive with respect to external nucleophiles. This is no doubt a result of the high functionalization of the molecule which combines steric hindrance with an unfavourable inductive effect. Substitution, practically impossible with former techniques, only became a current operation following two innovations. The first was the introduction of polar aprotic solvents; the favourite solvent is N,N-dimethylformamide since this is the easiest to eliminate at the end of the reaction, by extracting it with water from an ether solution.

Abstract Synthesis Aromatic polyetherimides are usually prepared from: (a) bisphenoxide salts and aromatic dinitrobisimides via nucleophilic nitro-displacement reactions (b) bisphenoxide salt and a bis(N -chlorophthalimido) compound via nucleophilic chloro-displacement reactions in a solvent of low polarity (e.g. o-dichlorobenzene), and in the presence of a thermally stable, phase transfer catalyst stable at the polymerization temperatures, e.g. hexaalkylguanidinium chloride; microwave irradiation accelerated polymerization process; (c) two-step polycondensation of aromatic diamines and ether-dianhydrides in a polar aprotic solvent, followed by thermal6 or chemical cyclodehydration of the polyamic acid precursors, (d) one-step, high temperature solution polymerization of aromatic diamines and ether-dianhydrides in a phenolic solvent, removing water of condensation azeotropically. Certain polyetherimides can also be synthesized via direct melt polymerization.

Time Dependent Fluorescence Shift (TDFS) of a polar compound (AMBO) dissolved in a sequence of polar (protic or aprotic) solvents has been studied. This compound (AMBO) is: shown on fig. 1. In the ground state, the solvent cage surrounding the polar compound has a well defined topology which minimizes the solute-solvent interactions. In the excited state, as shown on fig. 1, large change occurs (charge distribution, dipole moment). As a consequence, the solvent cage has to reorganize in order to minimize this new interaction. The TDFS reflects this change and particularly, its dynamics, related to the motion of the cage. The kinetic theory of TDFS has recently gained new interest {1,2} and our results provide a test of the validity of theoretical approach.

Liquid dynamics has been studied by various time-domain techniques such as photon echoes [1] and dynamic Stokes shift measurement [2], which utilize electronic absorption as a probe. Attempts to measure electronic dephasing times in solution using the standard photon echo technique have been hampered by the rapidity as well as the non-Markovian nature of the dynamics. Much emphasis has been directed toward employing shorter and shorter pulses. However, photon echoes using short pulses often simply measure the ultrafast break up of the intra-molecular vibrational wavepacket created by the large spectral bandwidth of the pulses. Recently, it has been shown that three pulse stimulated photon echo peak shift (3PEPS) measurements give accurate dynamical information on solute-solvent interaction [1,3]. In this technique, peak shifts are determined precisely by simultaneously measuring signals in the phase matching directions –k1+k2+k3 and k1−k2+k3. The peak shift reflects the ability of the system to rephase after evolving in a population state for time, T. That is, the decrease of 3PEPS mirrors the electronic transition frequency correlation function, M(t). Here we report 3PEPS studies on various polar protic and aprotic solvents using 22 fs and 90 fs pulses. It is shown the 3PEPS with pulses much longer than a typical electronic dephasing time still gives accurate information on ultrafast as well as slow dynamics in liquids. The experimental results are consistent with the numerical simulations including finite pulse duration.

A facile electrochemical anodization method was used for producing hierarchically textured surfaces based on TiO2 nanotubes in two different configurations. It was found that perfluoro-functionalized TiO2 nanotubes exhibit high static contact angles for a variety of liquids such as apolar, polar aprotic and polar protic solvents. Wenzel and Cassie-Baxter theories were applied for theoretical contact angle calculations for the present study. By using Cassie theories, it is shown that a drop of polar liquid was in a fakir or Cassie-Baxter (CB) state on perfluoro-functionalized nanotube surfaces. The fakir state prevents spreading of the liquid on the surface. On the other hand, the wetting of non-polar liquids such as hexane is characterized by either Wenzel states or transition states characterized by partial imbibition that lie in between the CB and Wenzel states.