Solvents protic, basic

Overlap Nu must approach the 6-i- carbon from behind and close to the pi plane. Media A weak base is needed the medium is usually weakly basic to neutral. Solvent Protic and polar to stabilize the anion formed. 

When the aldol-type addition is carried out under classical conditions (protic solvents and basic or acid catalysis), by-products such as dimers, polymers and a,P-unsaturated carbonyl compounds are frequently formed. Variants such as the Mukaiyama reaction [2] and the use of lanthanide triflates as catalysts [3] generally reduce these side-reactions. A significant contribution has also come from investigations on the use of water as the reaction medium. 

In a Lewis-acid catalysed Diels-Alder reaction, the first step is coordination of the catalyst to a Lewis-basic site of the reactant. In a typical catalysed Diels-Alder reaction, the carbonyl oxygen of the dienophile coordinates to the Lewis acid. The most common solvents for these processes are inert apolar liquids such as dichloromethane or benzene. Protic solvents, and water in particular, are avoided because of their strong interactions wifti the catalyst and the reacting system. Interestingly, for other catalysed reactions such as hydroformylations the same solvents do not give problems. This paradox is a result of the difference in hardness of the reactants and the catalyst involved... 

Thickeners. Thickeners are added to remover formulas to increase the viscosity which allows the remover to cling to vertical surfaces. Natural and synthetic polymers are used as thickeners. They are generally dispersed and then caused to swell by the addition of a protic solvent or by adjusting the pH of the remover. When the polymer swells, it causes the viscosity of the mixture to increase. Viscosity is controlled by the amount of thickener added. Common thickeners used in organic removers include hydroxypropylmethylceUulose [9004-65-3], hydroxypropylceUulose [9004-64-2], hydroxyethyl cellulose, and poly(acryHc acid) [9003-01-4]. Thickeners used in aqueous removers include acryHc polymers and latex-type polymers. Some thickeners are not stable in very acidic or very basic environments, so careful selection is important. 

The idea of kinetic versus thermodynamic control can be illustrated by discussing briefly the case of formation of enolate anions from unsymmetrical ketones. This is a very important matter for synthesis and will be discussed more fully in Chapter 1 of Part B. Most ketones, highly symmetric ones being the exception, can give rise to more than one enolate. Many studies have shown tiiat the ratio among the possible enolates that are formed depends on the reaction conditions. This can be illustrated for the case of 3-methyl-2-butanone. If the base chosen is a strong, sterically hindered one and the solvent is aptotic, the major enolate formed is 3. If a protic solvent is used or if a weaker base (one comparable in basicity to the ketone enolate) is used, the dominant enolate is 2. Enolate 3 is the kinetic enolate whereas 2 is the thermodynamically favored enolate. 

The ultraviolet absorption spectra of the anhydro-bases in acid solution or in protic solvents are those of the 3,4-dihydro-)3-carbolinium ion (Ajnax 355 mp, for 438b and 438c). In alkaline solution and in nonionizing solvents absorption at a shorter wavelength (A ax 315 m/x) is observed. In general, solutions of the anhydro-bases in acid and in protic solvents are more deeply colored than their solutions in basic or in non-ionizing media. 

Hydrogen bonding with protic solvents or reagents occurs widely in azines even when they are not appreciably basic and the protic compounds are very poor acids. The latter do not have to be present... 

Solution (a) This is likely to be an SnjI reaction because the substrate is secondary and ben-zyiic, the nucieophiie is weakly basic, and the solvent is protic. 

El eliminations begin with the same uni molecular dissociation we saw in the Sfsjl reaction, but the dissociation is followed by loss of H+ from the adjacent carbon rather than by substitution. In fact, the El and SN1 reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a non-basic nucleophile. Thus, the best El substrates are also the best SN1 substrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 °C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (Sjql) and 2-methylpropene (El) results. 

Hydroxy-l-alkenyl diisopropylcarbamates 2 (X = OCb), in this respect, occupy a medium position since they are stable in strongly acidic and basic protic solvents. For deblocking vinyl carbamates, the presence of catalytic amounts of mercuric or palladium(II) salts is required. Due to this stability, several reactions of homoallylic alcohols, proceeding with high diastereo-selectivity, e g., epoxidation, are applicable in order to introduce further hetero-substituents. 

The reactions in a non-basic aprotic solvent CH2C12 provided solely 10, the product of carbon protonation, while those carried out in an acidic protic solvent HFIP give exclusively 8K, the product of oxygen protonation. The equilibrium protonation may be favored in a protic solvent having abundant protons available. In other basic solvents, the proton donor involved in the reaction should be the conjugate acid of the solvent, and many factors may delicately control the selectivity of the reaction. 

The donor number, DN, of a solvent, proposed by Gutmann, is a measure of the Lewis basicity of the solvent, i.e. its ability to donate a pair of electrons [16]. The DN is determined by measuring the negative enthalpy for the reaction of equimolar quantities of the solvent with the standard Lewis acid, SbCls, at room temperature in 1,2-dichloroethane (Scheme 1.1), and reflects the ability of the solvent to solvate Lewis acids. SbCls reacts with protic solvents such as alcohols... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Miscibility is an important consideration when selecting solvents for use in biphasic systems. Table 4.4 shows the miscibility of three ionic liquids with water and some organic solvents. [bmim][PFe] was found to be miscible with organic solvents whose dielectric constant is higher than 7, but was not soluble in less polar solvents or in water. Basic [bmim][AlCl4] was found to react with protic solvents, and the acidic form also reacted with acetone, tetrahydrofuran and toluene. 

Product 34 predominates in the polar aprotic solvent (acetonitrile), while in the polar protic solvent (methanol) products 35 are formed preferentially. The different products are caused by the relative rate of deprotonation against desilylation of the aminium radical, that is in turn governed by the action of enone anion radical in acetonitrile as opposed to that of nucleophilic attack by methanol. In an aprotic, less silophilic solvent (acetonitrile), where the enone anion radical should be a strong base, the proton transfer is favoured and leads to the formation of product 34. In aprotic solvents or when a lithium cation is present, the enone anion radical basicity is reduced by hydrogen bonding or coordination by lithium cation, and the major product is the desilylated 35 (Scheme 4). 

Nucleophilicities are affected by solvent, and any correlations with basicity can break down in protic solvents like methanol or ethanol. This is because anions are stabilized by hydrogen bonding, and become solvated. These solvation molecules must be lost before the anion can attack as a nucleophile. Accordingly,... 

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