Weak proton acids, reaction

Alkanes are rather unreactive compounds and are sometimes called paraffins (from the Latin for Tittle affinity ). They have strong C—C and C—H bonds, which are hard to break. All their electron pairs are in relatively stable sigma bonding MOs. They have no polar sites or unshared electrons to attract Lewis acid or base reagents. They are extremely weak proton acids. Therefore, only a few of the reactions covered in this text involve alkanes. 

A few of these reactions demand special comment. Dimethyl-formamide must act as a nucleophile in attacking an epoxide/ BF3 complex to give the ion (7), which suffers hydrolysis in water to give the 6j5-formate [152]. Boron trifiuoride and triethyl aluminium, both efficient Lewis acids, are able to promote reactions with substances which are only very weak proton acids e.g, alcohols and HCN). This may be due to... 

The reactions of 1 with weak protonic acids such as 2-thiol-pyridine or 2-sulfanylbenzothiazole that can act as ligands either in the protonated or in the de-protonated form, do not follow the latter reaction pathway and unexpectedly ... 

Why are acid-base properties presented for alkanes The answer is because it is important to define relative acidity as it relates to other compounds. Establishing that alkanes are considered to be extremely weak acids or bases gives a benchmark for categorizing other weak protonic acids and bases. In Chapter 15, a reaction will be discussed that generates an alkane as a conjugate acid. Categorizing alkanes as very weak acids rather than nonacids will help explain that reaction (see Sections 15.3 and 15.5.2). 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

J.ll In each of the following salts, either the cation or the anion is a weak acid or a weak base. Write the chemical equation for the proton transfer reaction of this cation or anion with water (a) NaC6H50 (b) KCIO (c) C,HSNHCI ... 

J.I3 Na As04 is a salt of a weak base that can accept more than one proton, (a) Write the chemical equations for the sequential proton transfer reactions of the anion with water. Identify the acid and the base in each reaction, (b) If 35.0 g of Na3As04 is dissolved in water to make 250.0 ml. of solution, how many moles of sodium cations are in the solution ... 

Another example of an acid is hydrogen cyanide, HCN, which transfers its proton to water when it dissolves to form the solution known as hydrocyanic acid, HCN(aq). However, only a small fraction of the HCN molecules donate their protons, and so we classify HCN as a weak acid in water. We write the proton transfer reaction with equilibrium half-arrows ... 

In a solution of a weak acid, the major species are water molecules and the acid, HA. The products of the proton transfer reaction, H3 0+ and A, are present in smaller concentrations as minor species. Figure 17-5 provides a molecular view. 

Each proton-transfer reaction creates HC2 O4, which is itself a weak acid. The values show that... 

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

As long as the buffer solution contains acetic acid as a major species, a small amount of hydroxide ion added to the solution will be neutralized completely. Figure 18-1 shows two hydroxide ions added to a portion of a buffer solution. When a hydroxide ion collides with a molecule of weak acid, proton transfer forms a water molecule and the conjugate base of the weak acid. As long as there are more weak acid molecules in the solution than the number of added hydroxide ions, the proton transfer reaction goes virtually to completion. Weak acid molecules change into conjugate base anions as they mop up added hydroxide. 

The palladium may then be considered as either Pd° ligated by a proton or as Pd11 ligated by a hydride,376 albeit that the very negative shift of the H-NMR signal is more indicative of a hydride linked to a palladium(II) center. This is why this reaction is also considered as an oxidative addition.376 When weak carboxylic acids are involved (e.g., acetic acid, formic acid), the reaction is regarded as an oxidative addition, i.e., insertion of palladium(0) into the H X bond (see Equation (2)) 367,376... 

The reactions are faster in the presence of weak protic acids, which can quench the initial products under conditions where the stannylcuprate reagents are not affected. This is illustrated in Scheme 2. With no internal proton source, the thermodynamically more stable result of Irans-addition is favored, but, in the presence of a proton source, the initial adducts resulting from m-metallostannation are trapped.116,117,118... 

Two types of electrogenerated carbon bases have commonly been used (1) dianions derived from activated alkenes, and (2) carbanions formed by reductive cleavage of halogen compounds or by direct reduction of weak carbon acids. In both cases, the efficiency of the proton transfer reaction relies on a thermodynamically favored proton transfer or a fast follow-up reaction of the deproto-nated substrate. 

In 2007, Tron and Zhu reported the multicomponent synthesis of 5-iminoox-azolines (42) starting from a,a-disubstituted secondary isocyano amides (41), amines, and carbonyl components (see Fig. 15) [155]. The reaction presumably follows a similar mechanism as in the 2,4,5-trisubstituted oxazole MCR (described in Fig. 11) however, because of the absence of a-protons at the isocyano amide 41, the nonaromatized product is obtained. As in the 2,4,5-trisubstituted oxazole MCR, toluene was found to be the optimal solvent in combination with a weak Brpnsted acid. The reaction was studied for a range of aldehydes and secondary amines. In addition, a variety of functionalities such as acetate, free hydroxyl group, carbamate, and esters are tolerated. Clean conversions were observed for this MCR as indicated by NMR analysis of the crude products (isolated yield 50-68%). The... 

Bransted acids (proton donors, HA) have significant influences on the reduction of organic compounds in aprotic solvents. If a weak Bronsted acid like water is added step-wise to the electrolytic solution, the height of the first polarographic wave increases at the expense of that of the second wave (Fig. 8.12). By the addition of a weak acid, the following reactions occur at or near the electrode ... 

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