Proton acid

Protonation of (benzo)pyridazines has been mentioned in Section 6.01.4.2 and was reviewed in CHEC-I 84CHEC-l(3B)l . Pyridazine (pA 2.3) is more basic than the other diazines due to reduced lone pair lone pair repulsion on formation of the cation. There is no equivalent to the benzene-like 

3-position of pyridine, and in the absence of marked steric effects all mesomeric electron donors will show a strong interaction with an ortho, or more especially a para nitrogen, and for example, 

Pyridines form stable salts with strong acids. Yellow ionic picrates were used for characterization in the past. Pyridine itself is often used to neutralize acid formed in a reaction and as a basic solvent. The basicity of pyridine (as measured by the dissociation constant of its conjugate acid, pKa 5.2) is less than that of aliphatic amines (cf. NH3, pA a 9.5 NMe3, pKd 9.8). This reduced basicity is probably due to the changed hybridization of the nitrogen atom in ammonia the lone electron pair is in an sp3-orbital, but in pyridine it is in an s/r-orbital. The higher the s character of an orbital, the more it is concentrated near the nucleus, and the less available for bond formation. Nitriles, where the lone electron pair is in an. vp-orbital, are of even lower basicity. 

The basicity of the diazines is sharply reduced from that of pyridine (pAfa 5.2) the pKa of pyrazine is 0.4, pyrimidine is 1.1 and pyridazine is 2.1. The significantly higher basicity of pyridazine as compared to pyrazine, unexpected for mesomeric and inductive effects, is attributed to the lone pair-lone pair repulsion which is removed in the cation. 

The basicities of triazines and tetrazines are also low (e.g., for 1,3,5-triazine pKa 1.0), but few quantitative data are available. 

A fused benzene ring has little effect on the pKa values in the cases of quinoxaline (ca. 0.6) and cinnoline (2.6). Quinazoline has an apparent pAfa of 3.3 which makes it a much stronger base than pyrimidine, but this is due to covalent hydration of the quinazolinium cation (see Section 3.2.1.6.3) the true anhydrous pK.d for equilibrium between the anhydrous cation and anhydrous neutral species of quinazoline is 1.95 (76AHC(20)128). 

Quino[7,8-/i]quinoline (50) and benzo[l, 2-h 4,3-/r Jdiquinoline (52) belong to the so-called proton sponges and possess abnormally high basicities, pKa= 12.8 and 10.3, respectively (89AG(E)84, 89AG(E)86). This is mostly due to strong destabilization of both bases because of electrostatic repulsion 

The sucrose inversion has been extensively studied from the viewpoint of electrolyte effects (Guggenheim and Wiseman, 2), the application of the Arrhenius equation to the reaction (Leininger and Kilpatrick, 3), and the catalytic effects of acid molecules (Hammett and Paul, 4). It is probable that, in aqueous solution, we are dealing with a case of specific hydrogen ion catalysis and can postulate the equilibrium (Gross, Steiner, and Suess, 5) 

When the source of the catalytically active hydrogen ion is a weak acid, one has to consider the weak electrolyte equilibrium involved and the change of the dissociation constant with electrolyte concentration, medium, and temperature. Br0nsted (7) termed this phenomenon secondary kinetic salt effect, but the writer would prefer to omit the word kinetic and substitute electrolyte for salt. The understanding of these 

The work of Brpnsted and Pedersen (23) on the catalytic decomposition of nitramide and the kinetic studies of Lowry and Faulkner (24) on the mutarotation of tetramethylglucose led to the formulation of a more general viewpoint on acids and bases which logically showed that the hydrogen ion and hydroxyl ion were not the unique carriers of acid and basic properties. An acid was defined as any substance capable of donating a proton, and a base any substance capable of accepting a proton. In accordance with this definition (Lowry, 25 Brpnsted, 26), the following substances are typical acids and bases  

In aqueous solution, water is both an acid and a base and we deal with a double acid-base equilibrium 

In the case of the strong acids HCIO4, HCl, HNO3, H2SO4 K 10), the equilibrium is shifted so far to the right in dilute aqueous solutions that the anion does not exhibit basic properties. 

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When we use any substance as a solvent for a protonic acid, the acidic and basic species produced by dissociation of the solvent molecules determine the limits of acidity or basicity in that solvent. Thus, in water, we cannot have any substance or species more basic than OH or more acidic than H30 in liquid ammonia, the limiting basic entity is NHf, the acidic is NH4. Many common inorganic acids, for example HCl, HNO3, H2SO4 are all equally strong in water because their strengths are levelled to that of the solvent species Only by putting them into a more acidic... 

He observed an f-factor of 3 and argued for the formation of the di-protonated acid. He interpreted the high electrical conductivity of these media in support of this. 

The catalysts for cationic polymerization are either protonic acids or Lewis acids, such as H2SO4 and HCIO4 or BF3, AICI3, and TiCl4 ... 

Protonic acids dissociate to some extent in the nonaqueous reaction mixtures to produce an equilibrium concentration of protons ... 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Acidic Cation-Exchange Resins. Brmnsted acid catalytic activity is responsible for the successful use of acidic cation-exchange resins, which are also soHd acids. Cation-exchange catalysts are used in esterification, acetal synthesis, ester alcoholysis, acetal alcoholysis, alcohol dehydration, ester hydrolysis, and sucrose inversion. The soHd acid type permits simplified procedures when high boiling and viscous compounds are involved because the catalyst can be separated from the products by simple filtration. Unsaturated acids and alcohols that can polymerise in the presence of proton acids can thus be esterified directiy and without polymerisation. 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

The low temperature limitation of homogeneous catalysis has been overcome with heterogeneous catalysts such as modified Ziegler-Natta (28) sohd-supported protonic acids (29,30) and metal oxides (31). Temperatures as high as 80°C in toluene can be employed to yield, for example, crystalline... 

Orientation in azole rings containing three or four heteroatoms Effect of azole ring structure and of substituents Proton acids on neutral azoles basicity of azoles Proton acids on azole anions acidity of azoles Metal ions... 

Proton acids on neutral azoles, basicity of azoles... 

Hydrogen peroxide has a rich and varied chemistry which arises from (i) its ability to act either as an oxidizing or a reducing agent in both acid and alkaline solution, (ii) its ability to undergo proton acid/base reactions to form... 

At least four series of periodates are known, interconnected in aqueous solutions by a complex series of equilibria involving deprotonation, dehydration and aggregation of the parent acid H5IO6 — cf. telluric acids (p. 782) and antimonic acids (p. 577) in the immediately preceding groups. Nomenclature is summarized in Table 17.24, though not all of the fully protonated acids have been isolated in the free state. The structural relationship between these acids, obtained mainly from X-ray studies on their salts, are shown in Fig. 17.24. H5IO6 itself (mp 128.5° decomp) consists of molecules of (HO)sIO linked into a three-dimensional array by O-H - O bonds (10 for each molecule, 260-278 pm). 

The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)]. ... 

In the case when boron fluoride or its etherate is employed, the protonic acid is formed according to the following schemes (water is produced in the reaction) ... 

Strong protonic acids can affect the polymerization of olefins (Chapter 3). Lewis acids, such as AICI3 or BF3, can also initiate polymerization. In this case, a trace amount of a proton donor (cocatalyst), such as water or methanol, is normally required. For example, water combined with BF3 forms a complex that provides the protons for the polymerization reaction. 

The rearrangement of 3 -benzylspiro[2//-1-benzothiopyran-3(4//),2 -oxirane]s 7, induced by Lewis or proton acid catalysts, gives the seven-membered ring dione systems 8. Compounds... 

The molecular ion can be very small or nonexistent. Esters where R is greater than methyl form a protonated acid that aids in the interpretation (e.g., m/z 47, formates m/z 61, acetates m/z 75. propionates m/z 89, butyrates etc.). Interpreting the mass spectra of ethyl esters may be confusing without accurate mass measurement because the loss of C2H4 can be confused with the loss of CO from a cyclic ketone. 

Molecular ion Although the molecular ion is always observed, the loss of 31 Daltons (OCFF) is the most intense ion. Generally, the acid and/or protonated acid is observed. Ortho substituents are distinguished by their large peaks at [M - 32]+, as well as [M - 31]+. A small peak is observed at [M - 60]+. 

Generally the acid or protonated acid is observed. The aromatic alcohols can be differentiated by the loss of 18 Daltons from the molecular ion. 

Activation Parameters of Non-catalyzed and Protonic Acid-catalyzed Esterifications and Polyesterifications. 83... 

It is not only in the field of kinetic relations that discrepancies exist. When the catalyst is a protonic acid and the reaction is carried out in dilute solution, the mechanisms describing the contribution of the catalyst are relatively well-known. But in most other cases and particularly when the catalyst is a metal derivative (see Chap. 4) none of the proposed mechanisms can be considered as definitive. 

Protonic acids Protonic Derivatives They have been used with most systems. Some 20-25,60. 

In all the preceding studies, the active species was supposed to be R OH however, later, many authors, using labelled molecules, considered that it is the protonated acid RC(OH) instead. 

In the cases where a strong protonic acid is added as a catalyst orders 2 with respect to acid and 0 with respect to alcohol have been found. It is assumed that free ions are present which leads to ... 

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