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Oxonium ions acidity

H3O" is strictly the oxonium ion actually, in aqueous solutions of acid this and Other solvated-proton structures exist, but they are conveniently represented as... [Pg.85]

The systematic name for the conjugate acid of water (HjO" ) is oxonium ion Its com mon name is hydronium ion... [Pg.34]

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... [Pg.247]

Once formed the tetrahedral intermediate can revert to starting materials by merely reversing the reactions that formed it or it can continue onward to products In the sec ond stage of ester hydrolysis the tetrahedral intermediate dissociates to an alcohol and a carboxylic acid In step 4 of Figure 20 4 protonation of the tetrahedral intermediate at Its alkoxy oxygen gives a new oxonium ion which loses a molecule of alcohol m step 5 Along with the alcohol the protonated form of the carboxylic acid arises by dissocia tion of the tetrahedral intermediate Its deprotonation m step 6 completes the process... [Pg.851]

Often the requisite THF oxonium ion is generated m situ by using a combination of reagents based on the Meerwein syntheses of trialkyl oxonium salts (150). These combinations include epichlorohydrin or a reactive haUde with a Lewis acid, a reactive hahde with a metal salt, or sometimes just a Lewis acid alone. The epoxide portion is often referred to as a promoter. [Pg.362]

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 ... [Pg.362]

Donation of a proton to the reactant often forms a carbenium ion or an oxonium ion, which then reacts ia the catalytic cycle. For example, a catalytic cycle suggested for the conversion of phenol and acetone iato bisphenol A, which is an important monomer used to manufacture epoxy resias and polycarbonates, ia an aqueous mineral acid solution is shown ia Figure 1 (10). [Pg.162]

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. [Pg.69]

Typical Lewis acids like BF3 and SbCls coordinate with oxirane oxygen to give (presumably) a cyclic oxonium ion (41) which reacts further (Scheme 28) (64HC 19-1)446, B-67MI50505). [Pg.105]

Coe et al. reported an efficient modification for the preparation of /-substituted indole analogs for biology screening in good yield. The intermediate P-nitrostyrene 44, prepared from the condensation of 43 with DMFDMA, underwent methanolysis and reduction to provide the aniline acetal intermediate 45. Alkylation of amine 45 was carried out employing standard conditions of reductive alkylation to provide A-alkyl analogs represented by 46. The indole 47 was generated by formation of the oxonium ion (from 46) under acidic conditions, followed by cyclization, accompanied by loss of methanol. [Pg.107]

Another similarity with water is that alcohols and phenols are both weakly basic and weaJkly acidic. As weak bases, they are reversibly protonated by strong acids to yield oxonium ions, TOFty. ... [Pg.603]

Nucleophilic addition of an alcohol to the carbonyl group initially yields a hydroxy ether called a hemiacetal, analogous to the gem diol formed by addition of water. HcmiacetaJs are formed reversibly, with the equilibrium normally favoring the carbonyl compound. In the presence of acid, however, a further reaction occurs. Protonation of the -OH group, followed by an El-like loss of water, leads to an oxonium ion, R2C=OR+, which undergoes a second nucleophilic addition of alcohol to yield the acetal. The mechanism is shown in Figure 19.12. [Pg.717]

Contained within intermediate 25 is an acid-labile mixed acetal group and it was found that treatment of 25 with camphorsulfonic acid (CSA) results in the formation of dioxabicyclo[3.3.0]octane 26 in 77 % yield. Acid-induced cleavage of the mixed cyclic acetal function in 25, with loss of acetone, followed by intramolecular interception of the resultant oxonium ion by the secondary hydroxyl group appended to C leads to the observed product. Intermediate 26 clearly has much in common with the ultimate target molecule. Indeed, the constitution and relative stereochemistry of the dioxabicyclo[3.3.0]octane framework in 26 are identical to the corresponding portion of asteltoxin. [Pg.328]

The relationship between 9 and its predecessor 10 is close. Oxidation of the allylic C-3 methylene group in 10 and elimination of the methoxy group could furnish enone 9. Retrosynthetic disassembly of ring E in 10 furnishes tertiary alcohol 11 as a viable precursor. That treatment of 11 with a catalytic amount of acid will induce the formation of a transient oxonium ion at C-12 which is then intercepted by the appropriately placed C-4 tertiary hydroxyl group is a very reasonable proposition. As we will see, the introduction of the requisite C-4 hydroxyl group is straightforward from intermediate 12. [Pg.455]

The hydroxylation reaction, whose stereochemical course is controlled by the strong inherent preference for the formation of a cis-fused 5,5 ring system, creates a molecule which would appear to be well suited for an intramolecular etherification reaction to give ring E of ginkgolide B (1). Indeed, when a solution of 11 in methylene chloride is exposed to camphorsulfonic acid (CSA), a smooth cycli-zation reaction takes place to give intermediate 10 in an overall yield of 75% from 12. The action of CSA on 11 produces a transient oxonium ion at C-12 which is intercepted intramolecularly by the proximal hydroxyl group at C 4. [Pg.461]

Cations which are covalently attached to the allyl anion part by a cr-bond and have sufficient Lewis acid properties offer the broadest versatility and highest levels of stereocontrol, since the C—C bond-forming step can occur in a pericyclic process9 accompanied by allylic inversion. It is reasonable to assume the prior assembly of both reaction partners in an open-chain complex, in which usually the (F )-oxonium ion, avoiding allylic 1,3-strain10, is predominant. [Pg.208]

Oxidation of either alkyl or aryl sulphoxides to sulphones in 65-90% yields may be accomplished by treatment with a nitronium salt15. In the case of aryl sulphoxides no nitration is observed (which is in contrast to the results of nitric acid oxidation). The reaction was shown to proceed through intermediate nitratosulphonium and nitritosulph-oxonium ions, as depicted in equation (7), which were studied by nmr spectroscopy. [Pg.971]

Ring expansion of tetrahydrofurans to dihydropyrans results when their 2-W-aziiidinyl imines are heated <96CC909> and when their 2-ca-alkyl bromides are treated with Ag20 in a nucleophilic acidic solvent <96JCS(P1)413>. Alkyl carbenes and bicyclic oxonium ion intermediates are invoked, respectively. [Pg.291]

For an 0-methylated oxonium ion, X-0(Me)H, we use a related equation based on the pXBe+ for dimethyl ether.Per acidic hydrogen... [Pg.41]

Bu2Sn(03SCF3)2.68 The Lewis acids promote ionization of the acetal to an oxonium ion that acts as the electrophile. The products are (J-alkoxy ketones. [Pg.85]

The use of 2-vinyldioxolane, the ethylene glycol acetal of acrolein, as a dienophile illustrates application of the masked functionality concept in a different way. The acetal itself would not be expected to be a reactive dienophile, but in the presence of a catalytic amount of acid the acetal is in equilibrium with the electrophilic oxonium ion. [Pg.493]

Aldehydes can be obtained by reaction of Grignard reagents with triethyl orthoformate. The addition step is preceded by elimination of one of the alkoxy groups to generate an electrophilic oxonium ion. The elimination is promoted by the magnesium ion acting as a Lewis acid.93 The acetals formed by the addition are stable to the reaction conditions, but are hydrolyzed to aldehydes by aqueous acid. [Pg.638]

To be of maximum synthetic value, the generation of the cationic site that initiates cyclization must involve mild reaction conditions. Formic acid and stannic chloride are effective reagents for cyclization of polyunsaturated allylic alcohols. Acetals generate oxonium ions in acidic solution and can also be used to initiate the cyclization of... [Pg.864]

Scheme 10.2 gives some examples of ene and carbonyl-ene reactions. Entries 1 and 2 are thermal ene reactions. Entries 3 to 7 are intermolecular ene and carbonyl-ene reactions involving Lewis acid catalysts. Entry 3 is interesting in that it exhibits a significant preference for the terminal double bond. Entry 4 demonstrates the reactivity of methyl propynoate as an enophile. Nonterminal alkenes tend to give cyclobutenes with this reagent combination. The reaction in Entry 5 uses an acetal as the reactant, with an oxonium ion being the electrophilic intermediate. [Pg.877]

The concentration of oxonium ions increases with increasing strength of the acid and the pH decreases. [Pg.65]

The case of the salt of a weak base and strong acid is treated in an analogous way. With decreasing strength of the base, the concentration of oxonium ions increases and the pH decreases. [Pg.65]

An exact description of the acidity of solutions and correlation of the acidity in various solvents is one of the most important problems in the theory of electrolyte solutions. In 1909, S. P. L. S0rensen suggested the logarithmic definition of acidity for aqueous solutions considering, at that time, of course, hydrogen instead of oxonium ions (cf. Eq. (1.4.11))... [Pg.74]

The conductivity also increases in solutions of weak electrolytes. This second Wien effect (or field dissociation effect) is a result of the effect of the electric field on the dissociation equilibria in weak electrolytes. For example, from a kinetic point of view, the equilibrium between a weak acid HA, its anion A" and the oxonium ion H30+ has a dynamic character ... [Pg.109]

First, we shall discuss reaction (5.7.1), which is more involved than simple electron transfer. While the frequency of polarization vibration of the media where electron transfer occurs lies in the range 3 x 1010 to 3 x 1011 Hz, the frequency of the vibrations of proton-containing groups in proton donors (e.g. in the oxonium ion or in the molecules of weak acids) is of the order of 3 x 1012 to 3 x 1013 Hz. Then for the transfer proper of the proton from the proton donor to the electrode the classical approximation cannot be employed without modification. This step has indeed a quantum mechanical character, but, in simple cases, proton transfer can be described in terms of concepts of reorganization of the medium and thus of the exponential relationship in Eq. (5.3.14). The quantum character of proton transfer occurring through the tunnel mechanism is expressed in terms of the... [Pg.364]


See other pages where Oxonium ions acidity is mentioned: [Pg.379]    [Pg.369]    [Pg.546]    [Pg.815]    [Pg.111]    [Pg.701]    [Pg.740]    [Pg.304]    [Pg.58]    [Pg.492]    [Pg.160]    [Pg.255]    [Pg.379]    [Pg.588]    [Pg.181]    [Pg.134]    [Pg.184]    [Pg.357]   
See also in sourсe #XX -- [ Pg.48 ]




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