Phenoxide anions

An alternative approach is to increase the nucleophihcity of the phenol by con verting it to its phenoxide anion m basic solution... 

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

Phenols that bear strongly electron withdrawing substituents usually give low yields of carboxylated products their derived phenoxide anions are less basic and the equilibrium constants for their carboxylation are smaller... 

Aryl ethers are best prepared by the Williamson method (Section 16 6) Alkylation of the hydroxyl oxygen of a phenol takes place readily when a phenoxide anion reacts with an alkyl halide... 

With pA a s of approximately 10 phenols are stronger acids than alcohols but weaker than carboxylic acids They are converted quantitatively to phenoxide anions on treatment with aqueous sodium hydroxide... 

Section 24 11 Phenoxide anions are nucleophilic toward alkyl halides and the prepara tion of alkyl aryl ethers is easily achieved under 8 2 conditions... 

All lation. In alkylation, the dialkyl sulfates react much faster than do the alkyl haHdes, because the monoalkyl sulfate anion (ROSO ) is more effective as a leaving group than a haHde ion. The high rate is most apparent with small primary alkyl groups, eg, methyl and ethyl. Some leaving groups, such as the fluorinated sulfonate anion, eg, the triflate anion, CF SO, react even faster in ester form (4). Against phenoxide anion, the reaction rate is methyl triflate [333-27-7] dimethyl sulfate methyl toluenesulfonate [23373-38-8] (5). Dialkyl sulfates, as compared to alkyl chlorides, lack chloride ions in their products chloride corrodes and requires the use of a gas instead of a Hquid. The lower sulfates are much less expensive than lower bromides or iodides, and they also alkylate quickly. 

Electrostatic potential map for phenoxide anion shows most negatively-charged regions (in red) and less negatively-charged regions (in blue). 

Nucleophiles can also act as acids and bases, and this behavior substantially alters their nucleophilicity. At pH 5, trimethylamine exists mainly as its conjugate acid, trimethylammonium cation. First draw a Lewis structure, and then examine the electrostatic potential for trimethylammonium ion. On the basis of the map, which is the better nucleophile, the cation or the corresponding neutral amine At pH 12, phenol exists mainly as its conjugate base, phenoxide anion. First draw a Lewis structure (or series of Lewis structures), and then examine the electrostatic potential map for phenoxide anion. Which is the better nucleophile, phenoxide or phenol ... 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

What controls selectivity Draw the products that would result from O addition and from ortho C addition (as well as "para C addition shown above) of phenoxide anion and phenyl diazonium ion. Compare the energies of these products (para C product, ortho C product and O product). Which is most stable Is this the observed product Can thermodynamics explain the outcome ... 

Finally, examine the highest-occupied molecular orbital (HOMO) of phenoxide anion. Is the HOMO the best electron-donor orbital Is the orbital localized primarily on oxygen or on carbon Is the observed product consistent with orbital control Explain your answers. 

HOMO of phenoxide anion reveals the most nucleophilic sites. 

The optimal pH-value for the coupling reaction depends on the reactant. Phenols are predominantly coupled in slightly alkaline solution, in order to first convert an otherwise unreactive phenol into the reactive phenoxide anion. The reaction mechanism can be formulated as electrophilic aromatic substitution taking place at the electron-rich aromatic substrate, with the arenediazonium ion being the electrophile ... 

Phenols (ArOH) are relatively acidic, and the presence of a substituent group on the aromatic ring has a large effect. The pKa of unsubstituted phenol, for example, is 9.89, while that of p-nitrophenol is 7.15, Draw resonance structures of the corresponding phenoxide anions and explain the data. 

Dichloro monomers can also be polymerized with bisphenols in the presence of fluorides as promoting agents.78 The fluoride ions promote the displacement of the chloride sites to form more reactive fluoride sites, which react with phenolate anion to form high-molecular-weight polymers. Adding 5-10 mol % phase transfer catalysts such as A-alkyl-4-(dialkylamino)pyridium chlorides significantly increased the nucleophilicity and solubility of phenoxide anion and thus shortened the reaction time to one fifth of the uncatalyzed reaction to achieve the same molecular weight.79... 

The stabilisation that can result by delocalisation of a positive or negative charge in an ion, via its n orbitals, can be a potent feature in making the formation of the ion possible in the first place (cf. p. 55). It is, for instance, the stabilisation of the phenoxide anion (23), by delocalisation of its charge via the delocalised n orbitals of the nucleus, that is largely responsible for the acidity of phenol (cf. p. 56) ... 

Reference has already been made to electron-donating and electron-withdrawing groups, their effect being to render a site in a molecule electron-rich or electron-deficient, respectively. This will clearly influence the type of reagent with which the compound will most readily react. An electron-rich species such as phenoxide anion (36)... 

The operation of the electron-withdrawing inductive effect can, however, be seen in the fact that the very small amount of m-attack (for which there is no specific stabilisation of the a complex by de-localisation) occurs more slowly than attack on benzene itself (cf. p. 158). In the case of the phenoxide anion,... 

The product from phenoxide ion (64) is, after acidification, very largely the o-aldehyde (salicylaldehyde, 65) plus just a small amount of the p-isomer. If both o-positions in the initial phenoxide anion are substituted, however, reaction then yields the p-aldehyde. 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

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