Why is phenol so reactive

Phenol is a very weak acid and the position of equilibrium lies well to the left. Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form. One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring. This overlap leads to a delocalization which extends from the ring out over the oxygen atom.

As a result, the negative charge is no longer entirely localized on the oxygen, but is spread out around the whole ion. Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen.

However, oxygen is the most electronegative element in the ion and the delocalized electrons will be drawn towards it. That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again.

That is why phenol is only a very weak acid. Why is phenol a much stronger acid than cyclohexanol? To answer this question we must evaluate the manner in which an oxygen substituent interacts with the benzene ring. As noted in our earlier treatment of electrophilic aromatic substitution reactions, an oxygen substituent enhances the reactivity of the ring and favors electrophile attack at ortho and para sites.

It was proposed that resonance delocalization of an oxygen non-bonded electron pair into the pi-electron system of the aromatic ring was responsible for this substituent effect. A similar set of resonance structures for the phenolate anion conjugate base appears below the phenol structures. The resonance stabilization in these two cases is very different. An important principle of resonance is that charge separation diminishes the importance of canonical contributors to the resonance hybrid and reduces the overall stabilization.

The facility with which the aromatic ring of phenols and phenol ethers undergoes electrophilic substitution has been noted. Two examples are shown in the following diagram. The second reaction is interesting in that it further demonstrates the delocalization of charge that occurs in the phenolate anion. Carbon dioxide is a weak electrophile and normally does not react with aromatic compounds; however, the negative charge concentration on the phenolate ring enables the carboxylation reaction shown in the second step.

The sodium salt of salicylic acid is the major product, and the preference for ortho substitution may reflect the influence of the sodium cation. This is called the Kolbe-Schmidt reaction , and it has served in the preparation of aspirin, as the last step illustrates.

Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxyl bearing carbon. Among the colored products from the oxidation of phenol by chromic acid is the dicarbonyl compound para-benzoquinone also known as 1,4-benzoquinone or simply quinone ; an ortho isomer is also known. These compounds are easily reduced to their dihydroxybenzene analogs, and it is from these compounds that quinones are best prepared. The browning reactions have generally been assumed to be a direct consequence of phenolic compound oxidation by PPO action [ 51 ].

However, at least a partial role may be attributed to the action of peroxidase [ 52 — 54 ]. The mixing of phenolic compounds with polyphenol oxidase and peroxidase enzymes in the presence of oxygen produces colored pigments [ 55 ]. When there is disruption of the cell, some compounds such as phenolic acids suffer the action of the polyphenol oxidases that induces oxidation of phenolics and results in dark compound formation [ 56 ]. In addition, polyphenol oxidases catalyze two different reactions.

This quinone formation suffers polymerization and causes yellow and brown coloring. The peroxidase enzyme is one the most important enzymes responsible for polyphenol degradation. This enzyme is generally considered as the reference enzyme for blanching treatments, due to its high thermal resistance and high concentration in most vegetables.

In order to inhibit the polyphenol oxidases and peroxidases some treatments have been used, including the addition of ascorbic acid or chemical agents sulfites , exclusion of oxygen, refrigeration, and nonthermal treatments. The blanching treatment thus demonstrated to be extremely effective in reducing the PPO activity and maximizing anthocyanin recovery.

This effect should be a result of the complete inactivation of the PPO enzyme, or of the greater extraction yield linked to the increase of fruit skin permeability caused by the heat treatment [ 60 ].

In some eggplant genotypes, cooking processes as boiling and grilling induced a drastic decrease in PPO activity, with a little residual PPO activity [ 61 ]. Whereas, in wheat flour, the PPO showed the maximum decrease in its activity when processed in microwave The strong decrease of the PPO activity, after microwave, can be attributed to the higher heating uniformity and higher penetration power of the microwave.

The activity of the enzymes PPO and POD are closely related, acting in a combined form in the darkening of fruits and vegetables. The phenol oxidation by PPO produces hydrogen peroxide H 2 O 2 , independently on the substrate used. The POD catalyzes the phenolic compound oxidation, since there is a high affinity between the H 2 O 2 , produced by the PPO, that acts as an electron acceptor and the vegetal phenolic compounds that work as electron donators substrates: catechin, quercetin and its glycosides.

This process promotes the oxidation of phenolic compounds and produces quinones that affect color, flavor, texture, and loss of the nutritional and functional quality [ 53 , 62 ]. In some cases, the inactivation can be obtained at 80 o C, which would explain, partially, the reason why the amount of phenolic compounds increases when the product is taken to high temperatures, as used during pasteurization or other procedures [ 62 — 65 ].

This process is regularly used in beverage industries to remove phenolic constituents that give color, astringency, and bitterness to the juices. To remove all undesirable molecules, the clarification process consists of the addition of gelatin, bentonite, polyvinylpolypyrrolidone PVPP , and kieselsol, among others.

Employing this method helps to avoid the formation of cloudy appearance during processing and storage. However, besides the direct reduction of phenolic compounds induced by clarification, the ultrafiltration process can remove the phenolic compounds that are complexed with proteins [ 67 ]. On the other hand, pasteurization is one of the methods used in industrial scale that causes the highest losses of bioactive compounds. According to Azofeifa, after the pasteurization of the blackberry Rubus adenotrichus Schltdl juice, many phenolics may be lost [ 69 ].

Another study showed that the increase of pasteurization temperatures promotes loss of phenolic compounds in orange juice [ 70 ]. New emerging technologies have been used in the beverage industry, such as high intensity pulsed electric field HIPEF , high pressure, and ultrasound.

Another processing that affects the amount of phenolic compounds is the fermentative process. Currently, in the food processing, there is a great concern about the residues formed by the action of some compounds used during the sanitation.

During harvest, transportation, or processing, the tissues may be mechanically injured that facilitates the food contamination [ 78 ]. In many countries, the use of chlorinated compounds, particularly the hypochlorite salts, is very common to minimize the pathogen infestation rate. Sodium hypochlorite is a potent sanitizer that has oxidant action and is used in domestic or industrial food processing. Due to these effects, its use has been forbidden in organic foods. The relation between products containing chlorine and the phenolic compound content has been investigated and there is still no consensus about the results.

In mushrooms, the use of sodium hypochlorite at room temperature induced the disappearance of phenolics and the formation of their oxidation products [ 80 ]. One of the alternatives for hypochlorite is ozone O 3 , used as an antimicrobial agent since the end of the nineteenth century to purify potable water. The use of O 3 has many advantages over other chemical oxidants, its precursors are numerous and economically profitable and can be used in gaseous or aqueous state, depending on the product [ 83 , 84 ].

Beyond its antimicrobial activity against a wide range of microorganisms, O 3 can destroy chemical residues and convert nonbiodegradable organic materials into biodegradable materials [ 85 ].

At the same time, due to its fast decomposition into oxygen and to the fact that it does not form residues in the treated products, its use in the food processing is authorized by the organic certification [ 86 ]. Various research groups have studied the relation of the ozone action with the phenolic compounds, but contradictory results about its action have been described. Certainly, some fruits and vegetables may be more susceptible to the action of this gas and may show different responses, mainly regarding the stress caused by the oxidant action.

Both the time and ozone concentration may cause different responses. In pineapple, banana, and guava, the application of gaseous O 3 induced a significant increase in total phenolics, whereas in bananas and pineapples, the flavonoid content increased in response to up to 20 min of ozone treatment.

For guava fruits, the flavonoid content increased and total phenolic decreased inversely when these fruits were exposed up to 10 min [ 87 ]. The increase of the phenolic compounds and flavonoid contents in these fruits may be caused by other factors, such as the modification of the cell wall that occurs when a plant cell is exposed to ozone, increasing the extractability and release of the phenolics bonded to the cell wall.

In kiwifruit stored in an enriched environment with ozone and under refrigeration for 30 days or for 3 months followed by 12 days of shelf life, there was an increase in the content of phenolic compounds [ 88 ]. The decrease in the content of some polyphenols caused by the treatment with ozone in grape juice has been described [ 89 ]. After the use of ozone at low concentration 1. However, at higher ozone concentrations 7.

The anthocyanin degradation influenced in the color of grape juice due to ozone processing can be attributed to the strong oxidizing potential of ozone [ 90 ]. Thus, ozone can induce increase or decrease of some phenolic content. In organic and conventional Palmer mangos, sanitized with ozonized water, no effect in phenolic content was observed [ 91 ]. The variations in the contents of these compounds were in function of the cultivation mode organic or conventional of the fruit. A similar result was described in organic or conventional cabbage treated with ozonized water.

There were no variations in the total phenols or flavonoid contents due to sanitizing treatment [ 92 ]. This treatment may also accelerate the ethylene production and, consequently, activate the expression of ethylene response factor ERF genes. The alteration of the ERF expression, through hormonal induction or abiotic stress, may induce secondary metabolic pathways, e.

In addition, the irradiation improved the antioxidant capacity, which is probably related to the increase of the phenolic compound content [ 99 ]. Among the gamma radiation effects, we can highlight the delay in the maturation, the reduction of the microorganisms in grains, cereals, fruits and spices, reduced storage losses, and extended shelf life.

However, the irradiation may induce stress signals and stress responses in fruits and vegetables that increases the antioxidant compounds [ ]. In juices of carrot Daucus carota var.

The influence of different radiation doses 1, 5, and 8 kGy were also verified on the color, organic acids, total phenolics, total flavonoids, and antioxidant activity of dwarf mallow Malva neglecta Wallr. Irradiation at 5 kGy increased the amounts of citric and succinic acids, and decreased the fumaric acid levels.

In contrast, in the decoction prepared, the antioxidant properties and levels of total phenolics and flavonoids were decreased with the 8 kGy dose.