4. The mechanism of phenol action as inhibitors

Involving phenols in radical reactions results in detachment of hydrogen atom from hydroxyl group on the first stage of the process with the formation of phenoxyl radical:

$$\text{PhOH} + \text{R}^\* \rightarrow \text{RH} + \text{PhO}^\* \tag{1}$$

The formation of phenoxyl radicals was confirmed by electron paramagnetic resonance (EPR) method, which was supported with kinetic isotopic effect, when hydrogen atom is changed for deuterium.

Phenoxylic radicals are formed in the reaction of phenols with peroxide radicals: alkoxyl, alkyl, carboxyl radicals, as well as with molecular oxygen. The subsequent behaviors of phenols in radical reactions depend on properties of the phenoxyl radicals formed.

The reaction of peroxide and phenolic radicals leads to the formation of quinolide peroxides. Another active radicals add to PhO\* similarly. The quinolide peroxides so formed could be decomposed at O-O bond to give active radicals even at moderate temperatures:

The alkoxyl radical could be subsequently dissociated to give quinone:

Hydroquinone, when reacting step by step with two peroxide radicals, transforms into quinone:

Together with the interaction of phenoxyl and peroxide radicals, the competitive reaction bimolecular loss of phenoxyl radicals takes place:

Modified Byproduct of Coke Phenols as Effective and Prospective Inhibitors for Petrochemical Industry http://dx.doi.org/10.5772/66886 307

$$\text{PhO}^\* + \text{PhO}^\* \rightarrow \text{Products} \tag{2}$$

The reaction direction depends on a structure of phenoxyl radical. Some radicals having at least one non-substituted "active" position are typically recombined into dimeric phenol compounds, for example:

4. The mechanism of phenol action as inhibitors

306 Phenolic Compounds - Natural Sources, Importance and Applications

deuterium.

none:

Involving phenols in radical reactions results in detachment of hydrogen atom from hydroxyl

The formation of phenoxyl radicals was confirmed by electron paramagnetic resonance (EPR) method, which was supported with kinetic isotopic effect, when hydrogen atom is changed for

Phenoxylic radicals are formed in the reaction of phenols with peroxide radicals: alkoxyl, alkyl, carboxyl radicals, as well as with molecular oxygen. The subsequent behaviors of phenols in

The reaction of peroxide and phenolic radicals leads to the formation of quinolide peroxides. Another active radicals add to PhO\* similarly. The quinolide peroxides so formed could be

Hydroquinone, when reacting step by step with two peroxide radicals, transforms into qui-

Together with the interaction of phenoxyl and peroxide radicals, the competitive reaction

bimolecular loss of phenoxyl radicals takes place:

PhOH þ R� ! RH þ PhO� (1)

group on the first stage of the process with the formation of phenoxyl radical:

radical reactions depend on properties of the phenoxyl radicals formed.

The alkoxyl radical could be subsequently dissociated to give quinone:

decomposed at O-O bond to give active radicals even at moderate temperatures:

2,4,6-Trisubstituted phenoxyls do not form stable dimers as a rule. If the radical has n-, or secalkyl substituents, the reaction of disproportionation is possible to obtain methylene quinone and to recover the initial phenol:

Methylene quinones are capable of dimerization with the formation of polynuclear phenols:

Methylene quinones could react with radicals to be added to unsaturated double methylene bond, generating phenoxyl radicals:

The activities of methylene quinones are almost equal to the same for the most reactive alkenes (diene, vinyl aromatic compounds), as well as sterically hindered quinones, but unlike the latter, they are very active to peroxides.

The disproportionations involving H-atoms of OH-groups are rather typical for phenol radicals, which have two hydroxyl groups in the circle, the so-called diatomic phenols—pyrocatechol and hydroquinone. The stabilization of phenoxyl radicals formed is possible owing to ortho- and para-quinones' generation. The disproportionation products in this case are quinoid compound and initial phenol:

The quinones obtained when compared with the starting ones are effective acceptors of alkyl radicals, which add the radicals according to the scheme. So, the products of phenol inhibitor oxidation turn to be the generator of secondary inhibitors.

Phenoxyl radicals could react with some molecules. The most important reactions are as follows:

$$\text{PhO}^\* + \text{ROOH} \rightarrow \text{PhOH} + \text{RO}\_2^\* \tag{3}$$

$$\text{PhO}^\* + \text{RH} \rightarrow \text{PhOH} + \text{R}^\* \tag{4}$$

$$\text{PhO}^\* + \text{PhOH} \rightarrow \text{PhOH} + \text{PhO}^\* \tag{5}$$

The inhibitor becomes ineffective, if the equilibrium of the phenoxyl radical and peroxide reaction is shifted to the formation of RO2 \* .

The reaction, where phenoxyl radical attacks C-H bond of hydrocarbon, is the most unwelcome for inhibition process. This reaction results in the formation of active alkyl radical and regeneration of a chain.

The processes of addition of phenoxyl radicals to unsaturated compounds lead to the formations of dimeric cyclohexadienones:

The activities of methylene quinones are almost equal to the same for the most reactive alkenes (diene, vinyl aromatic compounds), as well as sterically hindered quinones, but unlike the

The disproportionations involving H-atoms of OH-groups are rather typical for phenol radicals, which have two hydroxyl groups in the circle, the so-called diatomic phenols—pyrocatechol and hydroquinone. The stabilization of phenoxyl radicals formed is possible owing to ortho- and para-quinones' generation. The disproportionation products in this case are quinoid

The quinones obtained when compared with the starting ones are effective acceptors of alkyl radicals, which add the radicals according to the scheme. So, the products of phenol inhibitor

Phenoxyl radicals could react with some molecules. The most important reactions are as

PhO� þ ROOH ! PhOH þ RO�

The inhibitor becomes ineffective, if the equilibrium of the phenoxyl radical and peroxide

The reaction, where phenoxyl radical attacks C-H bond of hydrocarbon, is the most unwelcome for inhibition process. This reaction results in the formation of active alkyl radical

\* . <sup>2</sup> (3)

PhO� þ RH ! PhOH þ R� (4)

PhO� þ PhOH ! PhOH þ PhO� (5)

oxidation turn to be the generator of secondary inhibitors.

reaction is shifted to the formation of RO2

and regeneration of a chain.

latter, they are very active to peroxides.

308 Phenolic Compounds - Natural Sources, Importance and Applications

compound and initial phenol:

follows:

As the result of the reaction of phenoxyl radicals with molecular oxygen, the symmetric quinolide peroxides are formed:

Some reactions of phenoxyl radical dissociation with emissions of active radicals are realizable.

As it is demonstrated on the schemes shown earlier, the application of phenol compounds allows to increase the inhibiting effect due to partial regeneration in the system "phenol-quinone" [1].

The stability of forming phenoxyl radicals depends fundamentally on substituents in orthoposition to hydroxyl group, and it increases with the increasing branches and bulks of the substituents [2]. The properties of SHP become apparent, if the substituent is tert-alkyl at least. For the phenols having methyl substituents or having no substituents in ortho-position of hydroxyl group, the inhibiting effects are considerably low, and often it occurs to be missing or even having a negative value. That is why monoatomic phenols unsubstituted in orthoposition to hydroxyl group are practically unknown as inhibitors.

Diatomic phenols of resorcin series reveal their inhibiting activity as compounds with CHactivity, that is, according to another mechanism [3]. The distinctive features of the resorcin series compounds are their extreme manifestations of inhibiting activity, but in a very narrow concentration interval and in considerable less (1–2 order of magnitude) concentrations, than for other diatomic phenols.

During pyrolysis of plant raw materials besides monoatomic phenols, cresols and xylenols (32.8%) and insufficient amounts of pyrocatechine and methylpyrocatechine (7.1%), the derivatives of guaiacol (20.1%) and syringol (31.3%) are dominated (Table 1).


Table 1. Composition of phenols for pyrolysis of wood.

It is found that guaiacol and syringol derivatives in non-polar media do not reveal their inhibiting activities, so the hydrogen atom of phenol hydroxyl group forms the hydrogen binding with oxygen atom of neighboring (in ortho-position) methoxyl group, which prevents the phenoxyl radical formation:
