**2. Chemical structure and properties of phenols**

Phenols are compounds possessing one or more hydroxyl groups (–OH) directly connected to the aromatic system (e.g. phenyl, naphthyl) [8, 9]. **Figure 1** presents some examples of phenols with their systematic and common names.

All carbon atoms forming aromatic ring are sp2 hybridized. Therefore, phenyl has hexagonal planar structure with all bond angles 120° and delocalized π-electrons distributed over the ring. The C-O bond is formed from Csp2 -Osp3 and the O-H bond is formed from Osp3 -H1s. Two other orbitals of oxygen atom are occupied by two nonbonded electron pairs. For this reason, hydroxyl functional group C-O-H has a bent shape with almost the tetrahedral bond angle of 109.5° as it is represented in **Figure 2**. Oxygen is more electronegative than carbon and hydrogen that makes both the C-O and O-H bonds polar [10].

Moreover, electron pairs of oxygen atom are conjugated with aromatic system that causes partial transfer of negative charge from oxygen to the ring and delocalization of the charge. This effect additionally strengthens polarization of O-H bond. In consequence, phenol gains acidic character and ability to form phenoxide (phenolate) ion [9, 10]. Both phenol and its conjugate base are resonance stabilized. Dispersion of the negative charge over the molecule can be illustrated with the resonance structures or as a resonance hybrid as in **Figure 3**.

Chemical Structure of Phenols and Its Consequence for Sorption Processes http://dx.doi.org/10.5772/66537 5

**Figure 1.** Examples of phenolic compounds.

This chapter is devoted to review of current state of knowledge on sorption process of phenolic compounds. Many different types of sorbents are used for phenols in chromatographic columns and solid phase extraction devices. Their efficiency is diverse and depends on many factors. As the most important chemical structure of adsorbate, a type of sorbent and its porosity as well as properties of solvent (or eluent) should be mentioned. Also other properties such as pH, temperature and presence of oxygen influence the process. For this reason, sorption of phenols is a very complex phenomenon. Although many researchers try to explain the mechanism of interaction of phenols with adsorbents, it is still an open

Many groups of researchers tried to solve it using different scientific methods, for example, chromatography [1, 2], spectroscopy (UV, mass spectrometry [MS], Fourier transform infrared spectroscopy [FTIR]) [3–5], thermal analysis [6], and computer simulations [7]. Based on the obtained results, some authors proposed explanations of phenomena and mechanisms accompanying the sorption of phenol. Understanding the mechanism is important from scientific point of view. Elucidation of this process is essential for reasons of utility and finding an answer to the questions how to improve efficiency of sorption phenolic compounds in the processes of aqueous environment remediation. Effective removal of these species from industrial and urban waste water helps to protect aquatic ecosystem from toxic impact of phenols on the living organisms, which is an important aim all over

Phenols are compounds possessing one or more hydroxyl groups (–OH) directly connected to the aromatic system (e.g. phenyl, naphthyl) [8, 9]. **Figure 1** presents some examples of phenols

planar structure with all bond angles 120° and delocalized π-electrons distributed over the

Two other orbitals of oxygen atom are occupied by two nonbonded electron pairs. For this reason, hydroxyl functional group C-O-H has a bent shape with almost the tetrahedral bond angle of 109.5° as it is represented in **Figure 2**. Oxygen is more electronegative than carbon

Moreover, electron pairs of oxygen atom are conjugated with aromatic system that causes partial transfer of negative charge from oxygen to the ring and delocalization of the charge. This effect additionally strengthens polarization of O-H bond. In consequence, phenol gains acidic character and ability to form phenoxide (phenolate) ion [9, 10]. Both phenol and its conjugate base are resonance stabilized. Dispersion of the negative charge over the molecule can be illustrated with the resonance structures or as a resonance hybrid as in


hybridized. Therefore, phenyl has hexagonal


and the O-H bond is formed from Osp3

**2. Chemical structure and properties of phenols**

4 Phenolic Compounds - Natural Sources, Importance and Applications

and hydrogen that makes both the C-O and O-H bonds polar [10].

with their systematic and common names.

ring. The C-O bond is formed from Csp2

All carbon atoms forming aromatic ring are sp2

problem.

the world.

**Figure 3**.

**Figure 2.** The structure of phenol.

**Figure 3.** Resonance stabilized structures of phenol and phenoxide representing dispersion of negative charge and its resonance hybrid. Based on Refs. [9, 10].

All the above-mentioned structural features give the phenol specific physical and chemical properties. Phenol is a peculiar smelling, water-soluble crystalline solid, with low melting and high boiling point values. These attributes can be reinforced or weakened, if a substituent is connected to the aromatic ring. A chemical character (electron withdrawing or donating), the way of substitution—position and a number of functional groups attached to the ring—can alter properties of the phenol significantly. Some properties such as melting (MP) and boiling points (BP), density (D), water solubility (w.s.), acidity in water (pKa) and octanol-water partition coefficient (Log P) of phenol and its derivatives are presented in **Tables 1** and **2** [11, 12]. Comparison of these data allows to evaluate the impact of substituents on change of the properties.

Many monohydroxyl derivatives (alkylphenols, alkoxyphenols, halogenophenols, 2- and 3-nitrophenols) similar to phenol are crystalline solids with melting points below 100°C and boiling points ca. 200°C. Other nitrophenols, aminophenols, pentachlorophenol, hydroxybenzoic acids and species possessing two or more hydroxyl groups in a molecule have very high values both melting and boiling points. While some phenols are sublime, others decompose before reaching the boiling point. So, high temperatures are the consequence of intermolecular hydrogen bonds, which are formed between the molecules, thereby justifying water solubility of these compounds. The examples of intermolecular hydrogen bonds that phenols can form with their own molecules and with molecules of water are shown in the **Figure 4**.


All the above-mentioned structural features give the phenol specific physical and chemical properties. Phenol is a peculiar smelling, water-soluble crystalline solid, with low melting and high boiling point values. These attributes can be reinforced or weakened, if a substituent is connected to the aromatic ring. A chemical character (electron withdrawing or donating), the way of substitution—position and a number of functional groups attached to the ring—can alter properties of the phenol significantly. Some properties such as melting (MP) and boiling points (BP), density (D), water solubility (w.s.), acidity in water (pKa) and octanol-water partition coefficient (Log P) of phenol and its derivatives are presented in **Tables 1** and **2** [11, 12]. Comparison of these data allows to evaluate the impact of substituents on change of the

**Figure 3.** Resonance stabilized structures of phenol and phenoxide representing dispersion of negative charge and its

Many monohydroxyl derivatives (alkylphenols, alkoxyphenols, halogenophenols, 2- and 3-nitrophenols) similar to phenol are crystalline solids with melting points below 100°C and boiling points ca. 200°C. Other nitrophenols, aminophenols, pentachlorophenol, hydroxybenzoic acids and species possessing two or more hydroxyl groups in a molecule have very high values both melting and boiling points. While some phenols are sublime, others decompose before reaching the boiling point. So, high temperatures are the consequence of intermolecular hydrogen bonds, which are formed between the molecules, thereby justifying water solubility of these compounds. The examples of intermolecular hydrogen bonds that phenols can form with their own molecules and with molecules of water are shown in

properties.

resonance hybrid. Based on Refs. [9, 10].

6 Phenolic Compounds - Natural Sources, Importance and Applications

the **Figure 4**.

#### Chemical Structure of Phenols and Its Consequence for Sorption Processes http://dx.doi.org/10.5772/66537 7


**Table 1.**

Comparison of physical and chemical properties of phenol and its derivatives with electron-donating substituents.


**Compound**

**Case no.**

**M [g mol−1]**

**MP [°C]**

**BP [°C]**

**d [g cm−3]**

**p.s.**

**w.s. [g L−1]**

**pKa**

**LogP**

**μ**

**[D]**

**Chemical name**

**IUPAC name**

Mesitol

527-60-6

136.19

73

220

nda

s

1.01

10.88

2.73

nda

2,4,6-trimethylphenol

2-Ethylphenol

3-Ethylphenol

4-Ethylphenol

Guaiacol

2-Methoxyphenol

3-Methoxyphenol

4-Methoxyphenol

o-Aminophenol

95-55-6

109.13

170

153

1.328

c.s.

20

4.72

0.62

nda

9.71

−174

11 mmHg

2-Aminophenol

m-Aminophenol

591-27-5

109.13

123

164 at

1.195

c.s.

26.3

4.37

0.21

1.83

9.815

11mmHg

3-Aminophenol

p-Aminophenol

123-30-8

109.13

187.5

284

1.29

c.s.

1.6

5.48

0.04

nda

10.46

D.

4-Aminophenol

2,4-Diaminophenol

**Table 1.**

95-86-3

124.14

205

nda

nda M, molar mass; d, density; w.s., water solubility; MP, melting point; p.s., physical state; c.s., crystalline solid; l,– liquid; pKa acidity in water; BP, boiling point; D.,

decomposes; E., explodes; S, sublimes; Log P, partition coefficient; μ, dipole moment in benzene as solvent; nda, no data available.

Comparison of physical and chemical properties of phenol and its derivatives with electron-donating substituents.

c.s.

275

nda

nda

nda

D.

150-76-5

124.14

54-57

243

1.55

c.s.

40.0

10.05

1.41/

nda

1.34

150-19-6

124.13

−18

244

1.131

l

nda

nda

nda

nda

−16

620-17-7 123-07-9 8000-58-9

124.14

28-32

205

1.129

l/

18.7

9.98

1.32

nda

c.s.

122.16

46

217.9

1.011

c.s.

4.90

10.0

2.58

nda

8 Phenolic Compounds - Natural Sources, Importance and Applications

122.16

−4

218.4

1.028

l

nda

9.9

2.40

nda

90-00-6

122.16

18

204.5

1.015

l

5.34

10.20

2.47

nda

#### Chemical Structure of Phenols and Its Consequence for Sorption Processes http://dx.doi.org/10.5772/66537 9


**Table 2.** Comparison of physical and chemical properties of phenolic compounds with electron withdrawing substituents. Chemical Structure of Phenols and Its Consequence for Sorption Processes http://dx.doi.org/10.5772/66537 11

**Figure 4.** The intermolecular(purple dotted lines) and intramolecular hydrogen bonds (green dotted lines) formed by phenol and its derivatives. On the basis of Ref. [9].

**Compound**

**Case no.**

**M [g mol−1]**

**MP [°C]**

**BP [°C]**

**d [g cm−3]**

**p.s.**

**w.s. [g L−1]**

**pKa**

**Log P**

**μ [D]**

**Chemical name**

**IUPAC name**

2,3,4,5,6-Pentachlorophenol

o-Bromophenol

95-56-7

173.00

5.6

194.5

1.492

l

2.23

8.45

2.35

1.36

2.36d

2-Bromophenol

m-Bromophenol

591-20-8

173.00

33

236.5

nda

c.s.

23

9.03

2.63

nda

10 Phenolic Compounds - Natural Sources, Importance and Applications

3-Bromophenol

p-Bromophenol

160-41-2

173.00

66.4

238

1.840

c.s.

14

9.17

2.59

2.12

4-Bromophenol

o-Iodophenol

533-58-4

220.01

39–43

186

nda

c.s.

nda

8.47

nda

nda

100 mmHg

2-Iodophenol

m-Iodophenol

626-02-8

220.01

40

D.

nda

c.s.

nda

8.88

nda

nda

3-Iodophenol

p-Iodophenol

540-38-5

220.01

94

D.

nda M, molar mass; d, density; w.s., water solubility; MP, melting point; p.s., physical state; c.s., crystalline solid; l, liquid; pKa, acidity in water; BP, boiling point; D.,

decomposes; E., explodes; S, sublimes; Log P, partition coefficient; μ, dipole moment in benzene as solvent; d, dioxane; nda, no data available.

Comparison of physical and chemical properties of phenolic compounds with electron withdrawing substituents.

c.s.

nda

9.2

nda

nda

4-Iodophenol

**Table 2.**

87-86-5

266.34

174

309–310

1.978

c.s.

0.014

4.70

5.12

nda

D.

The possibility to form hydrogen bonds suggests that phenols should have good water solubility, but this is not the rule. The unsubstituted phenol is relatively well soluble in water (83 g L−1), while its substituted derivatives are not. For most of them, solubility does not exceed 30 g L−1. Alkyl and halogen groups enhance hydrophobic character of aromatic ring resulting in decrease of water solubility. For phenols possessing functional groups with strong polar character, differences in ability to dissolve are more pronounced. Even for isomers of the same compound, they can be significant. The close proximity of the –OH group to substituents such as -NO2 , -NH2 and -COOH lead to formation of intramolecular hydrogen bonds. In this way coordinated the hydroxyl group becomes less active in the solvation process. Due to this fact, solubility of ortho-isomer is lower than meta or para ones (e.g. nitrophenols, hydroxylbenzoic acids).

Very interesting behavior demonstrates benzenediols and benzenetriols. Increasing number of hydroxyl groups implies their solubility should be better in comparison to phenol. It is indeed, but hydroquinone and phloroglucinol are exceptions. Solubility of these two compounds is lower and amount 72 and 10 g L−1, respectively. This apparently abnormal behavior is the result of the presence of hydrogen bonds whose strength are additionally enhanced by symmetry of the molecules. The adjacent molecules form a kind of network in which each of them is strongly bonded with others by infinite chains of hydrogen bonds (**Figure 5**) [13–15]. In this way, a compact structure is created that prevents the penetration of solvent/water molecules into the interior. For this reason, dissolution process is significantly hindered. In

**Figure 5.** Hydrogen bonds enhanced by symmetrical structure of the molecules in (a) hydroquinone and (b) phloroglucine. Based on Refs. [13–15].

the similar way, slight solubility of other phenols (4-aminophenol/para-isomers, multi-substituted phenols) can be explained.

Acidity values (pKa) of most phenols are in the range of 8–10, which means they are acids stronger than water (pKa of water 15.7) but weaker than carbonic acid (pKa of 6.4). As it was mentioned above, acidity of phenol is the result of delocalizing of negative charge over the aromatic ring and resonance effect. Dipole moment is sensitive to this kind of changes. Entering *an electron donating* (-CH3 , -C2 H5 , -OCH3 , -NH2 , -OH) *or electronwithdrawing* (-COOH, -NO2 , -F, -Cl, -Br, -I) *group* to the ring causes changes in its electron density. As a consequence, altering of dipole moment of the molecule and its acidity is observed. For this reason, alkylphenols are less acidic and nitrophenols are more acidic than phenol. This effect is particularly well visible in case of picric acid. Three symmetrically situated nitro groups exert so strong electron-withdrawing effect that the pKa value for this phenol decreases up to of 0.42 and is comparable with acidity of mineral acids, for example, HBrO3 (pKa = −0.69), H<sup>3</sup> PO4 (pKa = 2.12) or strong organic acids CF<sup>3</sup> COOH (pKa = 0.2) and CCl3 COOH (pKa = 0.6). Similar effect is also observed for chlorophenols. The higher is a number of chlorine substituents the more acidic is character of phenol (decrease of pKa value) particularly in case of tri- and pentachlorophenols.
