Phenols

The compound \(C_6H_4(OH)CH_3\) is a methyl derivative of phenol. The carbon bearing \(-OH\) is numbered 1, so the structure with the methyl group on the next carbon (the first structure of the question) is 2-methylphenol (o-cresol), and the structure with the methyl group opposite to \(-OH\) (the second structure) is 4-methylphenol (p-cresol). The third isomer, with the methyl group at the meta position, is 3-methylphenol (m-cresol).

Phenol is much more polar than toluene. In phenol the oxygen is highly electronegative, so the \(C-O\) and \(O-H\) bonds are strongly polar, and the molecule has a dipole moment of about \(1.4\,D\). In toluene only the \(C-H\) bonds and the weak \(C(sp^2)-C(sp^3)\) bond are present, and the dipole moment is only about \(0.36\,D\), so toluene is almost non-polar.

\(C_6H_4(OH)_2\) is dihydroxybenzene, and it has three positional isomers according to the positions of the two \(-OH\) groups: benzene-1,2-diol (catechol, also called 2-hydroxyphenol), benzene-1,3-diol (resorcinol, 3-hydroxyphenol) and benzene-1,4-diol (quinol or hydroquinone, 4-hydroxyphenol).

Phenol is slightly soluble in water at room temperature (about \(8\) g in \(100\) mL), and it becomes completely miscible above \(65^\circ C\). Its \(-OH\) group forms hydrogen bonds with water molecules. Benzene is insoluble in water, because it is non-polar and cannot form hydrogen bonds. Both dissolve well in organic solvents. Phenol is less soluble than the small alcohols, because its large non-polar benzene ring reduces the interaction with water.

Phenol molecules are held together by strong intermolecular hydrogen bonds (\(O-H\cdots O\)), because of the polar \(-OH\) group. Benzene and toluene are non-polar and have only weak van der Waals (London) forces, so less energy is needed to separate their molecules. So the boiling point of phenol (\(182^\circ C\)) is higher than that of toluene (\(111^\circ C\)) and benzene (\(80^\circ C\)), although their molecular masses are nearly the same.

Benzene and toluene have only van der Waals forces, and toluene is higher because of its larger mass. Phenol and cresol are hydrogen bonded, so they boil higher, and cresol is the highest because of the larger molecular mass of the extra \(-CH_3\) group.

In \(o\)-nitrophenol the \(-OH\) group and the \(-NO_2\) group are close to each other, and the hydrogen of \(-OH\) is attached to an oxygen of \(-NO_2\) by intramolecular hydrogen bonding (a six-membered chelate ring). So its \(-OH\) group is not free to bond with other molecules or with water: the molecules are held together only by weak forces, and the compound is volatile with steam (boiling point about \(216^\circ C\)) and is less soluble in water.

In \(m\)- and \(p\)-nitrophenol the groups are too far apart for this, so the \(-OH\) group forms intermolecular hydrogen bonds with other molecules and with water. This gives a higher boiling point, a higher melting point and a greater solubility in water. The difference is used to separate \(o\)-nitrophenol from \(p\)-nitrophenol by steam distillation.

The \(-OH\) group pushes electrons into the ring by its \(+M\) effect and makes the ortho and para positions rich in electrons. An electron-withdrawing group such as \(-NO_2\), \(-CN\) or \(-COOH\) at the meta position pulls electrons out of the ring by its \(-I\) effect (and cannot give any resonance help to the ortho and para carbons, because it is meta). So the net electron density of the whole ring falls, and the ring becomes less reactive (deactivated) than phenol itself. Electrophilic substitution becomes slower, and more drastic conditions are needed. The \(-OH\) group still decides the positions, so the electrophile goes to the positions ortho and para to \(-OH\) that are not blocked.

Benzene has a uniform \(\pi\)-electron cloud and no group to increase its electron density. The non-polar \(Br_2\) molecule is not polarised enough by it, so a Lewis acid like \(FeBr_3\) or \(AlCl_3\) is needed to generate the electrophile: \(Br_2+FeBr_3\rightarrow Br^{\delta+}\cdots Br\cdots FeBr_3^{\delta-}\) (that is, \(Br^++FeBr_4^-\)).

Phenol has a lone pair on oxygen which is delocalised into the ring (\(+M\) effect), as the resonating structures show.

This increases the electron density of the ring, especially at the ortho and para positions. The ring is so rich in electrons that it polarises the \(Br-Br\) bond by itself (the ring acts as the nucleophile) and no catalyst is needed. In water the phenoxide ion is even more reactive, and 2,4,6-tribromophenol is obtained at once. The \(\sigma\)-complex (arenium ion) formed from phenol is also extra stabilised by the oxygen lone pair, so the activation energy is lower.

When phenol loses \(H^+\), the negative charge on the oxygen of the phenoxide ion is delocalised into the benzene ring through resonance. The lone pair on oxygen conjugates with the \(\pi\)-system, so the charge is shared by the oxygen and the ortho and para carbon atoms (structures I to IV, and the resonance hybrid). The spreading of charge over several atoms lowers the energy of the ion and makes it more stable.

In the ethoxide ion (\(CH_3CH_2O^-\)) the negative charge stays on the oxygen only (there is no resonance). The ethyl group also pushes electrons towards oxygen (\(+I\) effect), which increases the charge density and makes the ion less stable. So the phenoxide ion is more stable, and phenol gives up its proton more easily than ethanol.

Phenol is the strongest acid among the three because its conjugate base is resonance stabilised. Water is a little more acidic than ethanol, since the \(+I\) effect of the ethyl group destabilises the ethoxide ion compared with the hydroxide ion. Phenol turns blue litmus red slightly, reacts with \(Na\) and with \(NaOH\), but it is a weaker acid than carbonic acid and does not liberate \(CO_2\) from sodium bicarbonate. Ethanol and water do not react with \(NaOH\) in this way.

A substituent changes the acidity by changing the stability of the phenoxide ion.

Electron-withdrawing groups (\(-NO_2\), \(-Cl\)) pull electrons away from the ring and the oxygen (\(-I\) effect, and \(-M\) effect for \(-NO_2\)). They spread the negative charge of the phenoxide ion further, so it is stabilised and the acidity increases. \(-NO_2\) is very effective, and \(-Cl\) works only by the \(-I\) effect, so it is less effective.

Electron-donating groups (\(-OCH_3\), \(-CH_3\)) push electrons into the ring (\(+M\) or \(+I\)), which increases the charge on oxygen and destabilises the phenoxide ion, so the acidity decreases.

This is the order of increasing acidic strength (the lower the \(pK_a\) the stronger the acid).

The oxygen of \(-OH\) has two lone pairs. One of them is delocalised into the \(\pi\)-system of the ring (\(+M\) effect), so phenol is a resonance hybrid of the structures above. In structures II, III and IV the negative charge sits on the carbons at the ortho (two positions) and para positions, and no negative charge ever appears at the meta positions. So the electron density is highest at ortho and para, and the electrophile (\(E^+\)) attacks there. The arenium ion formed by ortho or para attack has an extra resonance structure in which oxygen bears the positive charge and every atom has an octet, and this makes it more stable than the ion from meta attack.

The \(-OH\) group is a strong activating group. Through the \(+M\) effect the lone pair of oxygen increases the electron density of the whole ring (more at ortho and para), so the ring is a better nucleophile and attacks the electrophile more readily than benzene. The intermediate carbocation (\(\sigma\)-complex) is also stabilised by the lone pair of oxygen, so the activation energy is lower and the reaction is faster. That is why phenol is brominated and nitrated under milder conditions than benzene: it is nitrated by dilute nitric acid at room temperature, brominated without a catalyst and gives tribromophenol with bromine water, while benzene needs concentrated acids or a Lewis acid.

In water (a polar solvent) phenol ionises to a small extent to give the phenoxide ion, \(C_6H_5OH+H_2O\rightleftharpoons C_6H_5O^-+H_3O^+\). The \(O^-\) group is a much stronger electron donor than \(-OH\), so the ring is very strongly activated and all three positions (2, 4 and 6) are brominated at once. The product is the white precipitate of 2,4,6-tribromophenol.

In \(CS_2\) or \(CCl_4\) (non-polar solvents) phenol hardly ionises, and polar \(Br_2\) is not formed. The ring is only activated by the neutral \(-OH\) group, which is not enough for a second or third substitution at low temperature, so a monobromo product is obtained: \(p\)-bromophenol as the major and \(o\)-bromophenol as the minor product.

The question gives the structure of the first product (a benzene ring) and leaves the reagents and the other two products open. The sequence that fits the preparation methods of the unit (phenol, then benzene, then benzenesulphonic acid, then sodium phenoxide, which returns to phenol on acidification) is:

Step 1. Phenol is heated with zinc dust. The \(-OH\) group is removed as \(ZnO\) and benzene is formed: \(C_6H_5OH+Zn\xrightarrow{\Delta}C_6H_6+ZnO\).

Step 2. Benzene is heated with concentrated sulphuric acid (electrophilic substitution), and benzenesulphonic acid is formed: \(C_6H_6+H_2SO_4\xrightarrow{\Delta}C_6H_5SO_3H+H_2O\).

Step 3. Benzenesulphonic acid is fused with sodium hydroxide, and sodium phenoxide is formed (with sodium sulphite and water): \(C_6H_5SO_3H+2NaOH\xrightarrow{fusion}C_6H_5ONa+Na_2SO_3+2H_2O\). Acidification with \(HCl\) then gives phenol back: \(C_6H_5ONa+HCl\rightarrow C_6H_5OH+NaCl\).

Phenol is nitrated by dilute nitric acid at about \(298\,K\) to give a mixture of 2-nitrophenol (o-nitrophenol) and 4-nitrophenol (p-nitrophenol). The textbook notes take 2-nitrophenol as the major product and 4-nitrophenol as the minor product. The ortho isomer is volatile with steam (intramolecular hydrogen bonding), so the mixture is separated by steam distillation: \(C_6H_5OH+HNO_3\rightarrow C_6H_4(OH)NO_2+H_2O\).

With concentrated nitric acid in the presence of concentrated sulphuric acid all three activated positions (2, 4, 6) are nitrated, and 2,4,6-trinitrophenol (picric acid) is formed: \(C_6H_5OH+3HNO_3\xrightarrow{conc.\ H_2SO_4}C_6H_2(NO_2)_3OH+3H_2O\). The yield by this direct method is low, because nitric acid also oxidises phenol and gives tarry products.

Phenol is acylated on its oxygen, and the ester phenyl acetate (phenyl ethanoate) is formed: \(C_6H_5OH+CH_3COCl\xrightarrow{pyridine}C_6H_5OCOCH_3+HCl\). Pyridine is a base and takes up the \(HCl\) liberated, so the reaction goes to completion. It is a nucleophilic acyl substitution in which the oxygen of phenol attacks the carbonyl carbon.