Physical Properties of Phenols: Phenols are very important organic compounds in chemistry. Compounds that have a hydroxyl group directly attached to a benzene ring are called phenols. Thus, phenol is the specific name for hydroxyl benzene, and it is the general name for the family of compounds derived from hydroxyl benzene. In phenol, an \({\text{-OH}}\) group is attached to a \({\text{s}}{{\text{p}}^2}\) hybridised carbon atom, while in aliphatic alcohols, the \({\text{-OH}}\) group is attached to a \({\text{s}}{{\text{p}}^3}\) hybridised carbon atom.

A phenolic \({\text{OH}}\) is characterised by the appearance of violet, red or green colouration with a neutral \({\text{FeC}}{{\text{l}}_3}\) solution. Phenol, also known as carbolic acid, is an aromatic organic compound with a molecular formula \({{\text{C}}_6}{{\text{H}}_5}{\text{OH}}.\) It is a white crystalline solid that is volatile. The molecule consists of a phenyl group \(\left({{\text{-}}{{\text{C}}_6}{{\text{H}}_5}} \right)\) bonded to a hydroxyl group \(\left({{\text{-OH}}} \right).\) It is mildly acidic but requires careful handling due to its propensity to cause burns.

Due to the presence of hydroxyl groups in phenols, they are similar to alcohols in several ways. They can form strong intermolecular hydrogen bonding, for example, and so have higher boiling temperatures than hydrocarbons of similar molecular weight. Despite having nearly, the same molecular weight, phenol (bp \(182\,^\circ {\rm{C}}\)) has a boiling point more than \(70\,^\circ {\rm{C}}\) higher than toluene (bp \(110.6\,^\circ {\rm{C}}\)). Because of their propensity to create strong hydrogen bonds with water molecules, phenols are moderately soluble in water. Phenols are used for a variety of purposes.

Preparation of Phenol

Various methods of preparation of phenols are given below:

1. Hydrolysis of Chlorobenzene (Dow’s process)
2. Alkali Fusion of Sodium Benzene Sulphonate
3. From Cumene Hydroperoxide
4. Hydrolysis of Diazonium Salts
5. By Decarboxylation of Phenolic Acid

Physical Properties of Phenol

Physical State

Pure phenol is a white crystalline liquid with a distinct odour. It must be treated with considerable caution since it causes immediate white blistering on the skin. Frequently, the crystals are discoloured and wet. Generally, phenols are colourless liquids or crystalline solids that turn colour when exposed to air over time.

Smell

Phenol has an unpleasant, sickeningly sweet, and tarry odour. Phenol can be tasted and smelled at levels lower than those linked to hazardous consequences. It has a sour, scorching flavour.

Hydrogen Bonding

Phenols, like alcohols, have hydroxyl groups that can engage in intermolecular hydrogen bonding; however, phenols have stronger hydrogen bonds than alcohols. Phenols have substantially higher melting and boiling temperatures than hydrocarbons with similar molecular mass due to hydrogen bonding.

Melting and Boiling Points

The melting and boiling points of phenol and methylbenzene can be compared. Both molecules have the same number of electrons and are almost identical in shape. As a result, the intermolecular attractions caused by van der Waals dispersion forces will be extremely comparable.

The higher values for phenol owe in part to permanent dipole-dipole attractions caused by the electronegativity of the oxygen, but they are primarily due to hydrogen bonding. Hydrogen bonds can form between a lone pair of oxygen on one molecule and the hydrogen on the \({\text{-OH}}\) group of one of its neighbours.

Solubility

Phenol is water-soluble to a moderate extent; approximately \({\text{8 g}}\) of phenol will dissolve in \({\text{100 g}}\) of water. If you try to dissolve more than this, you’ll end up with two liquid layers. The top layer is a phenol in water solution, and the bottom layer is water in phenol solution. Phenol and water have a complicated solubility relationship. Because of its capacity to create hydrogen bonds with water, phenol is moderately soluble in water.
Certain \({\text{o}}\)–substituted phenol exhibits special properties due to intramolecular hydrogen bonding. For example, \({\text{o}}\)-nitrophenol is steam-volatile (lower solubility in water) and low boiling compound than its \({\text{m}}\)– and \({\text{p}}\)–analogues.
Due to intramolecular hydrogen bonding, \({\text{o}}\)-Nitrophenol and salicylic acid are no longer available for molecular association within themselves and with water molecules. Hence these compounds are less soluble in water and also have lower boiling points than the \({\text{m}}\)– and \({\text{p}}\)–isomers where there is intermolecular hydrogen bonding.

Phenol is a resonance hybrid of the following resonating structures:

Due to electron distribution, phenol shows a dipole moment of \(1.70\,{\text{D}},\) with the positive charge on the oxygen atom and negative charge dispersed in the ring. The structures also contribute certain properties as
(a) The hydrogen atom of the \({\text{-OH}}\) group should show acidic behaviour.
(b) The increased electron density in the ring predicts the electrophilic substitution at \({\text{o}}\)– and \({\text{p}}\)–position and also shows the strong activating effect of \({\text{-OH}}\) group.

Some Other Important Points

1. Because phenols are more acidic than water, so the reaction of phenol with sodium hydroxide goes essentially to completion and produces water-soluble sodium phenoxide. So, phenols are soluble in \({\text{aq}} – {\text{NaOH}}\) solution.
2. Most of the phenols are not soluble in aqueous sodium bicarbonate since \({{\text{H}}_2}{\text{C}}{{\text{O}}_3}\) is a stronger acid than most phenols.
3. The fact that phenols dissolve in aqueous sodium hydroxide, whereas most alcohols with six carbon atoms or more do not, gives us a convenient means for distinguishing and separating phenols from most alcohols.

Summary

1. Compounds that have a hydroxyl group directly attached to a benzene ring are called phenols.
2. Phenol is also known as carbolic acid, is an aromatic organic compound with a molecular formula \({{\text{C}}_6}{{\text{H}}_5}{\text{OH}}.\)
3. Due to the presence of hydroxyl groups in phenols, they are similar to alcohols in several ways.
4. Pure phenol is a white crystalline substance with a disinfecting odour.
5. Phenol has a distinct odour that is sickeningly sweet and tarry.
6. Similar to alcohols, phenols have hydroxyl groups that can participate in intermolecular hydrogen bonding; in fact, phenols tend to form stronger hydrogen bonds than alcohols.
7. The reason for the higher melting and boiling points of phenol is in part due to permanent dipole-dipole attractions due to the electronegativity of the oxygen – but is mainly due to hydrogen bonding.
8. Phenol is somewhat soluble in water because of its ability to form hydrogen bonds with water.
9. Certain \({\text{o}}\)–substituted phenol exhibits special properties due to intramolecular hydrogen bonding. For example, \({\text{o}}\)-nitrophenol is steam-volatile (lower solubility in water) and low boiling compound than its \({\text{m}}\)– and \({\text{p}}\)–analogues.
10. Due to intramolecular hydrogen bonding, \({\text{o}}\)-Nitrophenol and salicylic acid are no longer available for the molecular association within themselves and with water molecules.

FAQs on Physical Properties of Phenols

Q.1. What are phenols? How are they different from alcohol?
Ans: Compounds that have a hydroxyl group directly attached to a benzene ring are called phenols. Thus, phenol is the specific name for hydroxyl benzene, and it is the general name for the family of compounds derived from hydroxyl benzene. In phenol, an \({\text{-OH}}\) group is attached to a \({\text{s}}{{\text{p}}^2}\) hybridised carbon atom, while in aliphatic alcohols, the \({\text{-OH}}\) groups are attached to the \({\text{s}}{{\text{p}}^3}\) hybridised carbon atom. A phenolic \({\text{OH}}\) is characterised by the appearance of violet, red or green colouration with a neutral \({\text{FeC}}{{\text{l}}_3}\) solution. Phenol, also known as carbolic acid, is an aromatic organic compound with a molecular formula \({{\text{C}}_6}{{\text{H}}_5}{\text{OH}}.\)

Q.2. Do phenols form intermolecular hydrogen bonds?
Ans: Yes, phenol shows intermolecular hydrogen bonding. Similar to alcohols, phenols have hydroxyl groups that can participate in intermolecular hydrogen bonding; in fact, phenols tend to form stronger hydrogen bonds than alcohols. Hydrogen bonding results in higher melting points and much higher boiling points for phenols than for hydrocarbons with similar molecular weights.

Q.3. Discuss the melting and boiling points of phenols.
Ans: It is useful to compare phenol’s melting and boiling points with those of methylbenzene (toluene). Both molecules contain the same number of electrons and are very similar in shape. That means that the intermolecular attractions due to van der Waals dispersion forces will be very similar.

The higher values for phenol are partly due to permanent dipole-dipole attractions due to the electronegativity of the oxygen – but is mainly due to hydrogen bonding. Hydrogen bonds can form between a lone pair of oxygen on one molecule and the hydrogen on the \({\text{-OH}}\) group of one of its neighbours.

Q.4. Discuss the solubility of phenols in water.
Ans: Phenol is moderately soluble in water – about \({\text{8 g}}\) of phenol will dissolve in \({\text{100 g}}\) of water. If you try to dissolve more than this, you get two layers of liquid. The top layer is a solution of phenol in water, and the bottom one is a solution of water in phenol. The solubility behaviour of phenol and water is complicated. Phenol is somewhat soluble in water because of its ability to form hydrogen bonds with water.

Q.5. What is special about \({\text{o}}\)–substituted phenol?
Ans: Certain \({\text{o}}\)–substituted phenol exhibits special properties due to intramolecular hydrogen bonding. For example, \({\text{o}}\)-nitrophenol is steam-volatile (lower solubility in water) and low boiling compound than its \({\text{m}}\)– and \({\text{p}}\)–analogues. Due to intramolecular hydrogen bonding, \({\text{o}}\)-Nitrophenol and salicylic acid are no longer available for molecular association within themselves and with water molecules. Hence these compounds are less soluble in water and also have lower boiling points than \({\text{m}}\)– and \({\text{p}}\)–isomers, where there is intermolecular hydrogen bonding.

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