Physical and Chemical Properties of Phenols

Phenols are a rather interesting class of organic compounds that have enormous significance not only in any industrial process but also in everyday products. The basic characterization for phenols is the hydroxyl functional group (-OH), directly attached to an aromatic benzene ring. Phenols appear with some different and peculiar characteristics of physical and chemical properties in the aromatic benzene ring. Their unique structure makes them indispensable in many aspects and provides their reactivity. That is why phenols have been used as antiseptic and disinfectants in everyday life and medicine. For example, phenol has been utilized in earlier times as one of the primary antiseptics used in surgery, and today it is still applied in many disinfectants and antiseptics. Phenols are precursors of synthesis to various useful compounds in the chemical industry; these range from pharmaceuticals and plastics to dyes. The physicochemical properties of these compounds explain their behavior in various contexts. Their importance and versatility are underlined by chemical reactivity during the huge majority of reactions, such as nitration, carboxylation, and rearrangement.

Physical Properties of Phenols

Phenols are aromatic organic compounds with a hydroxyl functional group attached directly to the benzene ring. Due to this structural arrangement, which a chemist can easily predict, the physical properties differ uniquely. Chemically, phenols are colorless or a little yellow and readily soluble in water within a range of 0.1 to 4 g/L because they form weak hydrogen bonds with water. The hydroxyl group of phenol is more acidic than alcohols whereby this factor accounts for their chemical reactivity. It is this acidity that enables phenols to be dehydrogenated into phenoxide ions by strong bases, thereby making it act like an acid. For instance, phenol will react with sodium hydroxide to yield sodium phenoxide and water and hence act as a weak acid. Because of their reactivity and solubility aspects, phenols are rather significant in terms of various chemical and industrial processes.

Phenols are either colorless liquids or white crystalline solids.

1. Phenols are soluble in hot water.
2. The boiling points of phenols are much higher than the corresponding aromatic hydrocarbons and the haloarenes.
3. Phenols have anti-fungal and anti-bacterial properties. Thus used as disinfectants and antiseptics.

4. Phenols are poisonous but act as antiseptic and disinfectant.

Chemical Reactions Involving Phenols

Phenols contain both a hydroxyl group and an aromatic ring; therefore, they are highly prone to various chemical reactions. In the Reimer-Tiemann reaction, phenols form ortho-hydroxybenzaldehyde upon formylation with chloroform and base. It can be demonstrated as a significant reaction in the preparation of aromatic aldehydes finding applications in different chemistries. Kolbe's Schmidt carboxylation of phenols results in substituted important benzoic acids, which act as some of the intermediates in the synthesis of carboxylic acids. The Claisen rearrangement furnishes ortho-quinones upon heating the phenols with base and constitutes proof for the great reactivity of phenols in rearrangement reactions. When phenols react with concentrated nitric acid, they are nitrated to give nitrophenols, which find significance in the dye and pharmaceutical industries as intermediates. Reaction with phthalic anhydride gives a product called phenolphthalein, which is an indicator for titrations. Reactions with dilute nitric acid proceed to give nitrophenols or further oxidation products, hence the very great diversity in the chemical behavior of phenols.

Reaction with zinc dust: When phenol is distilled with zinc dust, benzene is obtained. The reaction occurs as follows:

Reaction with NH₃: Phenol reacts with ammonia in the presence of anhydrous zinc chloride at 573K or (NH₄)₂SO₃-NH₃ at 423K to form aniline. This conversion of phenol into aniline is called the Bucherer reaction.

Reaction with FeCl₃: Phenol gives violet coloration with ferric chloride solution due to the formation of a colored iron complex, which is characteristic of the existence of keto-enol tautomerism in phenols.

Riemer-Tiemann Reaction
Phenols for aromatic compounds containing EDG when refluxed with $\mathrm{CHCl}_3$ and alkali yield o- and p- hydroxybenzaldehyde. The Ortho product is the predominant product. It is an electrophilic substitution on $\mathrm{PhO}^{-}$ion. The electrophile is dichlorocarbene $\left(: \mathrm{CCl}_2\right)$which contains a C with only six electrons.

Mechanism
The mechanism of this reaction follows the following steps.

For example,

Koble-Schmitt Reaction
Phenol when heated at(390-410K) under pressure with $\mathrm{CO}_2$ and alkali gives salicylic acid after acidification in addition to some amount of p-isomer.

Mechanism

For example,

On heating aryl allyl or alkyl allyl ethers in an inert solvent, the allyl group migrates from the ortho position to the ring, preferably at the ortho position, but para if the ortho position is blocked.

Mechanism
The mechanism of this reaction occurs in three following steps:

1. Enolate formation
   
2. Nucleophilic attack
   
   
3. Removing of leaving group
   

Practical Relevance and Applications

Phenols represent vital reactive substances in industries that range from: Applications of phenols extend into medicine; it is their antiseptic properties in the means of production, both for disinfectants and antiseptics and some pharmaceutical agents. Particularly, skin care and mouthwashes are likely to use phenols or their derivatives since they may act against microbes. Chemical industries consider phenols as important precursors for the synthesis of polymers, pharmaceuticals, and dyes. For example, phenols play a role in the middle reaction in drug manufacturing in the Bayer-Landols reaction, while in the Kolbe-Schmidt reaction, they are used in the formation of important intermediates in the manufacturing process for drugs. Lastly, Claisen's rearrangement applies in synthesizing complex organic molecules. Again, phenol derivatives, such as bisphenol A, form the important precursor in the manufacture of epoxy resins and polycarbonates to form plastics in material science. If these applications came into realization, then one would say that is another reason why phenols find applications in several technological and industrial concerns with vast impact on modern science and industry.

Nitration
Anisole reacts with a mixture of concentrated sulphuric and nitric acids to yield a mixture of ortho and para-nitro anisole. The reaction occurs as follows.

With dilute nitric acid at low temperatures (298 K), phenol yields a mixture of ortho and para nitrophenols. The reaction occurs as follows.

Phthalic Anhydride
Phenol couples with phthalic anhydride in the presence of conc. H₂SO₄ to form phenolphthalein which is used as an indicator. The reaction occurs as follows.

The ortho and para isomers can be separated by steam distillation. O-nitrophenol is steam volatile due to intramolecular hydrogen bonding while p-nitrophenol is less volatile due to intermolecular hydrogen bonding which causes the association of molecules.

Recommended video(Physical and Chemical Properties of Phenols)

Some Solved Examples

Example 1 The organic compound that gives following qualitative analysis is :
Test & Inference
(a) $\mathrm{Dil} . \mathrm{HCl}$ & Insoluble
(b) NaOH solution & soluble
(c) $\mathrm{Br} _2 /$ water & Decolourization
1) (correct)

2)

3)

4)

Solution:

Therefore, option (1) is correct.

Example 2 Which of the following compounds gives pink color on reaction with phthalic anhydride in conc. H₂SO₄ followed by treatment with NaOH?

1) (correct)

2)

3)

4)

Solution:

Onlyphenolic compound has hydrogen at the para position of the phenolic group and the rest of the phenolic compounds not have hydrogen at the para position of the phenolic group.

So, option 1 compound gives pink colour on reaction with phthalic anhydride in conc. H₂SO₄ followed by treatment with NaOH.

Reaction:

Example 3 The major product of the following reaction is :

1)

2) (correct)

3)

4)

Solution:

Extraction of proton leads to the formation of substituted phenoxide ion which makes an SN₂ attack on methyl iodide to displace I^-

I^- will react with H⁺ and HI will be the side product.

Therefore, option (2) is correct.

Summary

Phenols have important applications based on their special physical and chemical properties due to the aromatic ring and hydroxyl group in their structure. Some of the general properties, like their solubility, acidity, and reactivity, were discussed in the paper with phenols. The Reimer-Tiemann reaction, Kolbe's Schmidt reaction, Claisen's rearrangement reactions, etc., are some important chemical reactions associated with phenols along with the use of various reagents. It was also considered the applied significance of phenols in medical, industrial, and materials sciences in their application for antiseptic, pharmaceutical, plastic production, and a variety of other very important compounds. Knowing these aspects makes one see the all-round view of the importance of phenols regarding research activity at an academic level and in real-life applications underlining their impact in various sectors that bring out their versatile nature.