Introduction to Phenol
Phenols are a type of organic compounds that contain a benzene ring which is bonded to a hydroxyl group. They are also called carbolic acids. They exhibit unique physical and chemical properties when compared to alcohol. These physical and chemical properties of phenol are mainly because of the presence of the hydroxyl group in them. Phenols are much similar to alcohols but form much stronger hydrogen bonds than the alcohols. Hence, they are more soluble in water than alcohols and have a higher boiling point. Phenols occur in either the form of colourless liquids or white solids at room temperature and can be highly toxic and caustic in nature. In this article, we will learn about the properties of phenol in detail and include both the physical properties of phenol and the chemical properties of phenol.
Properties of Phenol in Detail
1. Boiling Point of Phenol
Phenols usually have a higher boiling point when compared to the other hydrocarbons that have equal molecular masses. This is because of the presence of the intermolecular hydrogen bonding between the hydroxyl groups of the phenol molecules. In general, the boiling point of phenol increases when there is an increase in the number of carbon atoms.
2. Solubility of Phenol
The solubility of phenol in water is generally decided by the hydroxyl group that is present.
The hydroxyl group in phenol is involved to form the intermolecular hydrogen bonding.
Therefore, these hydrogen bonds are formed between water and phenol molecules that make phenol soluble in water.
However, the aryl group that is attached to the hydroxyl group is hydrophobic.
Hence, the solubility of phenol tends to decrease with an increase in the size of the aryl group present.
3. Acidity of Phenols
Phenols react with active metals like sodium, potassium, etc. and form the corresponding phenoxide. These reactions of phenols say about them that they are acidic in nature. In phenol, the sp2 hybridized carbon in the benzene ring which is attached directly to the hydroxyl group acts as the electron-withdrawing group. Therefore, it tends to decrease the electron density of oxygen.
Because of the delocalization of the negative charge in the benzene ring, the phenoxide ions are generally more stable than the alkoxide ions. Phenols, as a result, are more acidic in nature than alcohols. In the case of the substituted phenols, the acidity decreases if there is an electron-donating group which is attached to the ring when the acidity increases in case of the electron-withdrawing group.
4. Chirality of Phenols
Phenols exhibit the property of chirality within their molecules, for example, in catechin. This chirality is because of the absence of a planar and axial symmetry in the molecule of phenol.
The Application of Phenol in Day to Day Life
Phenol is an organic compound that is used in various fields, laboratory experiments and medical procedures. Even though it is a toxic organic substance, It is still contained in small amounts in mouth washed and in other day-to-day products.
Phenol is Used as an Injection
Phenol is used as an injection to treat muscle spasms. During severe cases of muscle stiffness, The person loses the ability to walk, phenol is used as it helps relax and limit the signals sent to the brain.
It is also used in the treatment of severe cases of an ingrown toenail. However, the news reports have said that hydrogen peroxide has fewer complications than phenol.
Phenol Used a Preservative
The vaccination doses for polio, smallpox, typhoid, meningitis and pneumonia are all stored in phenol for preservation.
Phenol Used in Chemical Peel
The compounds derived by the phenol are used in chemical peels as they let go of old and damaged skin from the surface. Although not very safe, The derived compounds might be highly mild when used on the skin. The phenol of trichloroacetic acid is used for this case. It can infiltrate deep into the skin.
Phenol is Used in Cosmetic and Food Preservation
Phenol is also used for food as well as cosmetic preservations. The safety is assured as it is FDA approved. Although, now the use of phenol derived BHT(butylated hydroxytoluene) is stopped as public awareness regarding its toxicity has come to the light.
Phenol and Health
Even though the toxicity of phenol is widely known, it is still used in the field of medicine. There are some rare benefits of phenol as well.
Did you know that flavonoids contained in teas, fruits, vegetables and wines are actually a derivative compound of phenol!
Apart from this, the resveratrol found in certain fruits, red wines as well nuts is also a phenol derivative.
Strangely, phenol derived compounds might have the properties to prevent cancer.
The Risk Involved in Using Phenol in our Day-To-Day Lives
If a human being is exposed to phenol for an extended period of time, it may lead to the risk of heart disease.
If phenol or the related compounds carrying any amount of phenol is physically ingested by a human being, it will severely damage the digestive parts of the body such as the esophagus, the stomach, the intestines as well as other body parts that are in contact with it.
When phenol comes in contact with the skin, it will immensely irritate and damage the skin. This will lead to severe skin conditions such as corrosion, blisters etc. Phenol is absorbed by the skin leading to other unidentified dangers to the human body.
Inhaling the fumes of phenol for a long time will lead to difficulty breathing and other lung-related conditions. It is also known to cause a lot of damage to the muscles.
Precautions to be Taken When Using Phenol
As a protective measure when using phenol, one must always wear long fitting trousers, shoes covering the toe, and protective eyewear. Also, a lab coat is recommended. Fully covering gloves is a must. The chloroprene gloves are not recommended, wear thicker gloves instead.
Classification of Phenols
Although there are various ways in which a phenol can be classified, It is generally done based on the number of phenol units present. The compound in which only one phenol unit is present is called a parent compound, and other compounds in which more than one unit of phenol are present are called polyphenols.
Phenol is used in chemical synthesis and as a disinfectant, it is a parent compound.
BHT- Butylated hydroxytoluene is a food additive.
Picric acid is trinitro-phenol and is a kind of explosive material. It is also known to be used as a dye.
Xylenol is also used in disinfectants and antiseptics.
Phenolphthalein has used an indicator of pH
Orthophenylphenol is used in waxing fruits and is a form of fungicide.
Phenolic Acid and Human Diet
Phenolic acid is composed of an organic carboxylic acid and a phenolic ring. It belongs to the group of aromatic compounds and is widely found in plant kingdoms.
Generally, it is also referred to as polyphenol and is a kind of phytochemical.
Phenolic acid is considered to be a crucial dietary supplement. It contains rich antioxidants and is known to improve human health. Flavonoid is a type of phenolic acid. For more information on phenolic acids, visit Vedantu.
Appearance of Phenol
In appearance, the phenol resembles a solid crystalline, is light pink coloured and has a sweet acrid odour to it.
FAQs on Properties of Phenol
1. What are the key physical properties of phenol?
Phenol exhibits several distinct physical properties. Understanding these helps in identifying and handling the compound:
- State and Colour: At room temperature, pure phenol is a colourless, crystalline solid with a needle-like structure. However, it often turns pink or reddish-brown upon exposure to air and light due to slow oxidation.
- Odour: It has a characteristic sweet and medicinal odour, often described as “carbolic.”
- Solubility: Phenol is sparingly soluble in water at room temperature due to its ability to form hydrogen bonds with water molecules. Its solubility increases significantly at higher temperatures. It is readily soluble in organic solvents like ethanol, ether, and chloroform.
- Boiling Point: The boiling point of phenol (181.7 °C) is considerably higher than that of hydrocarbons with similar molecular mass. This is due to the presence of intermolecular hydrogen bonding between phenol molecules.
2. Why is phenol acidic in nature?
Phenol is acidic because the hydroxyl (-OH) group is directly attached to a sp²-hybridised carbon of the benzene ring. This ring acts as an electron-withdrawing group, increasing the polarity of the O-H bond. Upon losing a proton (H⁺), phenol forms the phenoxide ion (C₆H₅O⁻). This ion is highly stable because the negative charge on the oxygen atom is delocalised across the benzene ring through resonance. This stabilisation of the conjugate base makes it favourable for phenol to donate a proton, thus exhibiting acidic character.
3. How does the acidity of phenol compare to that of ethanol and carboxylic acids?
The acidity of phenol is intermediate when compared to ethanol and carboxylic acids.
- Phenol vs. Ethanol: Phenol is more acidic than ethanol. This is because the phenoxide ion (phenol's conjugate base) is stabilised by resonance, spreading the negative charge over the benzene ring. In contrast, the ethoxide ion (ethanol's conjugate base) has its negative charge localised on the oxygen atom, which is destabilised by the electron-donating effect of the ethyl group.
- Phenol vs. Carboxylic Acids: Phenol is less acidic than carboxylic acids (like acetic acid). Although both phenoxide and carboxylate ions are resonance-stabilised, the negative charge in a carboxylate ion is delocalised over two highly electronegative oxygen atoms. In the phenoxide ion, the charge is spread over one oxygen and less electronegative carbon atoms, making the carboxylate ion more stable and the parent acid stronger.
4. Why is the hydroxyl group in phenol activating and ortho-, para-directing?
The hydroxyl (-OH) group in phenol is an activating group because the lone pair of electrons on the oxygen atom participates in resonance with the benzene ring. This delocalisation increases the electron density within the ring, making it more susceptible to attack by electrophiles compared to benzene itself. This effect is strongest at the ortho (positions 2, 6) and para (position 4) positions. Consequently, incoming electrophiles are directed to these electron-rich sites, making phenol an ortho-, para-directing substituent.
5. What are some important electrophilic substitution reactions of phenol?
Due to the activating nature of the -OH group, phenol readily undergoes electrophilic substitution. Key examples include:
- Nitration: With dilute nitric acid at low temperatures, phenol gives a mixture of ortho- and para-nitrophenols. With concentrated nitric acid, it undergoes extensive nitration to form 2,4,6-trinitrophenol, commonly known as picric acid.
- Halogenation: When phenol reacts with bromine water, it gives a white precipitate of 2,4,6-tribromophenol. The reaction is so fast that it does not require a Lewis acid catalyst.
6. What is the importance of the Reimer-Tiemann reaction for phenol?
The Reimer-Tiemann reaction is a crucial method for introducing a formyl group (-CHO) onto the benzene ring of phenol, a process known as formylation. When phenol is treated with chloroform (CHCl₃) in the presence of an aqueous alkali like sodium hydroxide (NaOH), salicylaldehyde (o-hydroxybenzaldehyde) is formed as the major product. This reaction is an important synthetic route for preparing aromatic aldehydes, which are valuable intermediates in the pharmaceutical and fragrance industries.
7. Explain the chemical property that allows phenol to be converted into aspirin.
The conversion of phenol to aspirin involves two key chemical reactions. First, phenol undergoes the Kolbe's reaction (or Kolbe-Schmitt reaction), where it is treated with sodium hydroxide and then carbon dioxide under pressure. This introduces a carboxylic acid group, forming salicylic acid. In the second step, the salicylic acid undergoes acetylation. The hydroxyl group of salicylic acid reacts with acetic anhydride in the presence of an acid catalyst to form acetylsalicylic acid, which is the chemical name for aspirin.
8. What are the primary industrial uses and applications of phenol?
Phenol is a vital industrial chemical used as a precursor to a wide range of products. Its primary applications include:
- Polymer Production: It is a key component in manufacturing phenolic resins like Bakelite (used in electrical switches and adhesives) and polycarbonates.
- Pharmaceuticals: It is used to synthesise various drugs, most notably aspirin. It is also used in the production of some antiseptics and disinfectants.
- Chemical Synthesis: Phenol is a starting material for producing other important chemicals, such as caprolactam (for nylon-6), bisphenol A (for epoxy resins), and various herbicides.
9. Since phenol has an -OH group, why doesn't it undergo nucleophilic substitution reactions like alcohols do?
Phenol does not typically undergo nucleophilic substitution reactions for two main reasons. Firstly, the C-O bond in phenol has a partial double-bond character due to resonance between the lone pairs on the oxygen and the benzene ring. This makes the bond stronger and shorter than the C-O single bond in alcohols, making it difficult to break. Secondly, the carbon atom of the benzene ring attached to the -OH group is sp² hybridised, which is more electronegative and less susceptible to nucleophilic attack compared to the sp³ hybridised carbon in alcohols.
