The aromatic hydroxy compounds in which one or more hydroxyl groups are directly attached to the aromatic nucleus are called phenols. Phenol is prepared from chlorobenzene, benzene sulphonic acid, cumene or aniline. In this article, we will study the chemistry of Phenols, Types of Phenols, IUPAC Nomenclature of Phenols, Methods of Preparation, Physical and Chemical Properties with Reactions, Formulae, Examples, and Uses.
What are Phenols?
In organic chemistry, aromatic hydroxy compounds in which one or more hydroxyl groups are directly attached to the aromatic nucleus (i.e. benzene like a ring) are called phenols. The class name ‘phenols’ is adopted from the simplest member, monohydric benzene (in Greek Phene means benzene, ol means OH).
Phenols are about a million times more acidic than alcohols but less acidic than carboxylic acids. Phenol undergoes electrophilic aromatic substitution like bromination, nitration, sulphonation etc. Due to the resonance effect, the -OH group in phenol directs the incoming electrophile to ortho and para positions.
Phenol yields salicylic acid by Kolbe’s reaction and salicylaldehyde by Reimer – Tiemann reaction.
Classification of Phenols
Phenols are classified as monohydric, dihydric and trihydric depending upon the presence of one, two or three hydroxyl groups attached to the aromatic rings. Their structures are as follows:
Nomenclature of Phenols
The Nomenclature of Alcohols is done in two ways:
Common or Trivial Names
Phene was the old name of benzene and its hydroxyl derivative was
called phenol. Systematic name of phenol is benzenol. The prefixes used are ortho (o) for 1, 2-disubstituted.meta (m) for 1.3-disubstituted and para (p) for 1,4-disubstituted compounds of phenol.
In the IUPAC system the numbering of the ring starts with –OH substituted carbon as 1. Other substituted carbon is indicated by the lowest number.
Substituents are cited in alphabetical order.
A number of phenols are assigned special names. The name phenol for the hydroxy derivative of benzene is accepted by the IUPAC system.
Compounds having one substituent are named as the derivative of phenol.
When carboxyl group, a carbonyl group or ester group is present along with -OH group then, phenols are named as derivatives of these compounds.
Read about Amines here
Structure of Phenols
In phenols, the -OH group is attached to the sp2 hybridised carbon atom of the aromatic ring. The carbon-oxygen bond length (136pm) in phenol is slightly less than in methanol.
This is due to (i) partial double bond character on account of the conjugation of unshared (lone) electron pair of oxygen with \(\pi\) electrons of the aromatic ring and (ii) sp2 hybridized state of carbon to which oxygen is bonded.
Preparation of Phenols
Phenols can be prepared by the following approaches:
From Chlorobenzene: (Dow Process)
In this method, chlorobenzene is heated with excess aqueous sodium hydroxide at 613K under the pressure of 300 atmospheres. Sodium phenoxide formed in the process is hydrolyzed using dilute hydrochloric acid to get phenol. Alternatively, a current of carbon dioxide is passed through aqueous sodium phenoxide to get phenol.
From Chlorobenzene (Raschig Method)
In this method, phenol is obtained by heating chlorobenzene with steam at 698K using \(Ca_3(PO_4)_2\) or \(SiO_2\), as the catalyst. The hydrolysis can also be affected by water in the presence of copper as a catalyst as 673K and under pressure,
From Benzene Sulphonic Acid
Benzene is sulphonated with oleum to give benzene sulphonic acid. Benzene sulphonic acid is neutralized using aqueous sodium hydroxide to obtain sodium benzene sulphonate.
Dry sodium benzene sulphonate is then fused with an excess of sodium hydroxide when sodium phenoxide is obtained along with sodium sulphite. Sodium phenoxide on treatment with dilute sulphuric acid gives phenol.
From Cumene (Commercial Method)
Nearly all phenols that are required today are obtained by this method. An alkaline solution of cumene (isopropyl benzene) in sodium carbonate is oxidized by passing air or oxygen using cobalt naphthenate as a catalyst at 423K, and cumene hydroperoxide is obtained. Cumene hydroperoxide on heating with dilute sulphuric acid decomposes to give phenol and acetone. Acetone is an important byproduct of the reaction and is separated by distillation. This reaction is called auto-oxidation.
From Aniline (Diazotization)
When aniline (a primary aromatic amine) is treated with nitrous acid (NaNO, and HCl) under ice-cold conditions, it gives benzene diazonium salt. This process is called diazotization. The aqueous solution of diazonium salt is hydrolyzed with dilute sulphuric acid to give phenol.
Physical Properties of Phenol
- Phenol has a melting point of 315K and a boiling point of 455K. The high boiling point of phenol is due to intermolecular hydrogen bonding.
- Pure phenol is a colourless crystalline solid. Phenol has a characteristic smell known as phenolic or carbolic odour.
- Phenol is sparingly soluble in water but completely soluble in alcohol, ether etc.
- Phenol is a weak acid, weaker than even carbonic acid. Phenol turns pink on exposure to air and light due to the formation of mixtures of quinones.
Reactions of Phenol
The -OH group attached to the aromatic ring is known as a phenolic group. This group activates the ortho and para positions of the benzene rings as these positions become electron-rich due to the resonance effect caused by the -OH group. Phenol thus gives a mixture of ortho and para products on substitution. Phenol is a bifunctional compound as it undergoes reactions due to the –OH group as well as electrophilic aromatic substitution.
Acidity of Phenols
Phenols react with strong bases like sodium hydroxide and metals like sodium, potassium, aluminium etc. These reactions indicate the acidic nature of phenols. Phenols
are about a million times more acidic than alcohols but less acidic than a carboxylic acid. Let us consider the ionization of alcohols and phenols,
In alcohols, the -OH group is attached to sp3 hybridized carbon while in phenols it is attached to sp2 hybridized carbon.
Due to the higher electronegativity of sp2 hybridized carbon, electron density on oxygen decreases. This increases the polarity of the -OH bond which results in more ionization of phenol than that of alcohols.
Further, in alkoxides ions, the negative charge is localized on oxygen while in phenoxide ions the negative charge is delocalized. Due to delocalization of the negative charge over the ortho and para positions of the aromatic ring, phenoxide ion is more stable than phenol.
There is also delocalization of charge in phenol, but its resonance structures (VI-X) have charge separation due to which the phenol molecule is less stable than phenoxide ion. This favours the ionization of phenol. Thus phenols are more acidic than alcohol and water.
Phenols with electron-withdrawing substituents like the nitro group are more acidic because these substituents delocalize the negative charge. This effect is more pronounced when such a group is present at ortho or para positions. It is due to the effective delocalization of negative charge in phenoxide ions. On the other hand, phenols with electron-donating substituents like alkyl groups are less acidic because these substituents do not delocalize the negative charge. Compounds with smaller \(pK_a\), values are more acidic.
Electrophilic Aromatic Substitution
Following reactions involve the Electrophilic Aromatic Substitution in Phenols.
Halogenation: Bromination of Phenol
The hydrogen atoms of the benzene ring of phenol are substituted by halogen. The products of the reaction depend on the nature of the solvent used for the reaction.
When phenol is treated with bromine at low temperature in a solvent, such as carbon disulphide or chloroform, a mixture of o-bromophenol and p-bromophenol is obtained. p-bromophenol is the major product.
When phenol is treated with bromine water, it gives a white precipitate of 2, 4, 6-tribromophenol.
Nitration of Phenol
Substitution of the hydrogen atom of the benzene ring of phenol by a nitro group is called nitration of phenol. The reagent used is nitric acid and the product depends upon the concentration of nitric acid and temperature.
When phenol is treated with dilute nitric acid at room temperature a mixture of o-nitrophenol and p-nitrophenol is obtained and o-nitrophenol is the major product.
By steam distillation, the ortho and para isomers can be separated. 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.
When phenol is heated with concentrated nitric acid in presence of concentrated sulphuric acid, it gives 2, 4, 6-trinitrophenol (also known as picric acid). However, the yield of the reaction is poor. The mixture of concentrated nitric acid and concentrated sulphuric acid is called a nitrating mixture.
Learn about Oxidation Numbers here.
Sulphonation of Phenol
Substitution of the hydrogen atom of the benzene ring of phenol by a \(-SO_3H\) group is called sulphonation of phenol. The product formed depends upon the temperature of the reaction. When phenol is treated with concentrated sulphuric acid at room temperature (298 K) it gives o-phenol sulphonic acid as the major product but at 373 K. p-sulphonic acid is a major product.
Phenol reacts with sodium hydroxide to give sodium phenoxide. Phenoxide ion undergoes electrophilic substitution with carbon dioxide (a weak electrophile) because phenoxide ion is more reactive than phenol. Salicylic acid is formed as the major product. At lower temperatures, ortho isomer predominates, whereas para isomer is obtained at a higher temperature.
Reimer – Tiemann reaction
Phenol in presence of sodium hydroxide reacts with chloroform to give salicylaldehyde.
Ortho isomer is a major product that can be separated from para isomer by steam distillation. The predominance of ortho isomer is due to its greater stability by intramolecular hydrogen bonding.
Reaction with Zinc Dust
When phenol is heated with zine dust, it gives benzene.
Phenol is oxidized by chromic acid to a diketone, benzoquinone. It is a conjugated diketone.
Distinguishing Test Between Alcohols and Phenol
Though both phenol and alcohols are hydroxyl compounds, some of their properties are different. Phenol is weakly acidic in nature and turns blue litmus paper red. Alcohols are almost neutral and have no action on litmus paper. When treated with aqueous neutral ferric chloride solution, phenol gives violet colour while alcohols do not give violet colour.
Uses of Phenols
- Phenol is used in the preparation of
- Phenol-formaldehyde polymer is used in a plastic Bakelite.
- Phenolphthalein and certain dyes.
- Dettol is used as an antiseptic.
- Drugs such as salicylic acid, salol, aspirin etc.
- 2,4-dichloro phenoxy acetic acid which is used as a selective weed killer.
- Picric acid which is used as explosive
Phenols in Everyday Life
- Some phenols are antioxidants and are used in foods. Phenols are also used in the manufacture of synthetic polymers and for the natural flavouring of tea and coffee.
- A number of phenols and phenolic ethers occur in nature. An oil from naturally occurring clove is used by dentists as an antiseptic (is phenolic). Thymol, phenol is used in mouth wash as a flavour and as an antiseptic.
- 2, 5-dichlorophenol is isolated from the defensive secretion of a species of grasshopper.
- Phenols occur widely throughout nature and also serve as intermediates in the industrial synthesis of products as various adhesives and antiseptics.
- Phenol itself is a general disinfectant found in coal tar.
- Methyl salicylate, a flavouring agent, is found in the oil of wintergreen.
So, this is all about the Phenols. Get some practice of the same on our free Testbook App. Download Now!
Q.1What do phenols do to the body?
Ans.1 Phenols are known to be antioxidants. They can stop the reaction of free radicals with other molecules in your body, preventing damage to your DNA. Plant-based compounds containing phenol are good for health.
Q.2What are phenols in food?
Ans.2 The aromatic hydroxy compounds in which one or more hydroxyl groups are directly attached to the aromatic nucleus (i.e. benzene like a ring) are called phenols.
Q.3What are examples of phenolics?
Ans.3 Ferulic acid, gallic acid, resveratrol, tannic acid etc. are examples of phenolics.
Q.4What does phenol do to the skin?
Ans.4 Phenols cause severe burns that may not be immediately painful or visible on the skin. They have an anaesthetic effect on the skin above 1.5% concentration.
Q.5Are phenols healthy?
Ans.5 Phenols are known to be antioxidants. They can stop the reaction of free radicals with other molecules in your body, preventing damage to your DNA. Phenolic acids are readily absorbed through intestinal tract walls. Hence, they are healthy.
Phenols are similar to alcohols but form stronger hydrogen bonds. Thus, they are more soluble in water than are alcohols and have higher boiling points. Phenols occur either as colourless liquids or white solids at room temperature and may be highly toxic and caustic.
The IUPAC name for Phenol is Monohydroxybenzene or C6H5OH.
Phenol | C6H5OH - PubChem.
- Phenol Injection. Phenol can be injected into your muscles to treat a condition known as muscle spasticity. ...
- Chemical matrixectomy. Phenol is commonly used in surgeries for ingrown toenails. ...
- Vaccine preservative.
- Sore throat spray. ...
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- Phenol liquid. ...
- Soap and antiseptic.
Phenol is an aromatic compound. The chemical formula of this organic compound is C6H6O. Phenol is also known as Carbolic acid. It consists of a hydroxyl group and phenyl group attached to each other. It considerably dissolves in water.
PHENOL (CHOH) ОН Phenol is also known as carbolic acid. It is an aromatic compound having molecular formula C HOH. The molecule consist of a phenyl group (CH) bonded to a hydroxy group (-OH).
Phenols are the compounds formed when a hydrogen atom in an aromatic ring, is replaced by a hydroxyl group (-OH group).
Compounds in which an OH group is attached directly to an aromatic ring are designated ArOH and called phenols. Phenols differ from alcohols in that they are slightly acidic in water. They react with aqueous sodium hydroxide (NaOH) to form salts. The parent compound, C 6H 5OH, is itself called phenol.
Phenyl groups are closely related to benzene and can be described as a benzene ring, minus a hydrogen, which can be substituted as a functional group by any other element or compound. Phenyl groups have six carbon atoms in a hexagonal planar structure, of which five are bonded to hydrogen atoms.
What is Phenol? Phenol is an aromatic, organic compound with a hydroxyl functional group, -OH. Phenols are a group of aromatic carbon compounds derived from alcohol. They are aryl alcohols. Essentially, the phenol group is a hydroxyl group bonded to a carbon atom in a benzene ring.
Phenol reacts with: A base (like NaOH) to form the phenoxide anion. This is a deprotonation reaction, due to the removal of the proton (hydrogen). Acetyl chloride or acetic anhydride to form an ester (OH group is replaced by an O-alkyl group)
phenols. Phenols are highly reactive toward electrophilic aromatic substitution, because the nonbonding electrons on oxygen stabilize the intermediate cation. This stabilization is most effective for attack at the ortho or para position of the ring; therefore, the hydroxyl group of a phenol is…
If bromine water is added to a solution of phenol in water, the bromine water is decolorized and a white precipitate is formed which smells of antiseptic. The precipitate is 2,4,6-tribromophenol.
- Alcohols are colourless.
- They generally give a sweet smell except for glycerol and a few lower alcohols.
- They are flammable and produce a blue flame.
- They don't produce any smoke while burning.
- They are generally liquid at room temperature.
What are the physical and chemical properties of alcohols and phenols? Lower alcohols are colourless liquids at normal temperature. The higher alcohols are colourless, odourless waxy solids. Phenols, like alcohols, are either colourless liquids or solids.
Phenol is a weak acid. It reacts with NaOH to produce salt.
Phenols are the compounds formed when a hydrogen atom in an aromatic ring, is replaced by a hydroxyl group (-OH group).
Phenol is prepared from chlorobenzene, benzene sulphonic acid, cumene or aniline.. Benzene sulphonic acid is neutralized using aqueous sodium hydroxide to obtain sodium benzene sulphonate.. Phenol thus gives a mixture of ortho and para products on substitution.. Phenol is a bifunctional compound as it undergoes reactions due to the –OH group as well as electrophilic aromatic substitution.. These reactions indicate the acidic nature of phenols.. Phenols. Thus phenols are more acidic than alcohol and water.. The products of the reaction depend on the nature of the solvent used for the reaction.. Sulphonation of Phenol Substitution of the hydrogen atom of the benzene ring of phenol by a \(-SO_3H\) group is called sulphonation of phenol.. When phenol is treated with concentrated sulphuric acid at room temperature (298 K) it gives o-phenol sulphonic acid as the major product but at 373 K. p-sulphonic acid is a major product.. A number of phenols and phenolic ethers occur in nature.. So, this is all about the Phenols.. Ans.2 The aromatic hydroxy compounds in which one or more hydroxyl groups are directly attached to the aromatic nucleus (i.e. benzene like a ring) are called phenols.
In this article, we will study the chemistry of Ethers, Types of Phenols, IUPAC Nomenclature of Ethers, Methods of Preparation, Physical and Chemical Properties with Reactions, Formulae, Examples, and Uses.. Like alcohols, ethers can form hydrogen bonds with water molecules.. Ethers are organic derivatives of water having two alkyl or aryl groups bonded to oxygen atoms.. The ethers, in which both the alkyl or aryl groups attached to an oxygen atom are the same, are called simple ethers.. The ethers, in which the two alkyl or aryl groups attached to the oxygen atom are different, are called mixed ethers.. Like alcohols, ethers can form a hydrogen bond with water molecules.. Like alcohols, ethers can form a hydrogen bond with water molecules.
Preparation of phenols from diazonium salts, benzene sulphonic acid, haloarenes, cumene.. They are weak acids and mostly form phenoxide ions by dropping one positive hydrogen ion (H+) from the hydroxyl group., phenol was mainly manufactured from coal tar.. Preparation of Phenols From Benzene Sulphonic Acid. Preparation of Phenols From Cumene Cumene is an organic compound acquired by Friedel-Crafts alkylation of benzene with propylene.. In this method, benzene sulfonic acid is reacted with aqueous sodium hydroxide.. The resulting salt is mixed with solid sodium hydroxide and reacted at a high temperature.. In the 1st step, a pair of electrons on the oxygen of the hydroperoxide's “hydroxyl group” is attracted to a proton of the H 3 O + molecule, making an oxonium ion.. It is also known as phenolic acid.. Phenols are organic compounds containing at least one -OH group directly attached to the benzene ring.
SOLUBILITY The hydroxyl groups in alcohols can form hydrogen bonds with water and many low molecular weight alcohols are miscible with water.. For isomeric alcohols the boiling points follow the order:- primary alcohol > secondary alcohol > tertiary alcohol. Methyl alcohol > Primary alcohol > Secondary alcohol > Tertiary alcohol. Primary alcohol > Secondary alcohol > Tertiary alcohol. REACTIONS INVOLVING RUPTURE OF –C–O BOND Reaction with HX Alkyl halides are formed.. The reaction of ethyl alcohol with H2SO4 is very sensitive to reaction conditions. 3º alcohol > 2º alcohol > 1º alcohol. Distilled: Prepared by distillation of fermented liquids. The primary alcoholic groups are more reactive than secondary alcoholic group.. Reaction with nitric acid. nitric acid gives glyceric acid + tartronic acid. From phenolic acids Sodium salts of phenolic acids distilled with soda lime. ACYLATION – with acid chloride or acid anhydride. REACTIONS GIVEN BY PHENOLS ONLY
For isomeric alcohols the boiling points follow the order:- primary alcohol > secondary alcohol > tertiary alcohol. Methyl alcohol > Primary alcohol > Secondary alcohol > Tertiary alcohol. Primary alcohol > Secondary alcohol > Tertiary alcohol. REACTIONS INVOLVING RUPTURE OF –C–O BOND Reaction with HX Alkyl halides are formed.. 3º alcohol > 2º alcohol > 1º alcohol. Distilled: Prepared by distillation of fermented liquids. Reaction with nitric acid. nitric acid gives glyceric acid + tartronic acid. ACYLATION - with acid chloride or acid anhydride. REACTIONS GIVEN BY PHENOLS ONLY
LibGuides: CHE 120 - Introduction to Organic Chemistry - Textbook: Chapter 2 - Alcohols, Phenols, Thiols, Ethers ›
Does CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 have a functional group?. Note that there are four butyl alcohols in the table, corresponding to the four butyl groups: the butyl group (CH 3 CH 2 CH 2 CH 2 ) introduced in Chapter 1 "Organic Chemistry Review / Hydrocarbons", Section 1.5 "IUPAC Nomenclature", and three others:. Condensed Structural FormulaClass of AlcoholCommon NameIUPAC NameCH 3 OH—methyl alcoholmethanolCH 3 CH 2 OHprimaryethyl alcoholethanolCH 3 CH 2 CH 2 OHprimarypropyl alcohol1-propanol(CH 3 ) 2 CHOHsecondaryisopropyl alcohol2-propanolCH 3 CH 2 CH 2 CH 2 OHprimarybutyl alcohol1-butanolCH 3 CH 2 CHOHCH 3 secondary sec -butyl alcohol2-butanol(CH 3 ) 2 (CH 3 ) 2 CHCH 2 OHprimaryisobutyl alcohol2-methyl-1-propanol(CH 3 ) 3 COHtertiary tert -butyl alcohol2-methyl-2-propanol. The alcohol 1-decanol (CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 OH) is essentially insoluble in water.. Alcohols of four or fewer carbon atoms are soluble in water because the alcohol molecules engage in hydrogen bonding with water molecules; comparable alkane molecules cannot engage in hydrogen bonding.. Chemical reactions in alcohols occur mainly at the functional group, but some involve hydrogen atoms attached to the OH-bearing carbon atom or to an adjacent carbon atom.. The reaction removes the OH group from the alcohol carbon atom and a hydrogen atom from an adjacent carbon atom in the same molecule:. Note that in oxidation of both primary (RCH 2 OH) and secondary (R 2 CHOH) alcohols, two hydrogen atoms are removed from the alcohol molecule, one from the OH group and other from the carbon atom that bears the OH group.. Tertiary alcohols (R 3 COH) are resistant to oxidation because the carbon atom that carries the OH group does not have a hydrogen atom attached but is instead bonded to other carbon atoms.. This alcohol has the OH group on a carbon atom that is attached to only one other carbon atom, so it is a primary alcohol.. This alcohol has the OH group on a carbon atom that is attached to three other carbon atoms, so it is a tertiary alcohol.. This alcohol has the OH group on a carbon atom that is attached to two other carbon atoms, so it is a secondary alcohol; oxidation gives a ketone.. Condensed Structural FormulaNameMolar MassBoiling Point (°C)Intermolecular Hydrogen Bonding in Pure Liquid?CH 3 CH 2 CH 3 propane44–42noCH 3 OCH 3 dimethyl ether46–25noCH 3 CH 2 OHethyl alcohol4678yesCH 3 CH 2 CH 2 CH 2 CH 3 pentane7236noCH 3 CH 2 OCH 2 CH 3 diethyl ether7435noCH 3 CH 2 CH 2 CH 2 OHbutyl alcohol74117yesEther molecules do have an oxygen atom, however, and engage in hydrogen bonding with water molecules.. The carbon groups on either side of the oxygen atom are propyl (CH 3 CH 2 CH 2 ) groups, so the compound is dipropyl ether.. Why does diethyl ether (CH 3 CH 2 OCH 2 CH 3 ) have a much lower boiling point than 1-butanol (CH 3 CH 2 CH 2 CH 2 OH)?
For most basic substituent groups (such as alkyl groups and halides), the base name for phenols is simple phenol.. This acidity owes to the ability of the benzene ring to stabilize a deprotonated hydroxyl group through electron delocalization (resonance forms).. The ion intermediate in this case then reacts with water, yielding phenol and a hydroxide ion.. Reactions of Aryl Halides and Phenols. Practice Problem: Propose an explanation of why single chlorination of phenol occurs para to the hydroxyl group and why multiple chlorination occurs para and ortho to the hydroxyl group.
We have already learnt that the nucleophilic substitution reactions of alkyl halides with dilute alkali, conversion of alkenes to alcohols by hydration and the preparation of alcohols using Grignard reagent in XI standard.. When alcohols are treated with Lucas agent (a mixture of concentrated HCl and anhydrous ZnCl 2 ) at room temperature, tertiary alcohols react immediately to form a turbidity due to the formation of alkyl chloride which is insoluble in the medium.. Secondary alcohols react within 10 minutes to form a turbidity of alkyl chloride where primary alcohols do not react at room temperature.. Among isomeric alcohols primary alcohols have higher boiling point and the tertiary alcohols have lower boiling points.. Alcohols undergo nucleophilic substitution reaction with hydro halic acids to form alkyl halides.. When alcohols are heated with a suitable dehydrating agents like sulphuric acid, the H and OH present in the adjacent carbons of alcohols are lost, and it results in the formation of a carbon – carbon double bond.. Alcohols react with active metals such as sodium, aluminium etc… to form the corresponding alkoxides with the liberation of hydrogen gas and similar reaction to give alkoxide is not observed in the reaction of alcohol with NaOH.. 1° alcohol > 2° alcohol > 3° alcohol. Ethers having primary alkyl group undergo SN 2 reaction while tertiary alkyl ether undergo SN 1 reaction.