Physical and chemical properties of phenols

Phenols are formed by the substitution of one H atom from aromatic hydrocarbons with a –OH group. Phenol is a derivative of benzene, whereby a –OH group is hooked up to the benzene formula. Relying upon the no. of OH hooked up to the ring, the phenols are classified as mono or dihydroxy or polyhydroxy derivatives. The name ‘phenol’ is given to the primary derivative, and also the group members are named as derivatives of phenol. For instance, o-cresol, with –CH3 group and one –OH, is termed 2-methyl phenol.

Structure of Phenol

Phenol incorporates a ring structure, and thus, the carbon atom within the ring (with alternate double bonds) is sp2 hybridized. The –OH group is hooked up to the sp2 hybridized atom present within the aromatic ring. Hence, the C–O bond is made from the overlapping of the sp3 orbital of oxygen and also the sp2 hybridized orbital of carbon within the aromatic ring takes place.

Physical Properties of Phenol

The structure of phenols consists of 2 parts:

1. The aromatic ring or the aryl group
2. The useful group (OH) or the hydroxyl

In phenols, the presence of substituents within the ring can alter or modify their properties slightly or significantly relying upon the sort and range of substituents present.

Some vital physical properties of phenols are:

1. Physical state: Phenols are colourless liquids or solids. However, they generally turn reddish-brown in the atmosphere due to oxidation.

1. Boiling point: Boiling points of phenols increase with a rise within the range of carbon atoms due to the increase of the van der Waals forces. Also, the hydroxyl group is in an intermolecular H bonding and, therefore, exists as associated molecules, thereby increasing the B.P(boiling point) of phenols.

III. Solubility in water: Due to the ability to make H bonding, phenols are promptly soluble in water. However, with the addition of different hydrophobic rings within the ring, the solubility decreases.

Chemical Properties of phenols and their compounds 

Again here, the chemical reactions of phenols are often classified beneath 2 headings – one involving the cleavage of the O–H bond and the other involving the cleavage of the C–O bond.

1.Reactions involving O–H bond cleavage

2. With metals: Phenols react with metals like atomic number sodium(Na), Potassium (K) and Aluminium (Al), etc., to make phenoxide with the discharge of hydrogen gas.

Apart from active metals, phenol gives sodium phenoxide in reaction with aqueous  NaOH,  releasing water molecules as a by-product.

The acidity of phenols is due to the discharge of H+ ions from the hydroxyl. The explanation behind the discharge of a proton is that the OH is concerned in resonance, and thus, the chemical element gets a partial ‘+ve’ charge. This permits the H+ particle to move out simply, thereby creating phenols, a Bronsted acid. Also, phenols are stronger acids than their counterparts, alcohols, due to the very fact that the phenoxide particle (formed when the discharge of a proton) is stabilised by resonance. This makes the removal of H+ ions easier in phenols.

Formation of Esters (Esterification)

Phenols react with carboxylic acids, acid anhydride and acid chlorides to make esters. The esterification reaction is dispensed within the presence of sulphuric acid. The reaction is reversible. The water molecule obtained from the reaction is instantly removed to facilitate the completion of the reaction. Esterification reaction with acid chloride takes place within the presence of a base (pyridine) to neutralize the HCl formed throughout the reaction.

Acetylation

The reaction of phenols with an acid anhydride and also the introduction of acetyl radical (CH3CO–) are known as acetylation, and also the product obtained is aspirin.

II. Electrophilic Aromatic Substitution

The –OH group within the phenol activates the benzene ring and pushes it towards electrophilic substitution. Also, the substituents or the coming groups are directed by the OH  group towards the ortho and para positions of the ring. The cause behind this can be a result of the resonance impact of the ring structure making these positions electron-rich.

Nitration Reaction

Addition of an –NO2 group to the ring is termed nitration. Phenol reacts with dilute acid at 298K (low temperature) yields ortho and para nitrophenols. The ortho and para nitrophenols therefore obtained are often separated using steam distillation. The intramolecular H bonding in ortho nitrophenol makes it steam volatile, whereas para nitrophenol is a smaller amount volatile as a result of its intramolecular H bonding. Phenol yields a pair of,4,6- trinitrophenol or acid once treated with conc. Nitric acid. The yield is poor. 

Halogenation Reaction

Phenols react with bromine( atomic no.35) to yield completely different products by different experimental conditions.

It reacts with bromine in low polarity solvents, like chloroform or CS2, at low temperatures to make mono bromophenols. The explanation is that the non-polar or low-polar solvents will solely activate the ring within the one and four positions. Hence, mono-substituted products are only formed.

Kolbe’s Reaction

Phenol reacts with sodium hydroxide and greenhouse gas(carbon dioxide)  to make sodium salicylate,  then acidifies to yield benzoic acid. The phenoxide particle formed with NaOH has higher reactivity than phenol towards aromatic, electrophilic substitution, and thus, acidifies to supply hydroxy acid.

Reimer-Tiemann Reaction

Phenol, on reaction with chloroform (CHCl3), within the presence of NaOH, forms an aryl aldehyde. A –CHO group is introduced into the ring. This reaction is called or known as the Reimer-Tiemann reaction. The electrophile shape during this reaction (: CCl2) is termed dichlorocarbene. Intermediate, substituted benzyl chloride is made, which is then hydrolyzed into salicylaldehyde, the product, within the presence of alkali.

Uses of Phenol

Phenols and their derivatives are utilized in many applications, such as:

1. Phenols are used as disinfectants, antiseptics in soaps and for manufacturing of acid. It is also used in making medicine in the pharmaceutical trade
2. In addition to being widely used in household products, phenols are also commonly used as intermediates in industrial synthesis. For example, phenol itself is used as a disinfectant in household cleaners and mouthwashes (at low concentrations) to prevent the spread of bacteria. 
3. Phenol is used in industry as a starting material for the production of plastics, explosives such as picric acid, and pharmaceuticals such as aspirin. 
4. When exposed silver bromide crystals are exposed to the light, phenol hydroquinone, a common component of photographic developer, transforms them into black metallic silver. 
5. A variety of substituted phenols are used in the dye industry to produce dyes that are intensely coloured. 
6. Phenol mixtures (particularly the cresols) are used as components in wood preservatives such as creosote, which is made from phenols.

Conclusion

Phenols have an –OH group hooked up to the benzene formula, that exhibits some characteristic physical and chemical properties. They jibe in their chemical and physical properties to alcohol-attributable to the presence of the OH group. However, they have their distinctive chemical properties due to the ring structure and resonance, like electrophilic substitution reactions. They’re used as disinfectants, antiseptics in soaps, preservatives for inks, etc.