Phenol definition-Phenol structure and Identification in chemistry

Phenol definition in organic chemistry  

The hydroxyl derivatives of aromatic hydrocarbon which contain hydroxyl group  are known as phenols . 

Phenol is an organic as well as aromatic compound with molecular formula C₆H₅OH . 

Phenol is also known as benzenol, carbolic acid, or as hydroxylbenzene. It is white or colourless crystalline  and also a volatile solid compound. 

Phenol was first extracted from coal tar . But now a days, phenol  is produced  in a large scale from petroleum .

Phenol structure and molecular formula   

Phenol is an organic as well as aromatic  compound with molecular formula C₆H₅OH. 
That is it consists of  one phenyl  group  ( – C₆H₅ ) and one  hydroxyl  group ( –  OH ), attached with phenyl  carbon atom. 

Each carbon atom of phenol is sp2 hybridized. The oxygen atom of phenolcontains two lone pair of electrons. 

The ‘O’-atom  shows sp3 hybridization. Phenol  has  hexagonal planar structure . 

The hexagonal planar structure and chemical or molecular formula of phenol is shown  below.  

Phenol resonating structure

The lone pair on oxygen atom of O–H bond in phenol participate in resonance with the pi electron of benzene ring .

There are five resonating structure of phenol through which phenol gets stability .

The resonating structures are ,

Identification of phenol with ferric chloride .

In addition of few drops of neutral ferric chloride ( FeCl₃ ) solution to phenol , the entire solution becomes violet in color . 

Because in neutral solution,phenolreacts with ferric chloride ( FeCl₃) to form violet color complex ion [ Fe ( OC₆H₅ )₆ ]^3-

Hence the color of the solution becomes violet. Phenol is identified with the help of this reaction .

Indeed, all the aliphatic or aromatic compounds with enol group create a characteristic color with neutral ferric chloride solution .

Depending on the structural shape of phenol the color of the solution may be green, blue, even red including violet.

Identification of phenol 

When phenol is dissolved in concentrated sulfuric acid and a few drops of aqueous  sodium nitrite is added ,the color of entire solution change into red on dilution. 

Finally the color turns blue when made alkaline with aqueous sodium hydroxide. 

With the help of the above experiment phenol is identified . The above  experiment is known as Liebermann reaction .

Why o-nitro phenol is more acidic than o-methoxy  phenol ?

In case of o-nitro phenol, the compound contains strong electron withdrawing nitro group( –NO₂ ). Due to –R and –I effect of nitro group , the density of electron on phenolic oxygen atom decreases. 

As a result , the ‘O’-atom attract the O – H bonded electron towards itself . Hence the O–H bond becomes more polar as well as weak .

Consequently, the weak O–H bond dissociate easily to leave proton in aqueous solution and  thus o-nitro phenol shows acidic nature .

On the other hand , in o-methoxy phenol, due to +R effect of electron repulsive –OCH3 group , the density of electron on oxygen atom as well as ‘O’-linked carbon atom increases.

As a result, the displacement of O–H bonded electron towards ‘O’-atom does not happen . Hence , the O–H bond becomes less polar that is more stronger with compare to  O–H bond of o-nitro phenol.

Therefore ,O–H bond does not break easily . Hence o-methoxy phenol shows less acidic property with compare to o-nitro phenol.

Summary 

• Phenol definition in organic chemistry  
• Phenol  structure and molecular formula   
• Identification of phenol with ferric chloride .
• Identification of phenol in chemistry
• Why o-nitro phenol is more acidic than o-methoxy  phenol ? 

Paracetamol chemistry-click here 

Aspirin-use of aspirin and aspirin from phenol .

What is aspirin?

Aspirin is a very important organic as well as aromatic compound. Aspirin is also known as acetyl salicylic acid.The systematic IUPAC name of aspirin is 2-acetoxy benzoic acid.  

Which one is more basic aniline or 4-bromoaniline ?

Which one is more basic between aniline and 4-bromo aniline?

Which one is more basic between aniline and 4-bromo aniline that depends on the tendency to donate electron. 4-bromo aniline contains an electron withdrawing group, –Br. 

friedel-crafts acylation reaction-friedel-crafts acylation of benzene-phenol-anisole and chloro benzene

What is Friedel crafts acylation?

There are few chemical reactions, where benzene or benzenoid compounds react with acyl chloride (RCOCl) in presence of anhydrous aluminium chloride ( AlCl3 ) catalyst, through which the hydrogen atom of benzene  ring be substituted  

by acyl group (RCO–) to form acyl benzene or benzenoid compounds that is aromatic ketone. This reaction is known as Friedel crafts acylation reaction.

Mechanism for Friedel crafts acylation reaction.

This is an example of very important aromatic electrophilic substitution reaction.

In this reaction , acyl chloride ( RCOCl ) is used as acylating agents . 

Besides acyl chloride , acetic anhydride is also used as acylating agent. The mechanism of Friedel crafts acylation reaction of benzene is shown below.

Application of Friedel crafts acylation

( I ) Although poly alkylation of benzene takes place easily yet poly acylation of benzene does not . 

Because acyl group being electron withdrawing group , can attract electron towards itself . As a result , the density of electron of benzene nucleus decreases. 

Since ,acylation of aromatic compound is an example of electrophilic substitution reaction , so further acylation of acyl benzene does not occur.

That is why poly acylation of benzene does not happen .There fore , in Friedel crafts acylation reaction pure aromatic ketone is prepared .

( II) In Friedel crafts acylation reaction, rearrangement reaction of carbon chain of acyl halide takes place.  

Consequently, in that case  no any other isomers ofexpected aromatic ketone is obtained . That is in this case we get the targeted product.

For example, acylation of benzene by propanoyl chloride gives only one product, ethyl phenyl ketone, which on clemension reduction produced  the expected product, propyl benzene .

What is intra molecular Friedel crafts acylation reaction ?

There are very few example of Friedel crafts acylation reaction , which occur in the absence of acylating agents. 

In that case, rearrangement of different part of a specific molecule takes place and give the expected product. 

This type of reaction is called Friedel crafts acylation reaction.

For example, Friedel crafts acylation of 4-phenyl butanoyl chloride in presence of  aluminium chloride ( AlCl3 ) produced  α-tetralone.

Friedel crafts acylation reaction of benzene 

Friedel crafts acylation reaction is an example of aromatic electrophilic substitution reaction.Benzene under goes electrophilic substitution reaction easily . 

Because due to participate in aromatic electrophilic substitution reaction , aromaticity of benzene thatis stability of benzene remain unchanged.

Consequently, benzene  exhibits Friedel crafts acylation reaction.

Few example of Friedel crafts acylation reaction of benzene molecule are shown above.

Friedel crafts acylation reaction of phenol 

Friedel crafts acylation of phenol is an example of aromatic electrophilic substitution reaction. Here electrophilile is acyl group ( RCO– ). Now phenol is an anbidant  nucleophile. 

 The lone pair electron on oxygen atom of phenol can attact the acyl electrophile . 

So, there are two type of Friedel crafts acylation reaction of phenol may be occur ,namely ‘O’-acylation and ‘C’- acylation. 

Therefore ,when phenol react with actyl chloride or acetic anhydride, two different type of reaction is happened.  

When O-acylation takes place, the product is phenyl acetate. Although phenyl acetate under goes fries rearrangement and ultimately convert into o-hydroxy aceto phenone and p-hydroxy aceto phenone.

But when C-acylation is occurred , the products are o-hydroxy aceto phenone and p-hydroxy aceto phenone.

Friedel crafts acylation reaction of anisole 

When anisole is react with acyl halide in presence of anhydrous AlCl₃, The acyl substituted anisole is obtained .  

Due to the + R effect of –OCH₃  group ofanisole the electron density in ortho and para position of benzene ring increases.

Hence , –OCH₃  group of anisole is ortho and para directing. That is electrophile attact on ortho and para carbon atom. 

Consequentlt in this reaction ortho methoxy toluene  and p-methoxy toluene  are obtained .

But para isomer is the main product. Because, in o-methoxy anisole, a steric hindrance is operated between – OCH₃  and – CH₃ group.

Friedel crafts acylation reaction of chloro benzene 

When chloro benzene is react with acyl halide in presence of anhydrous AlCl₃ , The acyl substituted chloro benzene is obtained.

Now,chlorine atom in chloro benzene have + R effect as well  – I effect . But +R effect is greater than – I effect .

 Due to +R effect of –Cl  group of chloro benzene, the electron density in ortho and para position of benzene ring increases . 

Hence , – Cl  group of chloro benzene is ortho and para directing. That is, electrophile attact on ortho and para carbon atom. 

Consequently in this reaction o- chloro alkyl phenyl ketone and p-chloro alkyl phenyl ketone are obtained.

But para isomer is the main product . Because, in o-chloro alkyl phenyl ketone, a steric hindrance is operated between – COCH₃  and – Cl group.



Summary

• What is  friedel-crafts acylation ?
• mechanism for friedel crafts acylation.
• friedel crafts acylation of benzene.
• friedel crafts acylation of anisole.
• friedel crafts acylation of phenol.
• friedel crafts acylation of chloro benzene.
• Application of friedel crafts acylation reaction .
• Intra-molecular friedel crafts acylation reaction.

Friedel-crafts alkylation reaction. Click here.

Why pyridine does not exhibits resonance and Why is phenol more acidic than cyclo hexanol ?

Why pyridine does not exhibits resonance?

Pyridine is a hetero cyclic aromatic compound. It has three alternating double bond. 

But,the lone of electron on nitrogenatom does not participate in the resonance with the pi electron of pyridine ring.