Phenols: Preparation & Properties

HINT:  Electron withdrawing group increases the acidity.

Explanation: 

Acidity of phenol / phenol derivatives depends on the stability of phenoxide ion. More the stability of phenoxide ion more is the acidity.

Electron releasing groups decreases the stability of conjugate base , decreasing it’s acidity.

Similarly, electron withdrawing groups increases the acidic nature by increasing the stability of conjugate base.

-NO₂ group increases the acidity of phenol. At p- position both -I effect and -M effect are present so it will be highly acidic. But at m- position only -I effect of -NO₂ will be there so m-nitro phenol is more acidic than phenol.

Due to -CH₃ group the acidity of  (A) will be least.

So correct order of acidity is  :  C > D > B > A

HINT: p-methyl diazonium chloride reacts with water to give p-cresol.

Explanation :

When aniline or it’s derivatives reacts with NaNO₂/HCl at very low temperature ($0^{0}C - 5^{0}C$) , then diazonium salts formation takes place.

Diazonium salts on reaction with water gives phenol / phenol derivatives. N₂ will be released during the reaction and -OH will replace $-N_{2}^{+}$ group.

So, p-methyl diazonium chloride reacts with water to give p-cresol.

Hint: Phenol acidity increases if it contains an electron-withdrawing group and phenol acidity decreases if it contains an electron-donating group at para or ortho position.

Explanation:
The acidic behaviour of phenols may be explained on the basis of two reasons:

(i) The value of the dissociation constant of phenols (K_a= 10^-10) and alcohols (K_a 10^-6 to 10^-18).

(ii) Due to resonance (which is not possible in alcohols),

the oxygen atom of the -OH group acquires a positive charge which helps in the release of a proton.

In the dissociation of phenol to phenoxide ion and a proton, the equilibrium lies mainly towards the right hand side  as the resulting phenoxide ion is more stable than phenol.

          Ar-OH ⇌ArO^– + H⁺

The acidic strength of phenols depends on the nature of substituents present in the benzene nucleus.

Electron withdrawing groups like -NO₂, -CN, -CHO and -COOH etc, when present at the ortho and para-positions, with respect to a phenolic group, increase the acidity of phenol due to greater stabilisation of phenoxide ion.

While the presence of electron releasing groups like -NH₂, -CH₃ etc, decrease the acidity of phenols. This explains the following order of acidity:

p-nitrophenol>phenol>p-cresol.

HINT: Reimer-Tiemann reaction.

Explanation: 
When phenol treated with chloroform in presence of aqueous sodium hydroxide at 340 K to form 2− hydroxybenzaldehyde or salicylaldehyde.  This reaction is called Reimer-Tiemann reaction.

HINT: KMnO₄ converts the carbon chain which is directly attached to the benzene ring, into the –COOH group

Zn dust acts as a reducing agent and converts the –OH group into – H due to which phenol is converted to benzene.

Benzene reacts with CH₃Cl in presence of anhy. AlCl₃ (Friedel –Craft’s alkylation) form toluene which subsequently gets

converted into benzoic acid by treatment with alkaline KMnO₄. The reactions are as follows:

HINT: See the effect of substituents on the stability of phenoxide ion.

Step 1:

Electron withdrawing group like -NO₂ increases the acidity of substituted phenol while the electron-donating group like -CH₃ decreases the acidity of substituted phenol by destabilizing the stability of phenoxide ion. 

Meta-nitrophenol is less acidic as compared to para-nitrophenol as the phenoxide ion in meta-nitrophenol is stabilised by inductive effect (-I) only but in para-nitrophenol it is stabilised by inductive as well as resonance effect which is more stabilising as compared to inductive effect only. 

Step 2:

So, the acidity order for the following compounds is :