Chemical Properties of Alcohols and Phenols - Reactions involving cleavage of O-H bond



Acidic nature of alcohol or phenol with active metals

Alcohols react with metal hydrides to form metal  alkoxides with the evolution of hydrogen gas.


Acidic character of alcohols and water

1. Those Alcohols where the conjugate base is resonance stabilized will be more acidic.
2. The classic example is cyclohexanol and phenol.
3. Cyclohexanol has the pKa of a typical alcohol (about 16).
4. The pKa of phenol, however, is about 10.
5.  In case of water it is amphoteric in nature.
6.  They can behave as acid as well as base.


Lower acidity of alcohols as compared to water

Alcohols behave as weak acids and ionize to a very small extent. It only give alkoxide ion and H+ ion on dissociation. The acidic nature of alcohols is due to the polarity of O-H bond. On account of this,  the shared pair shifts towards O atom and the O-H bond becomes weak. This facilitates the release of a proton from the molecule.This is why alcohols behave as weak acids. Their acidic strength is even less than that of water.


Effect of substituents on acidity of phenol

If stron electron withdrawing group is attached to phenol, the acidity of phenol increases, because electron-withdrawing group has the ability to de-localize negative charge, more than phenoxide ion itself.


Acidic strength of nitrophenols

1. The acidic nature of the three nitrophenols increases in the order: meta < ortho < para.
2. Para nitorophenol is the most acidic among the three. 
3. The nitro group attached to a conjugated system shows strong -M effect, and this decreases the electron density more at the ortho and para positions than at the meta position.
4.  This factor helps greater stabilization of the conjugate base (nitro phenoxide ion) of the ortho and para isomers than that of the meta isomer.
5.  So, meta nitrophenol is the weakest in terms of acidity.


Acidic strength of cresols

The decreasing order of acidic strength:  Phenol > m-Cresol > o-Cresol > p-Cresol.


acidic strength of methoxy and amino nitrophenols

As amino and methoxy both are electron donating group and exert +R effect. Such group pumps electrons into the ring and thus intensify the negative charge of the phenoxide ion. This groups decreases the acidic strength of the phenol.


Acidic strength of halophenols

As halogens are electron-withdrawing groups they withdraw electrons from the ring and results in dispersal of the negative charge of phenoxide ion and thus an increase in the stability of the ion. Consequently, acidic strength of the phenol increases.


Acylation of alcohols and phenols

Alcohols and phenols react with acid chlorides and acid anhydrides in the presence of bases like pyridine as catalyst to form esters. This reaction is known as acylation.


Mechanism of esterification

The esterification reaction of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution. Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol. Proton transfer in the tetrahedral intermediate converts the hydroxyl group into group, which, being a better leaving group, is eliminated as neutral water molecule. The protonated ester so formed finally loses a proton to give the ester.


Acidic character of alcohol

The acidic character of alcohols is due to the electronegative oxygen atom which withdraws the electrons of the O-H bond towards itself. As a result, the O-H bond becomes weak and hence a proton can be easily abstracted by a strong base. Actually, alcohols are very weak acids even weaker than water.


Chemical Properties of Alcohols - Reactions Involving Cleavage of C - O Bond


Electrophilic Aromatic Substitution Reactions in Phenols


Other reactions of Phenols


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