Nucleophilic substitution (SN) reaction of alcohols

In presence of HX :

R–OH + HX RX + H2O

This is a nucleophilic substitution (SN1/SN2)

Mechanism :

Note :     Mainly  - unsubstituted 1º alcohol give SN2 reaction with HX.

Ex.   

  Reactivity of HX is HI > HBr > HCl > HF.

Finkelstein reaction :

R—OH + HI  R—I  R—H + I2 (reduced) [Final product is Alkane (R-H) not R-I]

This is a problems that is why iodides are best prepared through halogen exchange reaction. It is also known as Finkelstein reaction. In this reaction R—Cl and R—Br is treated with sodium iodide in acetone.

         

         

  NaI is soluble in acetone. In this reaction acetone is used because sodium iodide is soluble in acetone but NaBr and NaCl are insoluble so precipitated out. This eliminates any possibility of reverse reaction.

  It is an SN2 reaction therefore only 1ºRX and 2ºRX is used.

Lucas reagent :  

The 1 : 1 mixture of anhydrous ZnCl2 : HCl (conc.) is called Lucas reagent which is used to distinguish between 1º, 2º and 3º alcohols.

Mechanism :         

  ZnCl2 increases the rate of reaction by making –OH group into a much better leaving group just through complexation.

  Reactivity of alcohols is 3ºROH > 2ºROH > 1ºROH

Ex.  

      

1.4     In presence of PX3 :

          R–OH  R–Br

          Mechanism :

         

 

          2R—OH + HOPBr2  2R–Br +

1.5     In presence of PX5 :

         

 

            Both the PX3 & PX5 proceeds via SN2 pathways, No.rearrangement occurs.

            Only 1ºR—OH and 2ºR—OH undergo this types of reaction. 3º R—OH undergoes elimination reaction.

 

Ex.      

 

1.6     In presence of SOCl2 (Thionyl chloride) :

          R—OH R—Cl + SO2 (SN2) ;         R—OH  R—Cl + SO2 (SNi)

 

          (A) Mechanism of SOCl2 with pyridine :

            SOCl2 with pyridine undergo SN2 reaction so inversion of configuration is obtained in product.

 

          (B) Mechanism of SOCl2 without pyridine : (Darzen method)    

                  

          SOCl2 without pyridine or base undergo SNi reaction, so retention of configuration is obtained in product.

Ex.         (a)  

               (b)  

 

1.7     Victor Mayer test :

          1° Alcohol%

         

 

          2° Alcohol%

         

 

          3° Alcohol %