Synthesis of Ethers

Sulfuric acid process

This method is used to make sterically hindered symmetrical ethers.

The mechanism of the sulfuric acid process involves the following five steps.

1. Sulfuric acid dissociates, giving a proton plus the bisulfate ion.

   
   

   
   

2. The alcohol's oxygen atom is protonated via an acid-base reaction, leading to the formation of an oxonium ion.

   
   

   
   

3. The oxonium ion decomposes, generating a 3° carbocation and water. Because carbocations are planar, this decomposition destroys the steric hindrance effect that the t-butyl group created.

   
   

   
   

4. In this step, the acid-base reaction between the carbocation and a second molecule of alcohol takes place, which forms an oxonium ion.

   
   

   
   

5. The oxonium ion liberates a proton to yield the ether.

   
   

   
   

Williamson method

The Williamson ether synthesis proceeds via an S_N2 mechanism, in which an alkoxide ion displaces a halogen ion.

This method cannot be used with tertiary alkyl halides, because the competing elimination reaction predominates. The elimination reaction occurs because the rearward approach that is needed for an S_N2 mechanism is impossible due to steric hindrance. An S_N1 mechanism is likewise unfavored, because as the 3° carbon attempts to become a carbocation, the hydrogens on the adjacent carbons become acidic. Under these conditions, the alkoxide ion begins to show less nucleophilic character and, correspondingly, more basic character. This basic character leads to an acid-base reaction, which results in the generation of an elimination product (an alkene).