Nucleophilic Substitution Reactions of Ethers

Like alcohols, ethers can be activated by protonation. Ethers, therefore, can undergo nucleophilic substitution reactions with HBr or HI. (HCl caimot be used because Cl is too poor a nucleophile.) The reaction of ethers with hydrogen halides, like the reactions of alcohols with hydrogen halides, is slow. The reaction mixture must be heated to cause the reaction to occur at a reasonable rate. 

What happens after the ether is protonated depends on the structure of the ether. If departure of ROH creates a relatively stable carbocation (such as a tertiary carbocation), an SnI reaction occurs. 

However, if departure of the leaving group would create an unstable carbocation (such as a methyl, vinyl, aryl, or primary carbocation), the leaving group caiuiot depart. It has to be displaced by the halide ion. In other words, an 8 2 reaction occurs. 

Ether cleavage forms only a substitution product because any alkene that would be formed in an elimination reaction would undergo electrophilic addition with HBr or HI to form the same alkyl halide that is obtained from the substitution reaction. 

The reagents (such as SOCI2, PCI3, or TsCl) used to activate alcohols so they can undergo nucleophilic substitution reactions cannot be used to activate ethers. When an alcohol reacts with one of these activating agents, a proton dissociates from the intermediate in the second step of the reaction and a stable product results. 

---

Nucleophilic substitution reactions of ethers (Section 11.6). The mechanisms are shown on pages 502 and 503. 

---