Ethers can be converted to alkyl halides when treated with HBr and HI in a sequence of two substitution reactions:

This transformation represents two substitution reactions. First, the ether is converted into an alkyl halide and an alcohol, which further reacts with the excess HX acid to form another equivalent of an alkyl halide:

When the reaction with HI is carried out at lower temperatures, the initial alcohol may be isolated as it does not further react with the acid.

What is the Mechanism of Ether Cleavage?

Ethers, just like the alcohols, are not suitable for direct nucleophilic substitution since the alkoxy groups are strong bases and therefore, poor leaving groups. For example, if we treat an ether with a good nucleophile, no reaction will occur since the cyanide cannot kick out the RO^–-alkoxide ion:

The analogy with alcohols is the poor leaving group ability of the OH group. However, again, like the alcohols, we can convert alkoxides into good leaving groups. And, while there are different ways of converting the OH into a good leaving group, such as the mesylation/tosylation, reaction with PBr₃ and SOCl₂, ethers are less reactive and it is only done by reacting them with strong acids HBr and HI.

The ether cleavage is a substitution reaction where the OR group is replaced with a halogen by converting it into a good leaving group first. This leaving group is an alcohol (initially formed as an oxonium ion in the ether), which is then replaced by the halide ion. It can occur by both S_N2 and S_N1 mechanisms depending on the identity of the R group.

If the R groups are methyl or primary alkyl groups, the reaction goes by the S_N2 mechanism:

Tertiary ethers react by an S_N1 mechanism and under milder conditions (lower temperatures, more dilute acid). This is explained by the higher stability of tertiary carbocations, which we have seen in the reaction of tertiary alcohols with HX acids and in the S_N1 mechanism in general

For example, when tert-butyl methyl ether is reacted with HI, a tertiary alkyl halide is formed with methanol because the S_N1 reaction occurs faster than competing S_N2 substitution.

The methyl (or primary alcohol) is converted into an alkyl halide by an S_N2 mechanism:

These patterns show how the reaction of ethers and alcohols with HX acids is nearly identical. You can check them in the following post for more comparison details:

Reactions of Alcohols with HCl, HB,r, and HI Acids

Check Also

Reactions of Alcohols

• Nomenclature of Alcohols: Naming Alcohols based on IUPAC Rules with Practice Problems
• Preparation of Alcohols via Substitution or Addition Reactions
• Reaction of Alcohols with HCl, HBr, and HI Acids
• Mesylates and Tosylates as Good Leaving Groups
• SOCl₂ and PBr₃ for Conversion of Alcohols to Alkyl Halides
• Alcohols in Substitution Reactions  Practice Problems
• POCl₃ for Dehydration of Alcohols
• Dehydration of Alcohols by E1 and E2 Elimination
• The Oxidation States of Organic Compounds
• LiAlH4 and NaBH4 Carbonyl Reduction Mechanism
• Alcohols from Carbonyl Reductions – Practice Problems
• Grignard Reaction in Preparing Alcohols with Practice Problems
• Grignard Reaction in Organic Synthesis with Practice Problems
• Protecting Groups For Alcohols in Organic Synthesis
• Oxidation of Alcohols: PCC, PDC, CrO₃, DMP, Swern, and All of That
• Diols: Nomenclature, Preparation, and Reactions
• NaIO4 Oxidative Cleavage of Diols
• The Pinacol Rearrangement
• The Williamson Ether Synthesis
• Alcohol Reactions Practice Problems
• Naming Thiols and Sulfides
• Reactions of Thiols
• Alcohols Quiz – Naming, Preparation, and Reactions
• Reactions Map of Alcohols

Ethers and Epoxides

• • Preparation of Epoxides
  • Ring-Opening Reactions of Epoxides
  • Reactions of Epoxides under Acidic and Basic Conditions
  • Reactions of Epoxides Practice Problems
  • The Grignard Reaction of Epoxides
  • Naming Ethers
  • The Williamson Ether Synthesis