Ether cleavage with HI: practice and mechanism
HI protonates the ether and iodide attacks the alkyl carbon via SN2. Aryl alkyl ethers always give phenol and an alkyl iodide; a dialkyl ether gives two alkyl iodides with excess HI.
Example
The ether oxygen is protonated and iodide attacks the alkyl group via SN2. Aryl alkyl ethers always give phenol and an alkyl iodide, because SN2 attack at an aromatic carbon is impossible. With excess HI, dialkyl ethers give two alkyl iodides.
Mechanism step by step
- 1
HI protonates the ether oxygen to an oxonium ion; OR becomes a good leaving group (ROH).
- 2
Iodide, a very good nucleophile, attacks the less hindered alkyl carbon via SN2. An alkyl iodide and an alcohol (or phenol) form.
- 3
With excess HI an alcohol is protonated again and also becomes an iodoalkane. Phenols do not react further.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Iodide cannot attack the aromatic carbon: an SN2 backside attack in the ring is impossible and an aryl cation is too unstable. The I goes to the methyl group, the O stays on the ring (phenol).
- With excess HI it does not stop at the alcohol: the alcohol formed is protonated as well and converted into a second alkyl iodide.
- HI does not substitute the ring (no SEAr). It cleaves the C–O bond of the ether.
- The phenol does not become iodobenzene. The Ar–O bond survives because neither SN1 nor SN2 is possible at an aromatic carbon.
- Water is not the nucleophile here. Iodide attacks, so iodoalkanes form, not two alcohols.
- Iodide is a good nucleophile and a weak base; HI does not eliminate to the alkene here but substitutes.
Often confused with: Alcohol to haloalkane (HBr)
Frequently asked questions
Why HI or HBr but not HCl?
Chloride is too weak a nucleophile to attack the oxonium ion. Iodide and bromide are large, highly polarisable nucleophiles.
And with tert-butyl ethers?
The tert-butyl group leaves as a stable carbocation (SN1). Such ethers cleave even with milder acids.