Cleavage of Ethers by HBr and HI

– In this topic, we will discuss the Cleavage of Ethers by HBr and HI 

Cleavage of Ethers by HBr and HI

– Unlike alcohols, ethers are not commonly used as synthetic intermediates because they do not undergo many reactions. This unreactivity makes ethers attractive as solvents. Even so, ethers do undergo a limited number of characteristic reactions.

– Ethers are cleaved by heating with HBr or HI to give alkyl bromides or alkyl iodides.

– Ethers are unreactive toward most bases, but they can react under acidic conditions.

– A protonated ether can undergo substitution or elimination with an alcohol serving as a neutral leaving group.

– Ethers react with concentrated HBr and HI because these reagents are sufficiently acidic to protonate the ether, while bromide and iodide are good nucleophiles for the substitution.

– Under these conditions, the alcohol leaving group usually reacts further with HX to give another alkyl halide.

– In effect, this reaction converts a dialkyl ether into two alkyl halides.

– The conditions are very strong, however, and the molecule must not contain any acid-sensitive functional groups.

– Iodide and bromide ions are good nucleophiles but weak bases, so they are more likely to substitute by S_N2 the mechanism than to promote elimination by the E2 mechanism.

– The following Mechanism shows how bromide ion cleaves the protonated ether by displacing an alcohol.

– In the following example, cyclopentyl ethyl ether reacts with HBr to produce cyclopentanol by this displacement.

– Cyclopentanol reacts further with HBr, though, so the final products are ethyl bromide and bromocyclopentane.

Mechanism: Cleavage of Ethers by HBr or HI

– Ethers are cleaved by a nucleophilic substitution of or on the protonated ether.

Step 1: Protonation of the ether to form a good leaving group

Step2: S_N2 cleavage of the protonated ether.

Step 3: Conversion of the alcohol fragment to the alkyl halide. (Does not occur with phenols.)

– This conversion can occur by either of the two mechanisms, depending on the structure of the alcohol and the reaction conditions.

– The protonated alcohol undergoes either S_N1 or S_N2 substitution by bromide ion.

Example: Cleavage of cyclopentyl ethyl ether by HBr.

Step 1: Protonation of the ether to form a good leaving group.

Step2: Cleavage of the protonated ether.

Step 3: Conversion of the alcohol fragment to the alkyl halide. First, the alcohol is protonated to form a good leaving group.

The protonated alcohol undergoes SN1 or SN2 substitution by bromide ion.

– Hydroiodic acid (HI) reacts with ethers the same way HBr does.

– Aqueous iodide is a stronger nucleophile than aqueous bromide, and iodide reacts at a faster rate.

– We can rank the hydrohalic acids in order of their reactivity toward the cleavage of ethers:

HI >HBr >> HCl

– Phenyl Ethers Phenyl ethers (one of the groups bonded to oxygen is a benzene ring) react with HBr or HI to give alkyl halides and phenols.

– Phenols do not react further to give halides because the sp²-hybridized carbon atom of the phenol cannot undergo the S_N2 (or S_N1) reaction needed for conversion to the halide.

References:

• Organic chemistry / L.G. Wade, Jr / 8th ed, 2013 / Pearson Education, Inc. USA.
• Fundamental of Organic Chemistry / John McMurry, Cornell University/ 8th ed, 2016 / Cengage Learningm, Inc. USA.
• Organic Chemistry / T.W. Graham Solomons, Craig B. Fryhle , Scott A. Snyder / 11 ed, 2014/ John Wiley & Sons, Inc. USA.