Addition Of Alcohols To Alkenes With Acid

by James

Description: Alkenes treated with acid in the presence of an alcohol (usually as solvent) form ethers.

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Notes: The acid used typically has a poorly nucleophilic counter-ion, like H2SO4 or p-TsOH. Often, H+ is just written as the acid. [If an acid with a relatively nucleophilic counter ion like HCl were written, then there might be some competition with Cl- adding to the carbocation that is formed].

The alcohol is generally used as solvent. Note that the addition is “Markovnikov” (H adds to least substituted carbon of the alkene, oxygen adds to most substituted carbon).

Examples:

2-examples alcohols

Notes: Note that the addition is always Markovnikov; the proton adds to the least substituted carbon of the alkene, and the alcohol adds to the most substituted carbon. Example 1 shows H+ as acid; note how there is essentially no difference in changing to H2SO4 or p-TsOH.

The reaction proceeds through a carbocation intermediate, so rearrangements can occur (example 4).

Intramolecular reactions are possible (example 5).

Mechanism: This reaction proceeds similarly to other “Markovnikov” additions to alkenes such as addition of hydrogen halides. In the first step the alkene is protonated by acid so as to produce the most stable carbocation (Step 1, arrows A and B). The carbocation then formed is attacked by the alcohol (Step 2, arrow C) leading to formation of the C-O bond. In the final step, the positively charged oxygen is deprotonated to give the neutral ether.

3-mechNotes:  Note that although here -OSO3H is shown as doing the final deprotonation, it is just as correct (if not more correct) to show the alcohol solvent performing this deprotonation.

{ 1 comment… read it below or add one }

Brian Butler

During the final deprotonation step, does one have to consider the pKa’s of both the R-OH and the H+ in order to properly write out the mechanism?

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