Addition of aqueous acid to alkenes to give alcohols

by James

Description: Addition of aqueous acid to alkenes leads to the formation of alcohols.

Notes: Note that the alcohol is formed at the most substituted carbon of the alkene (Markovnikoff selectivity!) Since this reaction proceeds through a carbocation, rearrangements can occur in some cases.

Aqueous acid is often written as “H3O(+)” . An alternative way to depict aqueous acid is H2O/H2SO4. There’s no essential difference for our purposes.

Examples:

Notes: Note that the last example is a rearrangement!

Mechanism: Protonation of the alkene by acid (Step 1, arrows A and B) leads to the more substituted (i.e. stable) carbocation, which is then attacked by water (Step 2, arrow C). Deprotonation of the oxygen then gives the neutral alcohol (Step 3, arrows D and E). Note that the acid is regenerated in step 3, so it acts as a catalyst.

Notes: It’s probably more reasonable to show water as the base in step 3, but HSO4(-) was used here for simplicity’s sake. Note that the acid is catalytic.

In this example a secondary carbocation is formed: if C-3 were a tertiary or quaternary carbon, a rearrangement would have occurred.

 

{ 7 comments… read them below or add one }

Dave Blackburn

Check your titles…

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james

Oh wow, this is a major brain fart. Thanks Dave.

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avraham

great stuff!!!

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Christopher Douglas

With problems where you’re given the reactant and the product and you’re to guess the reagent how do you know whether to choose aqueous acid or oxymercuration?

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James

Oxymercuration is generally the best and safest choice. Aqueous acid leads to carbocation formation, and carbocations can rearrange under certain conditions.

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Adrienne Kambouris

Would there be products with stereochemistry in examples 3 and 4?

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James

3 yes, because we make a new chiral centre. 4 no, because the carbon the OH is attached to is not a chiral centre.

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