8.4 Addition of Alcohols
PRE-HEALTH PREP
Acid-Catalyzed Addition of an Alcohol
| ACID-CATALYZED ADDITION OF AN ALCOHOL | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | H2SO4, ROH | |
| Regioselectivity | Markovnikov | |
| Stereospecificity | None | |
| Other Characteristics | Intermediate is a carbocation making rearrangements possible. | |
Acid-catalyzed addition of an alcohol results in the Markovnikov addition of a hydrogen (less substituted side) and an alkoxy group (more substituted side) across an alkene forming an ether. There is no stereospecificity associated with this reaction, but the intermediate is a carbocation so rearrangements are possible. The reagents are any alcohol with a catalytic amount of strong acid, most commonly sulfuric acid (H2SO4).
Acid-Catalyzed Addition of an Alcohol Mechanism
Step 1: The pi electrons of the alkene attack a hydrogen of the protonated alcohol* resulting in carbocation formation. The carbocation is a high energy intermediate making this the rate determining step (slow step) of the reaction.
*The protonated alcohol is the result of the strong acid protonating the alcohol as a strong acid will dissociate completely in an alcohol just as in water. This step is typically not included as part of the overall mechanism and the protonated alcohol is just assumed to already be in solution.
Step 2: When there is no favorable rearrangement the alcohol carries out nucleophilic attack on the carbocation forming a protonated ether.
Step 3: An alcohol molecule deprotonates the intermediate yielding the product ether.
Alkoxymercuration-Demercuration
| ACID-CATALYZED ADDITION OF AN ALCOHOL | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | 1. Hg(OAc)2, ROH 2. NaBH4 | |
| Regioselectivity | Markovnikov | |
| Stereospecificity | Anti | |
| Other Characteristics | Intermediate is a 3-membered ring (mercurinium ion). | |