8.3 Hydration of Alkenes
PRE-HEALTH PREP
Acid Catalyzed Hydration of Alkenes
| ACID CATALYZED HYDRATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | H2SO4, H2O (i.e. H3O+) | |
| Regioselectivity | Markovnikov | |
| Stereospecificity | None | |
| Other Characteristics | Intermediate is a carbocation making rearrangements possible. | |
Acid catalyzed hydration results in the Markovnikov addition of a hydrogen (less substituted side) and a hydroxyl group (more substituted side) across an alkene forming an alcohol. There is no stereospecificity associated with this reaction, but the intermediate is a carbocation so rearrangements are possible. The reagents are water with a catalytic amount of strong acid most commonly sulfuric acid (H2SO4) though the reagents can also simply be listed as H3O+ as shown below.
Acid-Catalyzed Hydration Mechanism
Step 1: The pi electrons of the alkene attack a hydrogen of H3O+ resulting in carbocation formation. The carbocation is a high energy intermediate making this the rate determining step (slow step) of the reaction.
Step 2: When there is no favorable rearrangement water carries out nucleophilic attack on the carbocation forming an oxonium ion.
Step 3: A water molecule deprotonates the oxonium intermediate yielding the product alcohol.
Oxymercuration-Demercuration
| OXYMERCURATION-DEMERCURATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | 1. Hg(OAc)2, H2O 2. NaBH4 | |
| Regioselectivity | Markovnikov | |
| Stereospecificity | Anti | |
| Other Characteristics | Intermediate is a 3-membered ring (mercurinium ion). | |
Oxymercuration-demercuration (a.k.a. oxymercuration-reduction) results in the Markovnikov addition of a hydrogen (less substituted side) and a hydroxyl group (more substituted side) across an alkene forming an alcohol. The reaction exhibits anti stereospecificity and is not subject to rearrangements as the intermediate is not a carbocation but a mercurinium ion instead. The reaction occurs in two steps with the first involving the addition of mercuric acetate and water (1. Hg(OAc)2, H2O). The resulting intermediate is then reduced with sodium borohydride (2. NaBH4).
Oxymercuration-Demercuration Mechanism
Undergraduate students are typically only responsible for the mechanism of oxymercuration involving the first set of reagents, mercuric acetate and water (1. Hg(OAc)2, H2O) as the mechanism for the demercuration (reduction) step is not precisely known. Mechanisms have been proposed for this reduction but there is no accepted consensus at this time.
Oxymercuration - 1. Hg(OAc)2, H2O
Step 1: Mercuric acetate ionizes to some extent and the pi electrons of the alkene attack a mercury cation which attacks one the of the alkene carbons back forming a three-membered ring with Hg referred to as a mercurinium ion.
While the mercurinium ion is more stable than a carbocation it is still a rather high energy intermediate, and this first step is the rate determining step (slow step) of the reaction. Also, the mercurinium ion intermediate does not undergo rearrangements such as are possible with carbocations.
Step 2: Ring-Opening of the Mercurinium Ion
Water carries out back-side attack (as in SN2) on the more substituted carbon of the mercurinium ion resulting in the bond to the Hg in the mercurinium ion breaking (analogous to loss of a leaving group in SN2 even though it is still attached to the adjacent carbon atom) and the opening of the 3-membered ring.
The back-side attack results in inversion of configuration at the carbon attacked helping to explain the nature of the anti stereospecificity.
Step 3: Proton Transfer
Acetate or water deprotonates the oxonium ion intermediate producing an alcohol.
Oxymercuration - 1. Hg(OAc)2, H2O
Dermercuration (Reduction) - 2. NaBH4
Hydroboration-Oxidation
| HYDROBORATION-OXIDATION | ||
|---|---|---|
| | | |
| Two Groups Added | | |
| Reagents Added | 1. BH3.THF 2. H2O2, OH-, H2O | |
| Regioselectivity | Anti-Markovnikov | |
| Stereospecificity | Syn | |
| Other Characteristics | Intermediate is a trialkylborane. | |
Hydroboration-oxidation results in the anti-Markovnikov addition of a hydrogen (more substituted side) and a hydroxyl group (less substituted side) across an alkene forming an alcohol. The reaction exhibits syn stereospecificity and is not subject to rearrangements as it does not involve a carbocation intermediate. The reaction occurs in two steps with the first involving the addition of borane which exists as a dimer but is often complexed with tetrahydrofuran (1. BH3.THF or 1. B2H6). The resulting intermediate is then oxidized with hydrogen peroxide and sodium hydroxide (2. H2O2, NaOH).
Hydroboration-Oxidation Mechanism
Undergraduate students are typically only responsible for the mechanism of the 1st step, hydroboration (1. BH3.THF) as the mechanism for the 2nd oxidation step is somewhat complicated, and therefore many textbooks and professors choose to omit the mechanism for the oxidation step from the curriculum. If you're taking a Ochem 1 you should pay particular attention to see if you're responsible for the oxidation mechanism.
Hydroboration - 1. BH3.THF
Boron does not have a filled octet making it electron deficient and a highly reactive electrophile. The pi electrons of the alkene attack the boron atom attaching it to the less substituted side of the alkene. Simultaneously, one of the B-H bonds breaks and this hydrogen attaches to the more substituted carbon of the alkene. The result is an alkylborane.
This single mechanistic step is essentially the entire mechanism for hydroboration though it does repeat twice to form a trialkylborane intermediate. Students are often shown and are responsible for the transition state for this step and it is included in the mechanism below. We can see that the boron and the hydrogen attach to both sides of the alkene at the same time and originate from the same BH3 molecule. This explains why they must add to the same face of the alkene making this a syn addition.
Oxidation - 2. H2O2, NaOH
The net result of this 2nd step is the conversion of the trialkylborane intermediate into three equivalents of the product alcohol.
Once again, most undergraduate students will not need to know the mechanism for this second step but for the few of you that do (and my apologies to you!) the mechanism for a generic trialkylborane is shown below.
Step 1: Proton Transfer - A hydroxide ion deprotonates hydrogen peroxide to form a hydroperoxide ion.
Step 2: Nucleophilic Attack - The hydroperoxide ion attacks the boron of the trialkylborane.
Step 3: Anionic Rearrangement - An uncommon anionic rearrangement takes place with the loss of hydroxide.
Steps 1-3 are repeated two more times.
Step 4: Nucleophilic Attack - A hydroxide ion attacks the boron of the intermediate.
Step 5: Loss of Leaving Group - The bond between boron and oxygen breaks with an alkoxide ion serving as the leaving group.
Step 6: Proton Transfer - The alkoxide ion is protonated by a water molecule to yield the alcohol product.
Steps 4-6 are repeated two more times two yield two more equivalents of the alcohol product.
