Question

Organic Chemistry I Homework Worksheet Alkenes: Three Hydration Methods Compare and contrast the three alkene hydration methods: acid catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation. Make sure to include each mechanism and how it affects regio-and stereochemistry. Give examples!! Label everything!!!

Answer 1

(1) Acid catalyzed hydration is a chemical reaction in which water adds to an unsaturated substrate under the influence of an acid catalyst.

An example is the hydration of ethene.

HSO e thene ethanol

The common acid catalysts are sulfuric acid and phosphoric acid. They react with water to form hydronium ions, which is the strongest acid that can exist in aqueous solution.

Mechanism

The reaction mechanism takes place in three steps

rdsh@ C-C · H, T,H ChCH3 OH2 2. CH2CH3 но- 3.

Step 1. Protonation of the double bond.

All the hydrogen atoms in the hydronium ion are fairly positively charged because they are attached to a very electronegative oxygen atom.

One of these hydrogen atoms is strongly attracted to the carbon-carbon double bond.

The π bond breaks, and the electrons in it move to make a new bond with the hydrogen atom.

That forces the electrons in the hydrogen-oxygen bond entirely onto the oxygen.

Step 2. Formation of an oxonium ion

The carbocation reacts with one of the lone pairs on a water molecule.

A bond forms between the positively-charged carbon atom and the oxygen atom of the water. The O atom gets a positive charge

Step 3. Deprotonation of the oxonium ion

Finally, reaction with water removes one of the hydrogen atoms on the oxygen.

(2) Oxymercuration-demercuration

Carbocation rearrangement is a process in which the carbocation intermediate can form a more stable ion. With carbocation rearrangement, the reaction would not be able to hydrate quickly under mild conditions and be produced in high yields. This reaction is very fast and proceeds with 90% yield.

HgOAc 、H HooAch H4, NaBH4 , NaOH, H20 H3C HaC OH OH H

This reaction involves a mercury acting as a reagent attacking the alkene double bond to form a Mercurinium Ion Bridge. A water molecule will then attack the most substituted carbon to open the mercurium ion bridge, followed by proton transfer to solvent water molecule.

Mechanism

OAc OAc :Hg OAc OAC Hg CO H3C H2O water attacks more substituted carbon OAc NaBH4 Hg с-с Hg он OAc он

The organomercury intermediate is then reduced by sodium borohydride - the mechanism for this final step is beyond the scope of our discussion here. Notice that overall, the oxymercuration - demercuration mechanism follows Markovnikov's Regioselectivity with the OH group is attached to the most substituted carbon and the H is attach to the least substituted carbon. The reaction is useful, however, because strong acids are not required, and carbocation rearrangements are avoided because no discreet carbocation intermediate forms.

(3) Hydroboration-Oxidation

Hydroboration-Oxidation is a two step pathway used to produce alcohols. The reaction proceeds in an Anti-Markovnikov manner, where the hydrogen (from BH₃ or BHR₂) attaches to the more substituted carbon and the boron attaches to the least substituted carbon in the alkene bouble bond. Furthermore, the borane acts as a lewisAnti-Markovnikov acid by accepting two electrons in its empty p orbital from an alkene that is electron rich. This process allows boron to have an electron octet. A very interesting characteristic of this process is that it does not require any activation by a catalyst. The Anti-MarkovnikovHydroboration mechanism has the elements of both hydrogenation and electrophilic addition and it is a stereospecific (syn addition), meaning that the hydroboration takes place on the same face of the double bond, this leads cis stereochemistry.

Alkynes can also be hydrated to form enols that immediately tautomerise. Terminal alkynes give aldehydes, internal alkynes give ketones. Typical reagents (two steps) 1. BH₃ or B₂H₆ then 2) H₂O₂ / NaOH. Electrophile is Boron atom

Regioselectivity : Anti-Markovnikov, since the B is ultimately converted to the -OH.

1. BH3 он H O 2. H202/aq. NaOH /\ alkyne enol

Mechanism

Step I

Part I:

In this first step the addittion of the borane to the alkene is initiated and proceeds as a concerted reaction because bond breaking and bond formation occurs at the same time.

Transition state BH2 Note that a carbocation is not formed. Therefore, no rearrangement takes place

Part II

The Anti Markovnikov addition of Boron. The boron adds to the less substituted carbon of the alkene, which then places the hydrogen on the more substituted carbon. Both, the boron and the hydrogen add simultaneously on the same face of the double bond (syn addition).

Step II

Part I

the first part of this mechanism deals with the donation of a pair of electrons from the hydrogen peroxide ion. the hydrogen peroxide is the nucleophile in this reaction because it is the electron donor to the newly formed trialkylborane that resulted from hydroboration.

HOOH он HOO HOH Hydrogen Peroxide Hydrogen Peroxide ion 尺 R-B Trialkylborane Hydroperoxide (An electrophile) (a nucleophile)

Part 2: In this second part of the mechanism, a rearrangement of an R group with its pair of bonding electrons to an adjacent oxygen results in the removal of a hydroxide ion.

RBO Two more of these reactions with hydroperoxide will occur in order give a trialkylborate H202 H202 NaOH NaOH Trialkylborate

Part 3: This is the final part of the Oxidation process. In this part the trialkylborate reacts with aqueous NaOH to give the alcohol and sodium borate.

(RO)3B +3 NaOH 3 ROH Na, ВОЗ + Trialkylborate Sodium Borate