Hydration Of Alkenes; Oxymercuration & Hydroboration - Chad's Prep®

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• Chapter 1 – Electrons, Bonding, and Molecular Properties
  • 1.1 Lewis Structures
  • 1.2 Formal Charges
  • 1.3 Valence Bond Theory and Hybridization
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  • 1.6 Intermolecular Forces
• Chapter 2 – Molecular Representations and Resonance
  • 2.1 Condensed Structures
  • 2.2 Bond Line Structures
  • 2.3 Functional Groups
  • 2.4 Resonance
• Chapter 3 – Acids and Bases
  • 3.1 Introduction to Acids and Bases
  • 3.2 Ranking Acids and Bases
• Chapter 4 – Alkanes
  • 4.1 Naming Alkanes
  • 4.2 Naming Complex Substituents
  • 4.3 Naming Bicyclic Compounds
  • 4.4 Drawing Constitutional Isomers
  • 4.5 Newman Projections
  • 4.6 Cycloalkanes and Cyclohexane Chair Conformations
• Chapter 5 – Isomers and Stereochemistry
  • 5.1 Overview of Isomerism
  • 5.2 Absolute Configurations | How to Assign R and S
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• Chapter 6 – Organic Reactions and Mechanisms
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  • 6.5 Reaction Mechanisms and Curved Arrow Pushing
• Chapter 7 – Substitution and Elimination Reactions
  • 7.1 SN2 Reactions
  • 7.2 SN1 Reactions
  • 7.3 SN1 vs SN2
  • 7.4 Introduction to Elimination Reactions [Zaitsev’s Rule and the Stability of Alkenes]
  • 7.5 E2 Reactions
  • 7.6 E1 Reactions and E1 vs E2
  • 7.7 Distinguishing Between SN1/SN2/E1/E2
• Chapter 8 – Alkenes
  • 8.0 Naming Alkenes
  • 8.1 Introduction to Alkene Addition Reactions
  • 8.2 Hydrohalogenation
  • 8.3 Hydration of Alkenes
  • 8.4 Addition of Alcohols
  • 8.5 Catalytic Hydrogenation
  • 8.6 Halogenation of Alkenes and Halohydrin Formation
  • 8.7 Epoxidation, Anti Dihydroxylation, and Syn Dihydroxylation
  • 8.8 Predicting the Products of Alkene Addition Reactions
  • 8.9 Oxidative Cleavage Ozonolysis and Permanganate Cleavage
• Chapter 9 – Alkynes
  • 9.1 Naming Alkynes
  • 9.2 Acidity of Alkynes
  • 9.3 Synthesis of Alkynes
  • 9.4 Reduction of Alkynes
  • 9.5 Hydrohalogenation of Alkynes
  • 9.6 Halogenation of Alkynes
  • 9.7 Hydration of Alkynes
  • 9.8 Ozonolysis of Alkynes
  • 9.9 Alkylation of Acetylide Ions
• Chapter 10 – Radical Reactions
  • 10.1 Free Radical Halogenation
  • 10.2 The Free Radical Halogenation Mechanism
  • 10.3 Allylic and Benzylic Bromination with NBS
  • 10.4 Addition of HBr and Peroxide
• Chapter 11 – Organic Synthesis (Retrosynthesis)
  • 11.1 Introduction to Organic Synthesis
  • 11.2 Common Patterns in Organic Synthesis (Involving Alkenes)
  • 11.3 Common Patterns in Organic Synthesis (involving Alkynes)
• Chapter 12 – Alcohols
  • 12.1 Naming Alcohols
  • 12.2 Properties of Alcohols
  • 12.3 Synthesis of Alcohols
  • 12.4 Grignard Reagents
  • 12.5 Protecting Alcohols
  • 12.6 Substitution Reactions of Alcohols
  • 12.6b Substitution with PBr3 and SOCl2
  • 12.7 Elimination Reactions (Dehydration) of Alcohols
  • 12.8 Oxidation of Alcohols
  • 12.9 Organic Synthesis with Alcohols
• Chapter 13 – Ethers, Epoxides, Thiols, and Sulfides
  • 13.1 Naming Ethers
  • 13.2 Synthesis of Ethers
  • 13.3 Reactions of Ethers
  • 13.4 Nomenclature of Epoxides
  • 13.5 Synthesis of Epoxides
  • 13.6 Ring Opening of Epoxides
  • 13.7 Nomenclature, Synthesis, and Reactions of Thiols
  • 13.8 Nomenclature, Synthesis, and Reactions of Sulfides
  • 13.9 Organic Synthesis with Ethers and Epoxides
• Chapter 14 – IR Spectroscopy and Mass Spectrometry
  • 14.1 Introduction to IR Spectroscopy
  • 14.2a IR Spectra of Carbonyl Compounds
  • 14.2b The Effect of Conjugation on the Carbonyl Stretching Frequency
  • 14.3 Interpreting More IR Spectra
  • 14.4 Introduction to Mass Spectrometry
  • 14.5 Isotope Effects in Mass Spectrometry
  • 14.6a Fragmentation Patterns of Alkanes, Alkenes, and Aromatic Compounds
  • 14.6b Fragmentation Patterns of Alkyl Halides, Alcohols, and Amines
  • 14.6c Fragmentation Patterns of Ketones and Aldehydes
• Chapter 15 – NMR Spectroscopy
  • 15.1 Introduction to NMR
  • 15.2 The Number of Signals in C 13 NMR
  • 15.3 The Number of Signals in Proton NMR
  • 15.4 Homotopic vs Enantiotopic vs Diastereotopic
  • 15.5a The Chemical Shift in C 13 and Proton NMR
  • 15.5b The Integration or Area Under a Signal in Proton NMR
  • 15.5c The Splitting or Multiplicity in Proton NMR
  • 15.6a Interpreting NMR Example 1
  • 15.6b Interpreting NMR Example 2
  • 15.6c Interpreting NMR Example 3
  • 15.6d Structural Determination From All Spectra Example 4
  • 15.6e Structural Determination From All Spectra Example 5
  • 15.7 Complex Splitting
• Chapter 16 – Conjugated Pi Systems
  • 16.1 Introduction to Conjugated Systems and Heats of Hydrogenation
  • 16.2a Pi Molecular Orbitals 1,3 Butadiene
  • 16.2b Pi Molecular Orbitals the Allyl System
  • 16.2c Pi Molecular Orbitals 1,3,5 Hexatriene
  • 16.3 UV Vis Spectroscopy
  • 16.4 Electrophilic Addition to Conjugated Dienes
  • 16.5 Diels Alder Reactions
  • 16.6 Cycloaddition Reactions
  • 16.7 Electrocyclic Reactions
  • 16.8 Sigmatropic Rearrangements
• Chapter 17 – Aromatic Compounds
  • 17.1 Naming Benzenes
  • 17.2 Aromatic vs Nonaromatic vs Antiaromatic
  • 17.3 The Effects of Aromaticity on Reactivity
  • 17.4 Pi Molecular Orbitals of Benzene
• Chapter 18 – Reactions of Aromatic Compounds
  • 18.1 Electrophilic Aromatic Substitution
  • 18.2 Friedel Crafts Alkylation and Acylation
  • 18.3 Activating and Deactivating Groups | Ortho/Para vs Meta Directors
  • 18.4 Catalytic Hydrogenation and the Birch Reduction
  • 18.5 Side-Chain Reactions of Benzenes
  • 18.6 Nucleophilic Aromatic Substitution
  • 18.7 Retrosynthesis with Aromatic Compounds
• Chapter 19 – Ketones and Aldehydes
  • 19.1 Nomenclature of Ketones and Aldehydes
  • 19.2 Synthesis of Ketones and Aldehydes
  • 19.3 Introduction to Nucleophilic Addition Reactions of Ketones and Aldehydes
  • 19.4a Formation of Hemiacetals and Acetals (Addition of Alcohols)
  • 19.4b Cyclic Acetals as Protecting Groups
  • 19.5 Formation of Imines and Enamines (Addition of Amines)
  • 19.6 Reduction of Aldehydes and Ketones
  • 19.7a Addition of Carbon Nucleophiles (Acetylide Ions, Grignard Reagents,etc.)
  • 19.7b The Wittig Reaction
  • 19.8 Baeyer Villiger Oxidation
  • 19.9 Retrosynthesis with Aldehydes and Ketones
• Chapter 20 – Carboxylic Acids and Acid Derivatives
  • 20.1 Naming Carboxylic Acids and Carboxylic Acid Derivatives
  • 20.2 Nucleophilic Acyl Substitution
  • 20.3 The Mechanisms of Nucleophilic Acyl Substitution
  • 20.4 Reaction with Organometallic Reagents
  • 20.5 Hydride Reduction Reactions
  • 20.6 Synthesis and Reactions of Acid Halides
  • 20.7 Synthesis and Reactions of Acid Anhydrides
  • 20.8 Synthesis and Reactions of Esters
  • 20.9 Properties, Synthesis, and Reactions of Carboxylic Acids
  • 20.10 Synthesis and Reactions of Amides
  • 20.11 Synthesis and Reactions of Nitriles
  • 20.12 Retrosynthesis with Carboxylic Acids / Acid Derivatives
• Chapter 21 – Substitution Reactions at the Alpha Carbon
  • 21.1 Acidity of the Alpha Hydrogen
  • 21.2 Mechanisms of Alpha Substitution Reactions
  • 21.3 Alpha Halogenation
  • 21.4 Alpha Alkylation (including the Stork Reaction)
  • 21.5 Aldol Reactions
  • 21.6 Claisen Condensation Reactions
  • 21.7 The Malonic Ester Synthesis and the Acetoacetic Ester Synthesis
  • 21.8 Michael Reactions
  • 21.9 The Robinson Annulation
  • 21.10 Retrosynthesis with Enolates and Enols
• Chapter 22 – Amines
  • 22.1 Naming Amines
  • 22.2 Basicity of Amines
  • 22.3 Synthesis of Amines
  • 22.4 Hofmann Elimination and Cope Elimination
  • 22.5 Sandmeyer Reactions
  • 22.6 EAS Reactions with Nitrogen Heterocycles
  • 22.7 Retrosynthesis with Amines

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8.2 Hydrohalogenation

8.4 Addition of Alcohols

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Acid Catalyzed Hydration of Alkenes

 

ACID CATALYZED HYDRATION
Less Substituted	More Substituted
Two Groups Added	H	OH
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
Less Substituted	More Substituted
Two Groups Added	H	OH
Reagents Added	1. Hg(OAc)2, H2O2. 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
Less Substituted	More Substituted
Two Groups Added	OH	H
Reagents Added	1. BH3.THF2. 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.

8.2 Hydrohalogenation

8.4 Addition of Alcohols