Chapter 8: Alkenes Reactions and Synthesis

Syn Stereochemistry

- A syn addition reaction is one in which the two ends of the double bond react from the same side. - A syn elimination is one in which the two groups leave from the same side of the molecule. - The opposite of anti stereochemistry

Anti Stereochemistry

- An anti addition reaction is one in which the two ends of the double bond are attacked from different sides. - An anti elimination reaction is one in which the two groups leave from opposite sides of the molecule. - The opposite of syn.

Hydroboration properties

- Anti-markonikov: alcohol goes on less substituted carbon - syn addition (OH and H are on the same side) - no carbocation intermediate - Enantiomers present

Addition Reactions of Alkenes

- Catalytic Hydrogenation - Hydrohalogenation - Halohydrin formation - Oxymercuration-demercuration - Hydroboration-oxidation - Epoxidation with a peroxyacid - Hydroxylation with OsO₄ - Addition of carbenes to yield cyclopropanes

Elimination Reactions

- Dehydrohalogenation - Dehydration

Addition of carbenes to yield cyclopropanes

- Dichlorocarbene addition - Simmons-Smith reaction - Hydroxylation by acid-catalyzed epoxide hydrolysis

Dehydrohalogenation

- Elimination reaction - The loss of HX from an alkyl halide - Occurs by reaction of an alkyl halide with a strong base

Dehydration

- Elimination reaction - The loss of water from an alcohol - Occurs by reaction of an alcohol with a strong acid

Addition of H₂O by Hydroboration-oxidation for Alkenes

- Non-Markovnikov - syn addition

Aldehyde (-al)

- Position is at the end of chain -

Acid-catalyzed hydration mechanism of alkenes

- Reagents: H₂SO₄, HClO₄, H₃PO₄; high temperature - Electrophilic H₃O⁺ is attacked by nucleophilic carbon double bond and loses a Hydrogen to the less substituted carbon - Carbocation intermediates - Nucleophilic H₂O donates electrons to carbocation - Anti-Addition - Markovnikov addition - No stereochemical preference

Hydroxylation by acid-catalyzed epoxide hydrolysis

- Syn addition reaction - No carbocation intermediate - reagents: OsO₄, NaHSO₃, H₂O

Result of dehydrohalogenation with a strong base (KOH) in ethanol solution

- loss of HX from an alkyl halide - Products: Alkene + H₂O

Result of dehydration treatment of an alcohol with a strong acid (H₂SO₄, H₂O)

- loss of H₂O from an alcohol - Products: Alkene + H₂O

Acid Catalyzed Hydration

- used to convert alkenes to alcohols - H-shift - electrophillic addition reaction - Markov regioselevtivity - carbocation rearrangements are possible - reagents: H₂O, H+, H₂SO₄

Acid Catalyzed Hydration Mechanism

1) protonation of alkene pi bond to form carbocation 2) reaction with H₂O to give alcohol product

Hydroboration formation mechanism

1. Addition of a B-H bond of borane (BH₃) to the side of the carbon double bond of an alkene that is least substituted (Non-Markonikov) to yield organoborane intermediate (R-BH₂) 2. Oxidation of the organoborane by basic hydrogen peroxide (H₂O₂) produces alcohol (OH) to replace BH₂

Cause for oxidation reaction

1. Bond formation between carbon and a more electronegative atom-forming C-O, C-N, C-X 2. Bond-breaking between carbon and a less electronegative atom-breaking C-H

Halohydrin Epoxide Formation reagents

1. Br₂ or Cl₂ + H₂O 2. NaOH + H₂O

Cleavage to Carbonyl Compounds

1. Compound is split at the C=C double bond 2. Ozone (O₃) adds 1 oxygen to each Carbon of the C=C double bond 3. Molozonide rearranges to form ozonide 4. Ozonide is treated with a reducing agent to produce carbonyl compounds

Dehydration (elimination of H₂O) reaction of alkens

1. Demercuration: via NaBH₄

Oxidation reactions of alkenes

1. Epoxidation 2. Cleavage to Carbonyl Compounds

Causes of reduction reactions

1. Formation between carbon and a less electronegative atom, forming C-H 2. Bond-breaking between carbon and a more electronegative atom-breaking C-O, C-N, C-X (X=Halogen)

Oxymercuration-demercuration mechanism

1. Initiated by electrophilic addition of Hg(OAc)₂ ion to the alkene. 2. Markovnikov addition of H₂O to most substituted carbocation. 3. Demercuration via NaBH₄ replaces Hg(OAc)₂ with a Hydrogen atom.

Methods of Epoxidation

1. Peroxyacids - mCPBA 2. Halohydrin formation - Br₂, H₂O - NaOH

Hydroxylation of Alkenes with OsO₄

Addition of a hydroxyl group via epoxidation and hydrolysis by treating an alkene with OsO₄ | Pyridine then treated with NaHSO₃ | H₂O - syn addition - product is a 1,2 -diol

Syn Addition

Addition of constituents to an alkene on the same side of the bond - Product has cis stereochemistry

Anti Addition

An addition reaction in which two substituents are added to opposite sides (or faces) of a double bond or triple bond. - Product has trans stereochemistry

Syn Elimination

An elimination reaction in which substituents being eliminated are removed from the same side of the molecule

Anti Elimination

An elimination reaction in which the substituents being eliminated are removed from opposite sides of the molecule

Non-Markovnikov regiochemistry in Hydroboration

Because attachment of Boron is favored at the less sterically crowded carbon atom of the alkene.

Halogenation of Alkenes

Bromine and Chlorine add to alkenes to give 1.2-Dihalides - Electrophilic addition - Anti addition - trans stereoisomer of the dihalide addition product. - does not need light or heat - intermediate

Halohydrin formation reagents and solvent

Br₂ | Cl₂ + H₂O =

Addition of Carbenes to Alkenes: Cyclopropane Synthesis

CHCl₃, KOH or CH₂I₂, ZnCu

Bromohydrin formation

Carried out using the reagent NBS as a source of Br₂.

The reaction of an alkene with dichlorocarbene is: 1. Regiospecific 2. Stereospecific 3. Markovnikov 4. Non-Markovnikov

Dichlorocarbene is stereospecific only

In the reaction of an alkene with dichlorocarbene, the dichlorocarbene is: Nucleophile or Electrophile

Dichlorocarbene is the electrophile

Addition of water to alkene attacks the more substituted carbon

Due to the carbon being more stable, it is able to allow the bond to break.

Bromine addition

Forms a Bromonium ion (Br⁺) instead of a carbocation - Formed by electrophilic addition of Br⁺ to the alkene

Stereochemistry of Bromine addition to alkene

Forms stereoisomers (each a pair of enantiomers):

Cleavage of 1,2-diols

HIO₄|H₂O

H₂O + H₂SO₄ →

H₃O⁺

Oxidative Cleavage of Alkenes: Reaction with KMnO₄ in Acidic Solution

KMnO₄|H₃O⁺: - Product is carboxylic acid or CO₂

Polymer

Large molecule consisting of repeating units of simpler molecules (monomers)

Trans-stereoisomer with cycloalkane halogenation

Occurs with anti-stereochemistry

Stereospecific

Only a single stereoisomer is formed as a product

Molozonide

Ozone (O₃) adds oxygen to C=C bond at a low temperature

Peroxyacid Epoxide Formation

Peroxyacid transfer an oxygen atom to the alkene - syn addition - Both C-O bonds form on the same face of the double bond - The oxygen atom furthest from the carbonyl group is the one transferred

mCBPA

RCOOOH

Halohydrin Formation of Alkenes

Reaction of alkenes with hypohalous acids (HO-Cl or HO-Br) yields 1,2-halo alcohol - Addition takes place by reaction of the alkene with either Br₂ or Cl₂ in the presence of water - Markovnikov regiochemistry - Anti stereochemistry (trans) - No carbocation

Oxidation

Reaction that results in a loss of electron density for carbon

Reduction

Reaction that results in gain of electron density for carbon

Causes a gain in Carbon electron density

Reduction reaction

Catalytic Hydrogenation: Selective or Non-selective

Selective Reason: Alkene are much more reactive toward catalytic hydrogenation than most other unsaturated functional groups

Hydrohalogenation

The addition of HCl or HBr to a multiple bond to give an alkyl halide product - Markovnikov - H-shift - Carbocation

Addition of H₂O by Oxymercuration-Demercuration

The addition of aqueous mercury acetate, Hg(OAc)₂ and H₂O to an alkene, followed by the removal of Hg(OAc)₂ via borohydride (NaBH₄) 1. Hg(OAc)₂, H₂O 2. NaBH₄

Oxidative Cleavage of Alkenes: Ozonolysis

The carbon double bond is cut in half, adding Oxygen on both halves of the double bond. O₃|Zn, H₃O⁺: - Product is aldehydes or ketones

Ozonide

The product initially formed by addition of ozone to a carbon-carbon double bond. - Ozonides are usually treated with a reducing agent, such as zinc in acetic acid, to produce carbonyl compounds.

Dichlorocarbene addition

The reaction between a carbene and an alkene to form a cyclopropane in the presence of KOH. - No intermediates form. - Anti stereochemistry

Simmons-Smith Reaction

The reaction of an alkene with CH₂I₂ and Zn-Cu to yield a cyclopropane.

Acid-catalyzed Epoxide opening of Alkenes

Undergoes an acid-catalyzed ring-opening reaction with water to give 1,2-dialcohol (-diol) 1. Protonation (H₃O⁺) of the epoxide to increase reactivity 2. Nucleophilic addition of water (similar to bromination: Br+) 3. Cycloalkene is opened and is a cycloalkane 4. Double addition of water results in trans-1,2-cycloalkanediol - RCO₃H [peroxyacids] - anti addition

Peroxyacid (mCPBA)

a carboxylic acid with an extra oxygen - aids in epoxide formation

alkene + water

alcohol

Catalytic Hydrogenation of Alkenes

reducing an alkene by adding H₂ to double bond with aid of metal catalyst. e.g. pt, pd, ni. - increases electron density of carbon - takes place on surface of metal - extremely sensitive to the steric environment around the double bond - very selective - syn addition

Carbene

sp² hybridized carbon atom, has a lone pair. - Electrophile because Carbon wants 8 electrons but only has 6.

alkene addition reaction

the C=C double bond opens up, allowing a new atom to be added to each carbon - Results in trans products (anti-stereochemistry)