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)
