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16 Cards in this Set

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What are some precursors to alkenes?
Biological systems - usually alcohols

Laboratory - either alcohols or alkyl halides

alkenes add H20 to form alcohols
alcohol eliminates water to form alkenes
What happens in a dehydrohalogenation reaction?
-loss of HX from alkyl halide

-usually occurs by reaction of an alkyl halide with a strong base

ex. Bromocyclohexane + KOH/EtOH --> Cyclohexene + KBr + H2O
What happens in a dehydration reaction?
loss of water from an alcohol

usually occurs by treatment of an alcohol with a strong acid

dehydration usually takes place on substrates in which -OH is positioned 2 carbons away from a carbonyl group

ex. 1-methylcyclohexanol + H2SO4, H20/ THF ---> 1-methylcyclohexene + H2O
What happens in a halogenation reaction in alkenes and cycloalkenes?
alkenes:
-addition reaction of alkenes
-addition of Br2 and Cl2 to alkenes to yield 1,2 dihalides

cycloalkenes:
-only trans dihalide product is formed
-reaction occurs with anti stereochemistry - two halogens come from opposite faces of the double bond

reaction for both occurs through an intermediate bromonium ion (R2Br+)
What is a common alkene halogenation reaction?
common laboratory reaction

limited primarily to marine organisms

carried out by enzymes called haloperoxidases

ex. Beta-Ocimene, Cl- ends up on the more stable carbocation
What is a halohydrin formation?
electrophilic addition

reaction of alkenes with hypohalous acids HO-Cl or HO-Br to yield 1,2 halo alcohols called halohydrins

Halide goes on the less substituted carbon
What happens in hydration of alkenes?
-alkenes undergo acid catalyzed addition reaction with water to yield alcohols, happens on double bond adjacent to carbonyl group
-not much use in laboratory due to high temps
-not electrophilic addition

Examples:
Oxymercuration:
-electrophilic addition of Hg2+ with Hg(OAc)2 in aqueous THF
-reaction yields alcohol
-corresponds to Markovnikov

Hydroboration/oxidation:
-addition of BH3 to an alkene
-occurs in SINGLE STEP
-no carbocation intermediate
-Syn stereochemistry
-NON-Markovnikov regiochemistry
What happens in Reduction of Alkenes?
addition reaction by which alkenes are reduced to alkanes

Reduction:
-increased electron density on carbon = by forming C-H, breaking C-C, C-O, C-N etc
-common catalysts = Platinum - PtO2 and Palladium Pd/C

ex. Catalytic Hydrogenation (occurs with Syn stereochemistry)

1,2 dimethylcyclohexane + H2, PtO2/CH3CO2H --> cis-1,2 dimethylcyclohexane
What are the steps of catalytic hydrogenation?
1. adsorption of H2 onto a catalyst surface - bound H

2. Complexation between catalyst and alkene occurs as a vacant orbital on metal interacts with filled pi orbitals

3. Hydrogen added to double bond

4. Saturated product diffuses away from catalyst
What are examples of hydrogenation?
unsaturated vegetable oils reduced to produce saturated fats used in margarine and cooking products

Vegetable oils:
-triesters of glycerol with 3 FA

FA:
-polyunsaturated carboxylic acids
-cis stereochemistry

polyunsaturated = vegetable oil
saturated = margarine

Biological hydrogenation:
-occurs in 2 steps
1. NADPH addes Hydride ion to double bond to produce anion
2. protonation of an anion by acid HA leading to an overall addition of H2
What happens in Epoxidation?
Oxidation reactions - a loss of electron density by carbon

decreases electron density by breaking C-H or C-C, and forming C-O, C-N, or C-X

alkenes treated with peroxyacid, RCO3H are oxidized to give epoxides

Syn stereochemistry, one step, no intermediates
How are epoxides synthesized from halohydrins?
-preparation of halohydrin through electrophilic addition of HO-X to alkene (trans product)

-treatment of halohydrin with base deprotonates OH

-O- nucleophile reacts with C-Cl electrophile substituting C-O for C-Cl

-Cl- is eliminated yielding the epoxide, Cl- is a good leaving group
What are examples of epoxides in biology?
converstion of squalene into 2,3 oxidosqualene; a key step in biosynthesis of steroids

oxidant is peroxide derived from a flavin cofactor, peroxyacids not involved

epoxide hydrolysis is common, this is a pathway used to detoxify harmful substances (Benzopyrene - found in cigarette smoke)
What happens in a hydroxylation reaction?
the addition of an -OH gropu to each of the double bond carbons

2 step process:
1. epoxidation
2. hydration

full process
-acid catalyzed epoxidation
-protonation of the epoxide to increase the electrophilicity of the carbon
-nucleophilic addition of water followed by deprotonation

ex in laboratory:
-carried out by KMnO4 or OsO4
-Syn stereochemistry
-no carbocation intermediate
What are some explanations for conjugate diene stability?
Valence Bond Theory:
-stability due to orbital hybridization
-sigma overlap is sp2 (shorter and stronger)

Molecular Orbital Theory:
-interaction between pi orbitals of double bonds
-both electrons occupy low energy bonding orbital leading to lower energy and more stability
What are some reactions of Conjugated Dienes?
-undergo electrophilic addtion reactions readily

-mixture of products obtained

-allylic carbocation is an intermediate - very stable C+
-secondary allylic carbocation is more stable than primary because it is stablized by resonance