Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
16 Cards in this Set
- Front
- Back
|
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 |
