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Alkenes Lecture 4
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Isomerism 1. Structural isomers 2. Geometrical isomers
(E)-3-methylpent-2-ene (Z)-3-methylpent-2-ene entgegen zusammen
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Methods of preparation
1. Dehydrogenation of alkanes – elimination of hydrogen the process is usually not selective 2. Dehalogenation of 1,2-dihalogenoalkanes – elimination of halogen 3. Dehydration of alcohols – elimination of water
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Zaitsev rule (1875) - elimination predominates in the direction that leads to the more highly substituted alkene. Aleksander Zaitsev
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4. From tetraalkylammonium salts (Hofmann reaction)
Hofmann rule – elimination from tetraalkylammonium salts predominates in the direction that leads to the less substituted alkene.
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5. Stereoselective hydrogenation of alkynes
Reduction of Lindlar catalyst (stereoselective) : Birch reduction (stereospecific):
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Mechanism of Birch reduction:
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6. Wittig reaction – reaction of an aldehyde or ketone with a triphenyl phosphonium ylide. The configuration of a double bond is predermined by the structure of the starting materials. Ylide – neutral molecule, in which an atom with a formal negative charge (carbon) is directly bound to an atom with a formal positive charge (N, P, S).
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Strong bases: n-butyl lithium phenyl lithium sodium hydride sodium amide sodium bis(trimethylsylil)amide
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Examples:
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Examples:
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Mechanism Georg Wittig,
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Industrial example– synthesis of vitamin А1 (BASF)
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Preparation of alkenes
Dehydrohalogenation of monohalogenoderivatives – elimination of hydrohalogenides Regioselectivity is controlled by Zaitsev rule
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Alkene derivatives – vinyl monomers
vinylchloride acrylic acid nitrile acrylic acid methyl acrylate methyl metacrylate PVC PAN PAA PMA PMMA polyvinyl alcohol vinylacetate PVA
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Production of methyl metacrylate
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Chemical properties of alkenes
1. Electrophilic addition reactions AdE 1.1. Addition of halogens Mechanism: 1) Formation of p-complex 2) Formation of s-complex – cyclic bromonium cation
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3) Addition of bromide-ion to s-complex
1.2. Addition of mixed halogenides
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1.3. Addition of hydrogen halogenides
Mechanism: 1) Formation of p-complex 2) Formation of carbomonium cation
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Vladimir Vasilyevich Markovnikov
3) Addition of bromide-ion to carbocation Markovnikov's rule In the addition of acids HX to an asymmetric alkene, the acid hydrogen (H) becomes attached to the carbon with more hydrogen substituents Vladimir Vasilyevich Markovnikov
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2. Radical addition reactions AdR
Mechanism: 1) chain initiation 2) chain propagation anti-Markovnikov addition 3) formation of product
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3. Radical substitution reactions SR
Mechanism: allyl chloride 1) chain initiation 2) chain propagation 3) formation of product allyl radical 4. Hydrogenation see preparation of alkanes
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peroxycarboxylic acid
5. Oxidation 5.1 Epoxidation – Prilezhaev reaction peroxycarboxylic acid alkene oxide epoxide oxiranes Nickolai Prilezhaev
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5. Oxidation 5.2 Hydroxylation
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5. Oxidation 5.3 Oxidative cleavage of alkenes Other reagents: K2Cr2O7 (H+), CrO3 the reaction is very unselective!
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5. Oxidation 5.4 Ozonolysis
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