Alkenes,dienes and alkynes

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Summarizes the nature and rections of Alkenes, Dienes and alkynes

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  • ALKENES
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  • Alkenes,dienes and alkynes

    1. 1. Presented by: JanineV. Samelo BSChem2
    2. 2.  Alkenes are a family of hydrocarbons (compounds containing carbon and hydrogen only) containing a carbon-carbon double bond.  General formula: CnH2n  Example:  Functional group = carbon-carbon double bond  sp2 hybridization => flat, 120o bond angles  Shape => trigonal planar  σ bond & π bond => H2C=CH2  C2H4 ethylene ethene C2H4 propene C3H6 C C H H H H
    3. 3.  Structural isomerism  All the alkenes with 4 or more carbon atoms in them show structural isomerism.This means that there are two or more different structural formulae that you can draw for each molecular formula.  For example, with C4H8, it isn't too difficult to come up with these three structural isomers:
    4. 4.  Geometric (cis-trans) isomerism  The carbon-carbon double bond doesn't allow any rotation about it.That means that it is possible to have the CH3 groups on either end of the molecule locked either on one side of the molecule or opposite each other.  These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides).  Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but- 2-ene.
    5. 5. Boiling Points  The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperature. All the rest that you are likely to come across are liquids.  It has a boiling point which is a small number of degrees lower than the corresponding alkane.The only attractions involved areVan derWaals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.
    6. 6. Solubility  Alkenes are virtually insoluble in water, but dissolve in organic solvents.  non-polar or weakly polar  no hydrogen bonding  relatively low mp/bp ~ (similar to alkanes)  water insoluble Importance:  common group in biological molecules  starting material for synthesis of many plastics
    7. 7. 1.Parent chain = longest continuous carbon chain that contains the C=C. alkane => change –ane to –ene • prefix a locant for the carbon-carbon double bond using the principle of lower number. 2.Alphabetize, name the substituents, etc. 3.If a geometric isomer, use E/Z (or cis/trans) to indicate which isomer it is.
    8. 8. Synthesis of Alkenes Alkyl halides are dehydrohalogenated with base to form alkenes. Alcohols are dehydrated with heat and acid to form alkenes.The product with the most number of carbons attached to the carbon- carbon double bond is formed in the higher yield.
    9. 9.  Polymers of Alkenes Ethylene is polymerized to polyethylene, which is used for bags, films, and bottles.  Propylene is polymerized to polypropylene, which is used for plastics.  Styrene is polymerized to polystyrene, which is used for plastics, plastic cups, and foam insulation.  Methyl α-methacrylate is polymerized to polymethyl α- methacrylate, which is used for plexiglass and Lucite paints.  Acrylonitrile is polymerized to polyacrylonitrile, which is used as Orlon or Acrylan fibers.  Tetrafluoroethylene is polymerized to polytetrafluoroethylene, which is used asTeflon.  Vinyl chloride is polymerized to polyvinyl chloride, which is used in plastics, films, and plumbing.  Vinylidene chloride is polymerized to polyvinylidene chloride, which is used in Saran.
    10. 10. addition reactions.  Example  The rather exposed electrons in the pi bond are particularly open to attack by things which carry some degree of positive charge.These are called electrophiles
    11. 11. 3. dehalogenation of vicinal dihalides | | | | — C — C — + Zn  — C = C — + ZnX2 | | X X example: CH3CH2CHCH2 + Zn  CH3CH2CH=CH2 + ZnBr2 Br Br  Not generally useful as vicinal dihalides are usually made from alkenes. May be used to “protect” a carbon-carbon double bond.
    12. 12. 1. dehydrohalogenation of alkyl halides | | | | — C — C — + KOH(alc.)  — C = C — + KX + H2O | | H X a) RX: 3o > 2o > 1o b) no rearragement c) may yield mixtures d) Saytzeff orientation e) element effect f) isotope effect g) rate = k [RX] [KOH] h) Mechanism = E2
    13. 13.  rate = k [RX] [KOH] => both RX & KOH in RDS  R-I > R-Br > R-Cl “element effect” => C—X broken in RDS  R-H > R-D “isotope effect” => C—H broken in RDS   Concerted reaction: both the C—X and C—H bonds are broken in the rate determining step.
    14. 14.  Mechanism = elimination, bimolecular E2  One step! “Concerted” reaction. base: C W C H C C + H:base + :W RDS
    15. 15.  CH3CHCH3 + KOH(alc)  CH3CH=CH2 Br isopropyl bromide propylene  CH3CH2CH2CH2-Br + KOH(alc)  CH3CH2CH=CH2 n-butyl bromide 1-butene  CH3CH2CHCH3 + KOH(alc)  CH3CH2CH=CH2 Br 1-butene 19% sec-butyl bromide + CH3CH=CHCH3 2-butene 81%
    16. 16. . Hydrocarbon containing two carbon-carbon double bonds Alkadienes Isolated dienes 1,4-pentadiene 1,5-Cyclo- octadiene Separated by one or more sp3-C atom. Conjugated dienes: 1,3-butadiene 1,3-cyclo- hexadiene Double bonds and single bonds alternate along the chain. Cumulated dienes Allene The C atom is common for two double bonds C C C C C C C C C H2C C CH2 Hydrocarbon containing two carbon-carbon double bonds
    17. 17. . cis,cis –2,4-hexadiene (2Z,4Z)-2,4-hexadiene (2Z,4E)-2,4-hexadiene cis,trans- C C C C CH3 H3C H HH H C C C C CH3 H3C H H H H •Suffix: ne diene • Cis-trans isomers:
    18. 18. . 4 C atoms are sp2- hybridized. C2-C3 σbond: sp2-sp2overlap C H H C C C H HH H 1,3-Butadiene: πbond:2p-2p overlap C2-C3 partially overlap by 2p-2p orbital 4 C atoms are coplanar C H H C C C H HH H 4 πelectrons are delocalized over 4 C atomsC Delocalization of πelectrons lowers the energy.
    19. 19. . Two possible planar conformation of 1,3-butadiene: s-Cis conformation s-Trans conformation
    20. 20.  Conjugated dienes have enhanced stability as compared to molecules without conjugated double bonds due to resonance. In general, this makes them slightly less reactive than other types of alkenes in general and dienes specifically. However, many reactions proceed through high-energy cation or radical intermediates; in these cases the resonance stabilization of the intermediate allyl species makes conjugated dienes more reactive than non-conjugated dienes or simple alkenes.  Hydrobromination:  Example:  Butadiene + HBr--> 3-bromobutene (LowTemperature) + 1-bromobut-2-ene (HighTemperature) + 1-bromobutene (Not Observed)
    21. 21.  Diels-Alder Reaction  One of the most important of all diene reactions is the Diels-Alder Reaction, in which a conjugated diene reacts with an dienophile to form a cyclohexene. Requirements:The diene must be able to access the s-cis conformation for the reaction to take place.  Example: H2C CH C O H H2C CH C O OCH2CH3 H2C CH C N HC C COOCH3
    22. 22.  - A Dienophile must contain a double or triple bond.Typically, an electron withdrawing group is conjugated to the dienophile to make it electron-poor (nitriles, ketones, and esters are common electron withdrawing groups). Because the reaction is highly stereospecific, the configuration of the dieneophile will determine the relative stereochemistry of the cyclohexene product.  The Diels-Alder reaction occurs most effectively with an electron-poor dieneophile and an electron-rich diene ('normal demand'). 'Inverse demand' Diels-Alder reactions can also be carried out, in which the dienophile is electron-rich and the diene electron-poor. A species which likes to attack Dienes
    23. 23.  Alkynes contain carbon-carbon triple bonds.  The carbon in an alkyne is sp, has a bond angle of 180o, and a linear shape. A carbon-carbon triple bond contains one sigma bond and two pi bonds.  A terminal alkyne contains at least one hydrogen attached to the carbon-carbon triple bond. An alkyne that is not terminal contains two alkyl groups attached to the carbon-carbon triple bond.
    24. 24.  Synthesis of Alkynes Vicinal dihalides are dehydrohalogenated twice with base to form alkynes. Geminal dihalides are dehydrohalogenated twice with base to form alkynes.  Reactions of Alkynes Hydrogen halide adds across a triple bond, via Markovnikov addition and with anti or syn addition, to form dihaloalkanes. Halogen adds across a triple bond to form a tetrahaloalkane. Hydrogen adds across a triple bond to make an alkane.
    25. 25.  Alkynes are compounds which have low polarity, and have physical properties that are essentially the same as those of the alkanes and alkenes.  They are insoluble in water.  They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene, carbon tetrachloride, etc.).  They are less dense than water.  Their boiling points show the usual increase with increasing carbon number.  They are very nearly the same as the boiling points of alkanes or alkenes with the same carbon skeletons.
    26. 26.  Alkynes Preparation  The carbon-carbon triple bond of the alkynes is formed in the same way as a double bond of the alkenes, by the elimination of atoms or groups from two adjacent carbons. W X W X HC - CH ==> HC = CH ==> HCCH X X Alkane Alkene Alkyne The groups that are eliminated and the reagents used are essentially the same as in the preparations of alkenes.
    27. 27. H3C C C CH3 xs H2 Pt CH3CH2CH2CH3 H2/Pd/BaSO 4 Quinoline (Lindlar's Catalyst) H3C C H C H CH3 Na NH 3 (liq) H3C C H C H CH3
    28. 28. H3C C C H HX H3C C C H H + X- H3C C C H H X Markovnikov addition a vinyl halide H3C C C H H X HX H3C C C H H HX + X - a heteroatom stabilized carbocation H3C C C H H HX + H3C C C H H X H X a geminal dihalide
    29. 29.  Keto-enol tautomerization
    30. 30.  Formation ofVicinal tetrahalides C C an alkyne X2 C C X X X X a vicinal tetrahalide carbon tetrachloride
    31. 31. R C CH Na R C C Na + R C C Na + + R'CH 2 X R C C CH2R' + NaX + H2 Terminal Alkyne Methyl or Primary Alkyl Halide  Alkynyl Anion Synthesis of Alkynes via Bimolecular Nucleophilic Substitution
    32. 32.  Six-membered Ring formation (4+2)  Diene + Dienophile

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