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5 Alkenes. Bonding, Nomenclature, Properties. Structure Hydrogen Deficiency Nomenclature Physical Properties Naturally Occurring Alkenes/Terpenes. 1. Alk ene: contains a carbon-carbon double bond and has the general formula C n H 2n. H. H. eth ene. C. C. H. H.

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Bonding, Nomenclature, Properties


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    1. 5 Alkenes Bonding, Nomenclature, Properties Structure Hydrogen Deficiency Nomenclature Physical Properties Naturally Occurring Alkenes/Terpenes

    2. 1. Alkene: contains a carbon-carbon doublebond and has the general formula CnH2n H H ethene C C H H UnsaturatedHydrocarbons

    3. 2.Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2 (Ch 7) ethyne (acetylene) Unsaturated Hydrocarbons

    4. 3. Arene: benzene and its derivatives (Ch 21-22) or Unsaturated Hydrocarbons

    5. Benzene and derivatives studied in Chapters 21 & 22, but some formulas may contain the phenyl group. Benzene C H - 6 5 Alternative representations for the phenyl group Benzene & Phenyl Group Ph-

    6. Double Bond: 1-bond formed by overlap of 2 sp2 hybrid orbitals 1 -bondformed by overlap of 2 parallel 2p orbitals -bond -bonded atoms all in a plane  trigonal - bond angles ~ 120° H C H perpendicular H C H to the plane Structure of Alkenes

    7. H H  C C H H 120o  H H C C  H H Valence Bond view of “olefins” or double bonds. x 109.5o 0.154 nm 88 kcals/mol 25 % s tetrahedral 120o 0.134 nm 146 kcals/mol 33 % s trigonal 180o 0.120 nm 200 kcals/mol 50 % s linear

    8. No rotation about a C=C bond - why? H C H H C H Structure of Alkenes Rotation requires breaking  bond ~63 kcal/mol

    9. Index of hydrogen deficiency (IHD): IDH =  (number of rings + number ofbonds) Compare HsofalkanewithHsin a compound CnH2n+2CnHx (14 - 10) 2 = ) (H - H reference molecule C6H2(6)+2 IDH = 2 Index of Hydrogen Deficiency = 2

    10. Other elements present F, Cl, Br, I add one H - (Group 7) O, S no correction (Group 6) N, P subtract one H (Group 5) Index of Hydrogen Deficiency 9 + 1 7 + 1 8 + 1 - 1

    11. Problem: isopentyl acetate has a molecular formula of C7H14O2. Calculate its IHD reference hydrocarbon C7H16 IHD = (16-14)/2 = 1 Problem: calculate the IHD for niacin, molecular formula C6H6N2O reference hydrocarbon C6H14 IHD = [14 - (6-2)]/2 = 5 O O Isopentyl acetate Index of Hydrogen Deficiency

    12. Number chain - olefin lowest number #- 1 6 2 5 3 4 Nomenclature IUPAC: Root name - longest continuous chain containing the olefin. alkene indicates C=C in chain Number of C’s in C=C chain 6 5 1 2 4 3 2-ethyl-4-methyl-1-pentene 2-hexene

    13. Number chain - olefin lowest number #-cycloalkene #- #- alkene x Nomenclature IUPAC: Root name - longest continuous chain containing the olefin. alkene indicates C=C in chain Number of C’s in chain with an C=C Cyclic Olefin (functional group) - positions 1,2 Number around the ring to best accommodate substituents

    14. IUPAC names? 7-bromo-4-(2-iodoethyl)-6-methyl-3-octene (7S)-7-bromo-4-(2-iodoethyl)-6-methyl-3-octene (S)-6-ethyl-1-fluoro-5,5-dimethyl-1-cyclooctene

    15. Cis/trans isomerism (recall cycloalkanes - cis/trans) same side of -bond Cl Cl Cl H Cl H C C C C C C H Cl H H H Cl opposite side -bond cis-1,2-dichloroethene trans-1,2-dichloroethene transgenerally more stable thancis - dipoles and 1,2 interactions

    16. Configuration is determined by the orientation of atoms of the main chain The Cis,Trans System trans-3-hexene cis-3,4-dimethyl-2-pentene

    17. 5 Alkenes Bonding, Nomenclature, Properties Structure Hydrogen Deficiency Nomenclature Physical Properties Naturally Occurring Alkenes/Terpenes

    18. Common Names used lab & lecture not on tests

    19. C8 limited stability as trans Cycloalkenes- 3 to 7 “cis” olefins rings not large enough to accommodate trans double bonds

    20. uses priority rules (Chapter 3) higher priority groups - same side, Z higher priority - opposite sides, E E,Z Configuration E Z

    21. H C C H C C H H C C C C C C H C C H C C H H E/Z - priorities of groups on ends of C=C (1) Atom assigned a priority, higher atomic number higher priority. (2) Isotopes - higher atomic mass, higher priority 1H < 2H < 3H [H < D < T] (3) If priority the same, go to the next set of atoms: -CH2-H < -CH2-O-H < -CH2-Cl (4) double (triple) bonds replaced by single bonds.

    22. Example:name each alkene and specify its configuration by the E,Z system The E,Z System

    23. Stereochemistry? IUPAC names? ?(trans)-longest chain? (3Z,7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene (7S)-7-bromo-4-(2-chloroethyl)-6-methyl-3-octene (S)-6-ethyl-1-fluoro-5,5-dimethylcyclooctene

    24. If same atoms, priority goes to the next point of difference - atomic number 9-bromo-5-(2-methylpropyl)-4-nonene (E)- (4E)- cis-

    25. buttrans-1,3-dimethylcyclopentane could be R,R- or S,S-1,3-dimethylcyclopentane relative cis/trans configurationmay be absolute or relative e.g.cis-1,3-dimethylcyclopentane or R,S-1,3-dimethylcyclopentane absolute E,Z and R,S absolute

    26. 1 2 7 3 4 6 5 Nomenclature - more than 1 unsaturated group Diene (or diyne, en-yne) longest chain (ring) with both groups: #,#-alkadiene #,#-alkadiyne #-alken-#-yne number so enes (enyne, ynes) have the lowest possible number (if the same ene > yne). 1,4-cycloheptadiene

    27. alkenes with n double bonds, which can be cis,trans isomers, 2n stereoisomers are possible example 2,4-heptadiene: 22 = 4 cis,trans (E,Z) Dienes, Trienes, and Polyenes

    28. Dienes - E or Z per olefin with number 1,6-dichloro-2,4-dimethyl-2,6-decadiene ( 2Z, 4S, 6E)- ( 2Z, 4S, 6E)-

    29. vitamin A, a biologically important, sight/color four double bonds w/ cis,trans isomerism 24 = 16 stereoisomers Dienes, Trienes, and Polyenes

    30. Alkenes are nonpolar compounds attractive forces between molecules are dispersion forces The physical properties of alkenes are similar to those of alkanes Physical Properties to 6.1