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  • 1.Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson

2. Alkenes: Structure and Nomenclature Chapter 5 3. Unsaturated Hydrocarbons

  • Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds
  • Alkene: contains a carbon-carbon double bond and has the general formula C n H 2n
  • Alkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2

4. Unsaturated Hydrocarbons

  • Arenes: benzene and its derivatives (Ch 21-22)
    • we do not study arenes until Chapters 21 & 22
    • however, we show structural formulas of compounds containing the phenyl group before that time
    • the phenyl group is notreactive under any of the conditions we describe in Ch 6-20

5. Structure of Alkenes

  • A double bond consists of
    • one sigma bond formed by the overlap ofsp 2hybrid orbitals and onepi bond formed by the overlap of parallel 2 porbitals
    • the two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120

6. Structure of Alkenes

    • it takes approximately 264 kJ (63 kcal)/mol to break the pi bond in ethylene; that is, to rotate one carbon by 90 with respect to the other so that there is no overlap between 2 porbitals on adjacent carbons

7. Cis,TransIsomerism in Alkenes

  • Cis,transisomers:isomers that have the same connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond

8. Index of Hydrogen Deficiency

  • Index of hydrogen deficiency (IHD):the sum of the number of rings and pi bonds in a molecule
  • To determine IHD, compare the number of hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds
    • the molecular formula of the reference hydrocarbon is C n H 2n+2

9. Index of Hydrogen Deficiency

    • for each atom of a Group 7 element (F, Cl, Br, I), add one H
    • no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon
    • for each atom of a Group 5 element (N, P), subtract one hydrogen

IDH = 2 (H reference -H molecule ) 10. Index of Hydrogen Deficiency

  • Problem:isopentyl acetate has a molecular formula of C 7 H 14 O 2 . Calculate its IHD
    • reference hydrocarbon C 7 H 16
    • IHD = (16-14)/2 = 1
  • Problem:calculate the IHD for niacin, molecular formula C 6 H 6 N 2 O
    • reference hydrocarbon C 6 H 16
    • IHD = (16 - 6)/2 = 5

O O Isopentyl acetate 11. IUPAC Nomenclature

    • 1. Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers
    • 2. Locate the double bond by the number of its first carbon
    • 3. Name substituents
    • 4. Number the carbon, locate and name substituents, locate the double bond, and name the main chain

12. Common Names

  • Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

13. Common Names

    • the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups

14. TheCis , TransSystem

  • Configuration is determined by the orientation of atoms of the main chain

15. TheE , ZSystem

    • uses priority rules (Chapter 3)
    • if groups of higher priority are on the same side, the configuration isZ(German,zusammen )
    • if groups of higher priority are on opposite sides, the configuration isE(German,entgegen )

16. TheE , ZSystem

  • Example: name each alkene and specify its configuration by theE,Zsystem

17. Cis,TransIsomerism

  • Cycloalkenes
    • in small-ring cycloalkenes, the configuration of the double bond iscis
    • these rings are not large enough to accommodate atransdouble bond

18. Cis,TransIsomerism

    • trans -cyclooctene is the smallesttranscyclooctene that has been prepared in pure form and is stable at room temperature
    • thecisisomer is 38 kJ (9.1 kcal)/mol more stable than thetransisomer
    • thetransisomer is chiral even though it has no chiral center

19. Dienes, Trienes, and Polyenes

  • For alkenes containing two or more double bonds, change the infix - en - to - adien -, - atrien -, etc.
    • those containing several double bonds are often referred more generally as polyenes
    • following are three dienes

20. Dienes, Trienes, and Polyenes

    • for alkenes withndouble bonds, each of which can showcis,transisomerism,2 nstereoisomers are possible
    • example: 2 2= 4cis,transisomers are possible for 2,4-heptadiene

21. Dienes, Trienes, and Polyenes

    • vitamin A, a biologically important compound for which a number ofcis,transisomers is possible
    • there are four double bonds about whichcis,transisomerism is possible, for 2 4= 16 stereoisomers

22. Physical Properties

  • Alkenes are nonpolar compounds
  • The only attractive forces between their molecules are dispersion forces
  • The physical properties of alkenes are similar to those of alkanes

23. Terpenes

  • Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

24. Terpenes

  • Myrcene, C 10 H 16 , a component of bayberry wax and oils of bay and verbena
  • Menthol, from peppermint

25. Terpenes

    • -Pinene, from turpentine
    • camphor, from the camphor tree

26. Fatty Acids

  • Animal fats and vegetable oils are both triesters of glycerol, hence the nametriglyceride
    • hydrolysis of a triglyceride in aqueous base followed by acidification gives glycerol and three fatty acids
    • fatty acids with no C=C double bonds are called saturated fatty acid
    • those with one or more C=C double bonds are called unsaturated fatty acids

27. Fatty Acids

    • the most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain
    • the C=C double bonds in almost all naturally occurring fatty acids have acisconfiguration
    • the greater degree of unsaturation, the lower the melting point
    • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are calledoils
    • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are calledfats

28. Fatty Acids

    • the four most abundant fatty acids

29. Fatty Acids

    • carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation
    • because of their high degree of order, they pack together well and are held together by dispersion forces
    • as a result both saturated fatty acids and triglycerides derived from them are solids at room temperature
    • following is a saturated triglyceride

30. Fatty Acids

    • cisdouble bonds place kinks in the chains of unsaturated fatty acids
    • unsaturated fatty acids and the triglycerides derived from them do not pack as well in a crystal lattice as their saturated counterparts, and have weaker dispersion forces between their molecules
    • butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature
    • salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature

31. Alkenes: Structure and Nomenclature End Chapter 5