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  1. 1. Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson
  2. 2. Alkenes: Structure and Nomenclature Chapter 5
  3. 3. Unsaturated Hydrocarbons <ul><li>Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds </li></ul><ul><li>Alkene: contains a carbon-carbon double bond and has the general formula C n H 2n </li></ul><ul><li>Alkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2 </li></ul>
  4. 4. Unsaturated Hydrocarbons <ul><li>Arenes: benzene and its derivatives (Ch 21-22) </li></ul><ul><ul><li>we do not study arenes until Chapters 21 & 22 </li></ul></ul><ul><ul><li>however, we show structural formulas of compounds containing the phenyl group before that time </li></ul></ul><ul><ul><li>the phenyl group is not reactive under any of the conditions we describe in Ch 6-20 </li></ul></ul>
  5. 5. Structure of Alkenes <ul><li>A double bond consists of </li></ul><ul><ul><li>one sigma bond formed by the overlap of sp 2 hybrid orbitals and one pi bond formed by the overlap of parallel 2 p orbitals </li></ul></ul><ul><ul><li>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° </li></ul></ul>
  6. 6. Structure of Alkenes <ul><ul><li>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 p orbitals on adjacent carbons </li></ul></ul>
  7. 7. Cis,Trans Isomerism in Alkenes <ul><li>Cis,trans isomers: 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 </li></ul>
  8. 8. Index of Hydrogen Deficiency <ul><li>Index of hydrogen deficiency (IHD): the sum of the number of rings and pi bonds in a molecule </li></ul><ul><li>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 </li></ul><ul><ul><li>the molecular formula of the reference hydrocarbon is C n H 2n+2 </li></ul></ul>
  9. 9. Index of Hydrogen Deficiency <ul><ul><li>for each atom of a Group 7 element (F, Cl, Br, I), add one H </li></ul></ul><ul><ul><li>no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon </li></ul></ul><ul><ul><li>for each atom of a Group 5 element (N, P), subtract one hydrogen </li></ul></ul>IDH = 2 (H reference - H molecule )
  10. 10. Index of Hydrogen Deficiency <ul><li>Problem: isopentyl acetate has a molecular formula of C 7 H 14 O 2 . Calculate its IHD </li></ul><ul><ul><li>reference hydrocarbon C 7 H 16 </li></ul></ul><ul><ul><li>IHD = (16-14)/2 = 1 </li></ul></ul><ul><li>Problem: calculate the IHD for niacin, molecular formula C 6 H 6 N 2 O </li></ul><ul><ul><li>reference hydrocarbon C 6 H 16 </li></ul></ul><ul><ul><li>IHD = (16 - 6)/2 = 5 </li></ul></ul>O O Isopentyl acetate
  11. 11. IUPAC Nomenclature <ul><ul><li>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 </li></ul></ul><ul><ul><li>2. Locate the double bond by the number of its first carbon </li></ul></ul><ul><ul><li>3. Name substituents </li></ul></ul><ul><ul><li>4. Number the carbon, locate and name substituents, locate the double bond, and name the main chain </li></ul></ul>
  12. 12. Common Names <ul><li>Despite the precision and universal acceptance of IUPAC nomenclature, some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names </li></ul>
  13. 13. Common Names <ul><ul><li>the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups </li></ul></ul>
  14. 14. The Cis , Trans System <ul><li>Configuration is determined by the orientation of atoms of the main chain </li></ul>
  15. 15. The E , Z System <ul><ul><li>uses priority rules (Chapter 3) </li></ul></ul><ul><ul><li>if groups of higher priority are on the same side, the configuration is Z (German, zusammen ) </li></ul></ul><ul><ul><li>if groups of higher priority are on opposite sides, the configuration is E (German, entgegen ) </li></ul></ul>
  16. 16. The E , Z System <ul><li>Example: name each alkene and specify its configuration by the E,Z system </li></ul>
  17. 17. Cis,Trans Isomerism <ul><li>Cycloalkenes </li></ul><ul><ul><li>in small-ring cycloalkenes, the configuration of the double bond is cis </li></ul></ul><ul><ul><li>these rings are not large enough to accommodate a trans double bond </li></ul></ul>
  18. 18. Cis,Trans Isomerism <ul><ul><li>trans -cyclooctene is the smallest trans cyclooctene that has been prepared in pure form and is stable at room temperature </li></ul></ul><ul><ul><li>the cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer </li></ul></ul><ul><ul><li>the trans isomer is chiral even though it has no chiral center </li></ul></ul>
  19. 19. Dienes, Trienes, and Polyenes <ul><li>For alkenes containing two or more double bonds, change the infix - en - to - adien -, - atrien -, etc. </li></ul><ul><ul><li>those containing several double bonds are often referred more generally as polyenes </li></ul></ul><ul><ul><li>following are three dienes </li></ul></ul>
  20. 20. Dienes, Trienes, and Polyenes <ul><ul><li>for alkenes with n double bonds, each of which can show cis,trans isomerism, 2 n stereoisomers are possible </li></ul></ul><ul><ul><li>example: 2 2 = 4 cis,trans isomers are possible for 2,4-heptadiene </li></ul></ul>
  21. 21. Dienes, Trienes, and Polyenes <ul><ul><li>vitamin A, a biologically important compound for which a number of cis,trans isomers is possible </li></ul></ul><ul><ul><li>there are four double bonds about which cis,trans isomerism is possible, for 2 4 = 16 stereoisomers </li></ul></ul>
  22. 22. Physical Properties <ul><li>Alkenes are nonpolar compounds </li></ul><ul><li>The only attractive forces between their molecules are dispersion forces </li></ul><ul><li>The physical properties of alkenes are similar to those of alkanes </li></ul>
  23. 23. Terpenes <ul><li>Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene </li></ul>
  24. 24. Terpenes <ul><li>Myrcene, C 10 H 16 , a component of bayberry wax and oils of bay and verbena </li></ul><ul><li>Menthol, from peppermint </li></ul>
  25. 25. Terpenes <ul><ul><li> -Pinene, from turpentine </li></ul></ul><ul><ul><li>camphor, from the camphor tree </li></ul></ul>
  26. 26. Fatty Acids <ul><li>Animal fats and vegetable oils are both triesters of glycerol, hence the name triglyceride </li></ul><ul><ul><li>hydrolysis of a triglyceride in aqueous base followed by acidification gives glycerol and three fatty acids </li></ul></ul><ul><ul><li>fatty acids with no C=C double bonds are called saturated fatty acid </li></ul></ul><ul><ul><li>those with one or more C=C double bonds are called unsaturated fatty acids </li></ul></ul>
  27. 27. Fatty Acids <ul><ul><li>the most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain </li></ul></ul><ul><ul><li>the C=C double bonds in almost all naturally occurring fatty acids have a cis configuration </li></ul></ul><ul><ul><li>the greater degree of unsaturation, the lower the melting point </li></ul></ul><ul><ul><li>triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils </li></ul></ul><ul><ul><li>triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats </li></ul></ul>
  28. 28. Fatty Acids <ul><ul><li>the four most abundant fatty acids </li></ul></ul>
  29. 29. Fatty Acids <ul><ul><li>carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation </li></ul></ul><ul><ul><li>because of their high degree of order, they pack together well and are held together by dispersion forces </li></ul></ul><ul><ul><li>as a result both saturated fatty acids and triglycerides derived from them are solids at room temperature </li></ul></ul><ul><ul><li>following is a saturated triglyceride </li></ul></ul>
  30. 30. Fatty Acids <ul><ul><li>cis double bonds place kinks in the chains of unsaturated fatty acids </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature </li></ul></ul><ul><ul><li>salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature </li></ul></ul>
  31. 31. Alkenes: Structure and Nomenclature End Chapter 5