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Ch5 Ch5 Presentation Transcript

  • Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson
  • Alkenes: Structure and Nomenclature Chapter 5
  • 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
  • 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 not reactive under any of the conditions we describe in Ch 6-20
  • Structure of Alkenes
    • A double bond consists of
      • 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
      • 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°
  • 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 p orbitals on adjacent carbons
  • Cis,Trans Isomerism in Alkenes
    • 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
  • 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
  • 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 )
  • 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
  • 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
  • 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
  • Common Names
      • the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups
  • The Cis , Trans System
    • Configuration is determined by the orientation of atoms of the main chain
  • The E , Z System
      • uses priority rules (Chapter 3)
      • if groups of higher priority are on the same side, the configuration is Z (German, zusammen )
      • if groups of higher priority are on opposite sides, the configuration is E (German, entgegen )
  • The E , Z System
    • Example: name each alkene and specify its configuration by the E,Z system
  • Cis,Trans Isomerism
    • Cycloalkenes
      • in small-ring cycloalkenes, the configuration of the double bond is cis
      • these rings are not large enough to accommodate a trans double bond
  • Cis,Trans Isomerism
      • trans -cyclooctene is the smallest trans cyclooctene that has been prepared in pure form and is stable at room temperature
      • the cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer
      • the trans isomer is chiral even though it has no chiral center
  • 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
  • Dienes, Trienes, and Polyenes
      • for alkenes with n double bonds, each of which can show cis,trans isomerism, 2 n stereoisomers are possible
      • example: 2 2 = 4 cis,trans isomers are possible for 2,4-heptadiene
  • Dienes, Trienes, and Polyenes
      • vitamin A, a biologically important compound for which a number of cis,trans isomers is possible
      • there are four double bonds about which cis,trans isomerism is possible, for 2 4 = 16 stereoisomers
  • 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
  • Terpenes
    • Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene
  • Terpenes
    • Myrcene, C 10 H 16 , a component of bayberry wax and oils of bay and verbena
    • Menthol, from peppermint
  • Terpenes
      •  -Pinene, from turpentine
      • camphor, from the camphor tree
  • Fatty Acids
    • Animal fats and vegetable oils are both triesters of glycerol, hence the name triglyceride
      • 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
  • 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 a cis configuration
      • the greater degree of unsaturation, the lower the melting point
      • triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils
      • triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats
  • Fatty Acids
      • the four most abundant fatty acids
  • 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
  • Fatty Acids
      • cis double 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
  • Alkenes: Structure and Nomenclature End Chapter 5