PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Conjugated Dienes•Conjugated dienes are compounds having two double bonds joined by one  bond. •Conjugated dienes are also called 1,3-dienes.•1,3-Butadiene (CH2=CH-CH=CH2) is the simplest conjugated diene.•Three stereoisomers are possible for 1,3-dienes with alkyl groups bonded to each end carbon of the diene.•Two possible conformations result from rotation around the C—C bond that joins the two double bonds.•Note that stereoisomers are discrete molecules, whereas conformations interconvert.Draw the three possible stereoisomers of 2,4-octadiene. Pick which one is (2E,4E) 2,4-octadiene.Draw the s-cis and s-trans conformations of (3Z,5Z)-4,5-dimethyl -3,5-octadienes-transs-cisThe Carbon—Carbon  Bond Length in 1,3-ButadieneFour features distinguish conjugated dienes from isolated dienes.1. The C—C single bond joining the two double bonds is unusually short.2. Conjugated dienes are more stable than similar isolated dienes.3. Some reactions of conjugated dienes are different than reactions of isolated double bonds.4. Conjugated dienes absorb longer wavelengths of ultraviolet light.The Carbon—Carbon  Bond Length in 1,3-ButadieneThe observed bond distances can be explained by looking at hybridization.A resonance argument can also be used to explain the shorter C—C  bond length in 1,3-butadiene.•Based on resonance, the central C—C bond in 1,3-butadiene is shorter because it has partial double bond character.•Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms.•Consequently, the  electrons are not localized between the carbon atoms of the double bonds, but rather delocalized over four atoms.•This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present.•This shortens the bond.Using hybridization, compare the C-C bonds of the following three compounds.H3C CH3H2C CH2HC CHsp325% s charactersp233% s charactersp50% s characterH3COO-Using resonance, why are the two C—O bonds the same length?H3COO-H3COO-The two resonance structures show how the electron density is delocalized over 3 atoms.Stability of Conjugated DienesWhen hydrogenation gives the same alkane from two dienes, the more stable diene has the smaller heat of hydrogenation.A conjugated diene has a smaller heat of hydrogenation and is more stable than a similar isolated diene.Figure 16.5Relative energiesof an isolated andconjugated diene•A conjugated diene is more stable than an isolated diene because a conjugated diene has overlapping p orbitals on four adjacent atoms. Thus, its  electrons are delocalized over four atoms.•This delocalization, which cannot occur in an isolated diene is illustrated by drawing resonance structures. For example, no resonance structures can be drawn for 1,4-pentadiene, but three can be drawn for (3E)-1,3-pentadiene (or any other conjugated diene).Electrophilic Addition: 1,2- Versus 1,4-Addition•The  bonds in conjugated dienes undergo addition reactions that differ in two ways from the addition reactions of isolated double bonds.1. Electrophilic addition in conjugated dienes gives a mixture of products.2. Conjugated dienes undergo a unique addition reaction not seen in alkenes or isolated dienes.•Recall that electrophilic addition of one equivalent of HBr to an isolated diene yields one product and Markovnikov’s rule is followed.•With a conjugated diene, electrophilic addition of one equivalent of HBr affords two products.•The 1,2-addition product results from Markovnikov addition of HBr across two adjacent carbon atoms (C1 and C2) of the diene.•The 1,4-addition product results from addition of HBr to the two end carbons (C1 and C4) of the diene. 1,4-Addition is also called conjugate addition.•The ends of the 1,3-diene are called C1 and C4 arbitrarily, without regard to IUPAC numbering.Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate.H3C CH3HClH3C CH3Cl+H3C CH3ClHClClHClClHClCl+ClCl++ClKinetic Versus Thermodynamic Products•The amount of 1,2- and 1,4-addition products formed in electrophilic addition reactions of conjugated dienes depends greatly on the reaction conditions.•When a mixture containing predominantly the 1,2-product is heated, the 1,4-addition product becomes the major product at equilibrium.•In the reactions we have learned thus far, the more stable product is formed faster—i.e., the kinetic and thermodynamic products are the same.•The electrophilic addition of HBr to 1,3-butadiene is different in that the kinetic and thermodynamic products are different—i.e., the more stable product is formed more slowly.•Why is the more stable product formed more slowly in this case?•Recall that the rate of a reaction is determined by its energy of activation (Ea), whereas the amount of product present at equilibrium is determined by its stability.Figure 16.6How kinetic andthermodynamic products formin a reaction: A → B + C•The 1,4-product (1-bromo-2-butene) is more stable because it has two alkyl groups bonded to the carbon-carbon double bond, whereas the 1,2-product (3-bromo-1-butene) has only one.•The 1,2-product is the kinetic product because of a proximity effect. •The proximity effect occurs because one species is close to another.•The overall two-step mechanism for addition of HBr to 1,3-butadiene to form both 1,2- and 1,4 addition products is illustrated in the energy diagram below.Why is the ratio of products temperature dependent?•At low temperature, the energy of activation is the more important factor. Since most molecules do not have enough kinetic energy to overcome the higher energy barrier at lower