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DIENES Dienes Are Alkenes with 2 Double Bonds. IUPAC: Same As

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DIENES Dienes Are Alkenes with 2 Double Bonds. IUPAC: Same As DIENES Dienes are alkenes with 2 double bonds. IUPAC: Same as alkene, but change -ene to -adiene and use two numbers to locate the two double bonds (number from the end of the chain which makes the smaller of these numbers smaller). Double bonds separated by more than one single bond are isolated. Compounds with isolated double bonds have the same chemical properties as alkenes. Double bonds that alternate with single bonds, eg C=C-C=C, are conjugated. Double bond arrangements like C=C=C are cumulated. Compounds containing two carbon-carbon cumulated double bonds are called allenes. Conjugated dienes differ from simple alkenes in that they are more stable, undergo 1,4-addition, and are more reactive. Stability Alkene Type ∆ Hhydrogenation RCH=CH2 30 kcal/mole,exotherm. R2C=CH2, RCH=CHR 28 R2C=CHR 27 ∆ For dienes with isolated double bonds, the Hhydrogenation calculated by adding the appropriate values from the table above together is very close to the experimental value. ∆ For conjugated dienes, the experimental Hhydrogenation is smaller than the calculated one, which is based on isolated double bonds. Conjugated Diene Calculated Experimental ∆ ∆ Hh Hh CH2=CH-CH=CH2 60 kcal/mole 57 CH2=C(CH3)-C(CH3)=CH2 56 54 Therefore, conjugated dienes are more stable than isolated dienes. This stability is reflected in the fact that, where possible, conjugated dienes are the preferred diene products of elimination reactions --- H H C C C C H H H C C H C C KOH, alcohol major product - conjugated C C heat Cl H H H C + C C + H O + KCl Formation of the conjugated C C 2 diene takes precedence over H the Saytzeff rule. minor product - isolated (Saytzeff product) This stability can be explained by electron delocalization in the conjugated diene using the resonance model --- or the molecular orbital model --- Energy (Huckel) of Two Isolated vs Conjugated Double Bonds π4 1.68_ 1.0_ π∗ π3 0.68_ Energy 0 π2 -0.68_ -1.0_ π π1 -1.68_ Two isolated π-bonds. Conjugated π-system. = −4.0 Eπ Eπ = −4.72 Electrophilic Addition to Conjugated Dienes: 1,2- vs. 1,4- Addition --- 1 1 2 4 YZ 2 4 2 1 3 3 3 4 Y YZ Z 1,2-addition 1,4-addition Note the "shift" of the π-bond. eg CH3CH=CH-CH=CHCH3 + HCl > CH3-CH2-CHCl-CH=CHCH3 via 1,2-addition + CH3-CH2CH=CH-CHCl-CH3 via 1,4-addition Mechanism --- H H H H 2 CH 4 CH3 HCl 4 3 1 3 CH 1 3 CH3 2 3 H H H H H ordinary 2o carbocation - VERY LITTLE FORMED + H H H H 1 1 2 4 CH3 2 CH 3 3 4 3 CH3 CH3 H H H H H H 2o allylic type of carbocation; ~ stability of 3o carbocation. Since the positive charge is shared between the carbons labeled 2 & 4 Cl- can attack either. 1,2- vs. 1,4-Addition: Rate vs. Equilibrium Control 2 4 1 3 HBr Low temp (-80o C) High temp (40o C) 1 1 2 4 2 3 3 4 H 80% H 80% Br Br 40oC 1 1 4 2 2 3 3 4 H 20% H 20% Br Br At -80o the 1,2-isomer At 40o the 1,2- and 1,4- converts to 1,4-isomer isomers interconvert, giving very slowly. Since 1,2- an equilibrium mixture. predominates here, The 1,4-isomer is more it is formed faster. stable and predominates. Reaction rates determine Equilibrium constant product here: determines product here: Kinetic Control Thermodynamic Control In this case, the less stable product is formed faster. How does this happen? Diels-Alder Reaction X X Y Y diene dieneophile a substituted cyclohexene (X, Y: typically one or both are electron withdrawing) There are other variations on this theme, eg--- X X Y Y Stereochemistry --- 1) With respect to the dieneophile, the addition is stereospecifically syn, eg H H H H Cl Cl Cl Cl H Cl + Cl H H Cl Cl H Cl H H Cl 2) With respect to the diene, the addition is stereospecific and syn, eg C6H5 C6H5 Cl Cl Cl Cl C6H5 C6H5 Cl Cl Cl Cl 3) The diene must be in the s-cis conformation for reaction to occur. [The mechanism is concerted --- no intermediates.] s-cis s-trans too far close close enough enough too far Dienes unable to attain the s-cis conformation do not react, eg 4) If the diene is cyclic and the dieneophile has an unsaturated substituent, the endo product is usually favored over the exo product [the reaction is stereoselective; one diasteromer (endo) is favored over another (exo)]. 1 2 3 2 b c 1 3 5 4 b a 5 d c H 4 a e d e H cyclopentadiene exo-dicyclopentadiene 1 2 3 2 H 1 3 c 5 4 H b b 5 c 4 a d a d e e cyclopentadiene endo-dicyclopentadiene.
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