Chapter 14 Lecture Conjugated Compounds and Ultraviolet Spectroscopy Organic Chemistry Vol II 9th ed John McMurry E Bryant PhD Houston Community College Houston, TX 1 Introduction: Alkenes . Hydrocarbons with carbon– carbon double bonds . Carbon–carbon double bond is the functional group of alkenes . Carbon–carbon double bond gives this group its reactivity . Called unsaturated compounds © 2013 Pearson Education, Inc. 2 Introduction: Alkene Nomenclature © 2013 Pearson Education, Inc. 3 Introduction: Alkene Nomenclature In a ring, the double bond is assumed to be between carbon 1 and carbon 2. CH3 3 1 CH3 2 2 1 1-methylcyclopentene 3-methylcyclopentene © 2013 Pearson Education, Inc. 4 Introduction: Alkene with Multiple Double Bonds . Give the double bonds the lowest numbers possible . Use di-, tri-, tetra- before the ending -ene to specify how many double bonds are present © 2013 Pearson Education, Inc. 5 Introduction: Alkene Cis-Trans Isomers . Also called geometric isomerism . Similar groups on same side of double bond, alkene is cis . Similar groups on opposite sides of double bond, alkene is trans . Not all alkenes show cis-trans isomerism . All cycloalkenes are assumed to be cis unless otherwise specifically named trans © 2013 Pearson Education, Inc. 6 Introduction: Alkene E-Z Nomenclature . Use the Cahn–Ingold–Prelog rules to assign priorities to groups attached to each carbon in the double bond 1. Look at the four atoms directly attached to the stereogenic center (X) 2. Assign priorities based on atomic number to all four atoms. Priority 1 is assigned to the atom or group of highest atomic number, priority 4 to the lowest 3. If two or more atoms are identical look at all the atoms directly attached to the identical atoms in questions Compare the highest priority atoms, if the highest priority atoms have the same priority,, then compare the second highest priority atoms and so on 4. If a difference still can not be found, move out to the next highest priority group (A- 1 and B-1 in the diagram) and repeat the process 5. Multiple bonds are considered as an equivalent number of single bonded atoms 7 Introduction: Alkene E-Z Nomenclature 1. Assign priorities to groups by the Cahn–Ingold–Prelog rules 2. If high-priority groups are on the same side, the name is Z (for zusammen) 3. If high-priority groups are on opposite sides, the name is E (for entgegen) 8 Introduction: Alkene E-Z Nomenclature Example . Assign priority to the substituents according 1 2 to their atomic number (1 is highest priority). If the highest priority groups are on opposite 1 2 sides, the isomer is E. If the highest priority E-1-bromo-1-chloropropene groups are on the same side, the isomer is Z. © 2013 Pearson Education, Inc. 9 Introduction: Alkene E-Z Nomenclature for Cyclic Stereoisomers . Double bonds outside the ring can show stereoisomerism © 2013 Pearson Education, Inc. 10 Introduction: E-Z Stereochemistry in Polyene Nomenclature . If there is more than one double bond in the molecule, the stereochemistry of all the double bonds should be specified. © 2013 Pearson Education, Inc. 11 Polyenes . Molecules contains more than one double bond © 2013 Pearson Education, Inc. 12 Polyene Systems • Double bonds separated by two or more single bonds are isolated • Double bonds separated by one single bond are conjugated • Conjugated double bonds are more stable than isolated ones © 2013 Pearson Education, Inc. 13 Is there a difference between isolated double bonds and conjugated double bonds? Yes, conjugated polyenes are more stable than isolated double bonds Determine by Hhydrog 14 What is Heat of Hydrogenation? . Compare heat given off on hydrogenation: Ho . Evaluate heat given off when C=C is converted to C-C . Less stable isomer is higher in energy . More stable alkene gives off less heat Hhydrog . trans-butene generates 5 kJ Hhydrog less heat than cis-butene 15 Stability of Conjugated Compounds . Conjugated dienes are more stable than nonconjugated based on heats of hydrogenation . Hydrogenating 1,3-butadiene produces 17 kJ/mol less heat than 1,4-pentadiene 16 Stability of Conjugated Compounds . What accounts for the stability of conjugated dienes? . Valence bond theory states it is due to orbital hybridization 17 Stability of Conjugated Compounds . Typical C-C bonds result from overlap of sp3 orbitals on both carbons . Conjugated C-C bonds result from overlap of sp2 orbitals on both carbons . sp2 orbitals have more s character (33% s) than sp3 orbitals (25% s) . Greater stability of a conjugated diene results from the greater amount of s character in the orbitals forming the C-C single bond 18 Stability of Conjugated Compounds . What accounts for the stability of conjugated dienes? . Valence bond theory states it is due to orbital hybridization . Molecular orbital theory states it is due to interaction between the orbitals of the two double bonds 19 Stability of Conjugated Compounds 1,3-Butadiene . Small amount of overlap across the central C2—C3 bond, giving it a partial double bond character . Electrons are delocalized over the molecule . The C2—C3 single bond is shorter than 1.54 Å © 2013 Pearson Education, Inc. 20 ADDITIONS TO CONJUGATED DIENES 21 Allylic Carbon 22 The Allylic Position . The allylic carbon is the one directly attached to an sp2 carbon . Allylic cations are stabilized by resonance © 2013 Pearson Education, Inc. 23 Allylic Cations The positive charge is delocalized over two carbons by resonance, giving the allylic cation more stability than nonconjugated cations © 2013 Pearson Education, Inc. 24 Stability of Carbocations . Stability of 1 allylic 2 carbocation . Stability of 2 allylic 3 carbocation © 2013 Pearson Education, Inc. 25 Electrophilic Addition to Conjugated Dienes 26 1,2- and 1,4-Addition to Conjugated Dienes . Electrophilic addition to the double bond produces the most stable intermediate . For conjugated dienes, the intermediate is a resonance-stabilized allylic cation . Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge 27 Conjugate Addition: 1,2- Addition and 1,4-Addition Addition by Markovnikov Rule Addition by allylic cation stabilization . Conjugated dienes undergo electrophilic addition . Products a mixture of 1,2 and 1,4 addition . Numbers refer to which carbon the addition occurs on 28 Conjugate Addition: 1,2- Addition and 1,4-Addition . Electrophilic addition . 1,2-addition is by Markovnikov Rule . 1,4 addition is by allylic cation stabilization 29 Conjugate Addition: 1,2-Addition and 1,4-Addition 30 Example 14-1 Give the structures of the likely products from reaction of 1 equivalent of HCL with 2-methyl- 1,3,-cyclohexadiene. Show both 1,2 and 1,4 adducts. SOLUTION 1. Protonate the two ends of the diene and draw resonance forms of the 2 allylic carbocations that result 2. React each resonance form with Cl- Most likely product More stable form of protonation Allylic cation more stable 31 Electrophilic Addition to Conjugated Dienes Kinetic or Thermodynamic Reaction 32 Conjugate Addition: Kinetic versus Thermodynamic Control . The percent of adduct available differs at different temperatures . At low temperatures, Markovnikov Rule product is in abundance . At higher temperature, the product ratio changes and the 1,4 adduct predominates 33 Conjugate Addition: Kinetic versus Thermodynamic Control 1,3 Butadiene Why would increasing the temperature after the product is formed at lower temperatures change the ratio of the products formed? 34 Conjugate Addition: Kinetic versus Thermodynamic Control . Kinetic control is an irreversible reaction that depends on relative rates, not stability . Thermodynamic control is reversible reaction that depends on stability not on relative rates 35 Conjugate Addition: Kinetic versus Thermodynamic Control . Kinetic control is an irreversible reaction that depends on relative rates, not stability . Thermodynamic control is reversible reaction that depends on stability not on relative rates 36 Kinetic Control at – 80oC . Transition state for the 1,2-addition has a lower Ea because it is a more stable secondary carbocation 37 Kinetic Control at – 80oC . Transition state for the 1,2-addition has a lower Ea because it is a more stable secondary carbocation . The 1,2-addition will be the faster addition at any temperature . The nucleophilic attack of the bromide on the C2 allylic carbocation is irreversible at this low temperature . The product that forms faster predominates (kinetic product) . Because the kinetics of the reaction determines the product, the reaction is said to be under kinetic control 38 Thermodynamic Control at 40oC . The 1,2-addition is still the faster addition, but at 40oC, the bromide attack is reversible . The 1,2-product ionizes back to the allylic cation . At 40oC an equilibrium is established, which favors the most stable product . The 1,4-addition is the most stable product (thermodynamic product) because it has a more substituted double bond . Because the thermodynamics of the reaction determines the product, the reaction is said to be under thermodynamic control 39 Diels-Alder Reaction 40 Diels–Alder Reaction . The reaction is between a diene and an electron-deficient alkene or alkyne (dienophile) dienophile diene 41 Diels–Alder Reaction . Conjugate dienes combine with alkenes to form six- membered cyclic compounds . Reaction called a cycloaddition . The formation of the ring involves no intermediate (concerted formation of two bonds) 42 Diels–Alder Reaction . Also called a [4 + 2] cycloaddition because a ring is formed by the interaction of four pi electrons of the alkene with two pi electrons of the alkene or alkyne . Two more sigma bonds . Two fewer pi bonds 43 Diels–Alder Reaction: Examples 44 s-Cis Conformation of the Diene . Positions of the two double bonds around the single bond in the diene are “cis” or “trans” to each other . Conformations are called s-cis and s-trans (“s” stands for “single bond”) . s-trans conformation is 12 kJ/mol more stable than the s-cis 45 Diels–Alder Reaction: Stereochemical Requirements .