11/5/ Conjugated Dienes. Conjugated Dienes. Conjugated Dienes. Heats of Hydrogenation

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1 8.12 Sites of unsaturation Many compounds have numerous sites of unsaturation If sites are well separated in molecule they react independently If sites are close together they may interact with one another Conjugated double bonds Double bonds that alternate with single bonds Heats of Hydrogenation Conjugated dienes are more stable than nonconjugated dienes Buta-1,3-diene is approximately 16 kj/mol (3.8 kcal/mol) more stable than expected 1

2 Explanations for conjugated diene stability 1) Valence Bond Theory Stability due to orbital hybridization Alkanes C-C single bonds σ overlap of sp 3 orbitals on both carbons Conjugated dienes σ overlap of sp 2 orbitals (shorter and stronger) 2. Molecular Orbital Theory Interaction between the p orbitals of the two double bonds Two p orbitals combine to form two p molecular orbitals Both electrons occupy the low-energy bonding orbital leading to a net lowering of energy and formation of a stable bond Four adjacent p atomic orbitals of a conjugated diene Four molecular orbitals of buta-1,3-diene 2

3 In a conjugated diene, the lowest p MO (y 1 ) has a favorable bonding interaction between C2 and C3 that is absent in the nonconjugated diene Certain amount of double-bond character to C2-C3 bond, making that bond stronger and shorter that a typical single bond 8.13 Reactions of Conjugated dienes Undergo electrophilic addition reactions readily Mixture of products obtained Addition of HBr to buta-1,3-diene yields mixture of two addition products Reactions of Allylic carbocation is an intermediate Allylic means next to a double bond When buta-1,3-diene reacts with H + electrophile two carbocation intermediates are possible: 1. A primary carbocation 2. A secondary allylic carbocation (stabilized by resonance between two forms) Secondary allylic carbocation is more stable and forms faster than the nonallylic carbocation 3

4 Reactions of Allylic carbocation reacts with Br - to complete the electrophilic addition Reaction can occur at C1 or C3 Both carbons share positive charge Mixture of 1,2- and 1,4-addition products results Worked Example 8.5 Predicting the Products of Electrophilic Addition to a Conjugated Diene Give the structures of the likely products from reaction of 1 equivalent of HCl with 2- methylcyclohexa-1,3-diene. Show both 1,2- and 1,4- adducts Conjugated dienes undergo reactions with alkenes to yield substituted cyclohexene products 4

5 Diels-Alder cycloaddition reaction is a Pericyclic reaction Pericyclic reactions take place in a single step by a cyclic redistribution of bonding electrons In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from sp 2 to sp 3 to form two new single bonds, while carbons 2 and 3 of the diene remain sp 2 hybridized to from the new double bond in the cyclohexene product Diels-Alder cycloaddition reaction occurs most rapidly if the alkene component, or dienophile ( diene lover ), has an electron-withdrawing substituent group 5

6 The double or triple bond of the dienophile is adjacent to the positively polarized carbon of an electron-withdrawing substituent The double-bond carbons in these substances are substantially less electron-rich than the carbons in ethylene Diels-Alder reaction is stereospecific Reactant stereochemistry is also maintained Diene must adopt an s-cis conformation, meaning cis-like about the single bond 6

7 Some dienes cannot adopt the s-cis conformation and cannot undergo Diels-Alder cycloaddition reactions Some dienes are fixed in the s-cis conformation and are highly reactive in Diels-Alder cycloaddition reactions Worked Example 8.6 Predicting the Product of a Diels-Alder Reaction Predict the product of the following Diels-Alder reaction 7

8 8.15 Alkyne Addition Reactions Alkynes behave similarly to alkenes Alkynes are less reactive than alkenes Various reactions can often be stopped at the monoaddition stage if one molar equivalent of reagent is used 8

9 Alkyne acidity Terminal alkynes (RC CH) are relatively acidic RC CH treated with a strong base NaNH 2 Terminal hydrogen is removed forming and acetylide anion Alkyne acidity BrØnsted-Lowry Acid A substance that donates H + Acidity order: Established by measuring acid dissociation constants and expressing the results as pka values Low pk a = strong acid High pk a = weak acid Amide ion (NH 2 - ), the conjugated base of ammonia (pka = 35), is often used to deprotonate terminal alkynes 9

10 Terminal alkynes more acidic than alkenes or alkanes Acetylide ions are more stable than vinylic (alkenyl) or alkyl ions Difference in acidities due to hybridization of negatively charged carbon atom Acetylide anion has sp-hybridized carbon Presence of negative charge and an unshared electron pair on carbon makes acetylide anions strongly nucleophilic Nucleophilic substitutions not limited to acetylene 10

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