University:
Rutgers University Newark
Course:
CHEM 336 | Organic Chemistry ||
Academic year:

2024
Views:

282

Pages:

79
Author:

Mahitab S.

CHAPTER 20 DIENES, CONJUGATED SYSTEMS, AND PERICYCLIC REACTIONS ©2018 Cengage Learning. All Rights Reserved. Chapter Outline • • • • • • (20.1) Stability of conjugated dienes (20.2) Electrophilic addition to conjugated dienes (20.3) UV-visible spectroscopy (not required) (20.4) Pericyclic reaction theory (20.5) The Diels-Alder reaction (20.6) Sigmatropic shifts (Claisen and Cope rearrangements) ©2018 Cengage Learning. All Rights Reserved. Dienes • Compounds that contain two carbon-carbon double bonds • Types • Unconjugated dienes: Double bonds are separated by two or more single bonds • Conjugated dienes: Double bonds are separated by one single bond • Cumulated dienes: Two double bonds share an sp hybridized carbon • 2p orbitals of the two double bonds do not overlap and are not conjugated because of the geometry of this carbon ©2018 Cengage Learning. All Rights Reserved. Example 20.1 - Conjugation • Which of these molecules contain conjugated double bonds? Unconjugatefne • Solution Conjugated Diene conjugated Diene mmmm Diene • Compounds (b) and (c) contain conjugated double bonds • The double bonds in compounds (a) and (d) are unconjugated ©2018 Cengage Learning. All Rights Reserved. Stability of Conjugated Dienes heatsofHydrogenationisused todeterminethe Stabilityof conjugateddieneandunconjugatediene • Relative stabilities of conjugated and unconjugated dienes can be compared using the heats of hydrogenation • The most stable olefins have the smallest heat of hydrogenation (low energy level) Thet smallestheat lowenergy b stableolefins Most stable more lessstable ©2018 Cengage Learning. All Rights Reserved. Conjugated Dienes – Stability of 1,3-Butadiene • Predicted heat of hydrogenation of 1,3-butadiene is −254 kJ (−30.3 kcal)/mol • Observed heat of hydrogenation of 1,3-butadiene is −237 kJ (−56.5 kcal)/mol • Conjugation of the double bonds in 1,3-butadiene gives an extra stability to the molecule of approximately 17 kJ (4.1 kcal)/ mol ©2018 Cengage Learning. All Rights Reserved. Figure 20.1 - Conjugation of Double Bonds in Butadiene Gives Additional Stability of Approximately 17 kJ/mol ©2018 Cengage Learning. All Rights Reserved. Figure 20.1 - Structure of 1,3-Butadiene: Molecular Orbital Model •Additional stability of conjugated dienes vs. unconjugated dienes arises from delocalization of electron density •In conjugated diene, 4 π electrons are delocalized over 4 parallel 2p orbitals •In unconjugated diene, each pair of π electrons is localized between 2 carbons 4 delocalizedover parallel tielectrons 4 2porbitals ©2018 Cengage Learning. All Rights Reserved. Stability of Conjugated Systems • Compounds containing conjugated double bonds (any), not just those in dienes, are more stable than isomeric compounds containing unconjugated double bonds conjugateddiene b moststable • Additional stability of dienes relative to unconjugated dienes arises from the delocalization of electron density in the conjugated diene ©2018 Cengage Learning. All Rights Reserved. Example 20.2 - Stability of Conjugated Systems • Using data from Table 20.1, estimate the extra stability that results from the conjugation of double bonds in trans-1,3-pentadiene (CH2=CH-CH=CH-CH3) • Solution (next slide) ©2018 Cengage Learning. All Rights Reserved. Example 20.2 - Stability of Conjugated Systems • Compare the sum of heats of hydrogenation of 1-pentene and trans-2butene with the heat of hydrogenation of trans-1,3-pentadiene (CH2=CHCH2-CH2-CH3, CH3-CH=CH=CH3) • Conjugation of double bonds in trans-1,3-pentadiene imparts an added stability of approximately 15 kJ (3.6 kcal)/mol NII ©2018 Cengage Learning. All Rights Reserved. Example • Which of the following is not conjugated? 1. 2. conjugateddiene unconjugated Diene 3. conjugated Diene 4. 5. Conjugated conjugated Diene ©2018 Cengage Learning. All Rights Reserved. Diene Example • How many π (pi) bonding molecular orbitals are present in 1,3,5-hexatriene ? (CH2=CH-CH=CH-CH=CH2) 1. 2. 3. 4. 5. 2 3 4 5 6 ©2018 Cengage Learning. All Rights Reserved. Example • Arrange the following alkenes in order of least stable to most stable Unconjugated 1. 2. 3. 4. 5. conjugated A