الشريحة 1 - Islamic University of Gaza

الشريحة 1 - Islamic University of Gaza

Organic chemistry B Chapter 11 Conjugation and Resonance By Prof. Dr. Adel M. Awadallah

Islamic University of Gaza Effect of double bond on reactions (Double bond as a substituent)

A) Radical and nucleophilic substitution H2C X A

B) Conjugated diens B Free Radical Halogenation of Alkenes

Reactions at the double bond Substitution at the Alkane

Free Radical addition does not occur at ?high temperature Why Because the allyl radical formed leading to substitution is more stable than the secondary one leading to addition

Free Radical substitution in alkenes orientation and reactivity Bromination of Alkenes at

the allylic position by NBromosuccinimide Br NBS, h CCl4

Br NBS, h CCl4

(CH3)3CO - K+ O N Br O

N-bromosuccinimide (NBS) Allylic Rearrangement

Orbital picture of the allyl Radical Hyperconjugation and stability of Radicals Hyperconjugation: Delocalization of electrons involving sigma bond orbitals over three atoms (two carbons and

one hydrogen). Three possible hyperconjugation structures for the ethyl radical, six for isopropyl and nine for tertiary butyl The allyl cation as a resonance hybrid

SN1 Substitution in allylic substrates The allyl carbocation is another example of a conjugated .system

.Conjugation stabilizes the allyl carbocation Example: SN1 substitution using a weak nucleophile (ethanol)

Example: SN2 substitution using a strong nucleophile (EtO-) Nucleophilic substitution in vinylic substrates is

very difficult To solve the problem, you need a) very good leaving group b) Electron release in the carbocation being formed

Example Resonance in Conjugated Dienes Having three or more p orbitals on adjacent atoms allows p orbitals

.to overlap and electrons to delocalize . Pentadiene is an isolated diene-1,4 .The bonds in 1,4-pentadiene are too far apart to be conjugated

Conjugation occurs whenever p orbitals can overlap on three or more .adjacent atoms .The four p orbitals on adjacent atoms make a 1,3-diene a conjugated system

Stability of substituted alkenes due to hyperconjugation The greater the number of alkyl groups attached to the doubly bonded carbon atoms, the more stable the alkene

Electrophilic addition to conjugated diene Mechanism

+ + H+

H H Br-

Br- Br

+ H Br H

Example: suggest a mechanism vs. 1,4-addition. Rate vs. equilibrium- 1,2 The 1,2-product is the kinetic product (formed

faster) The 1,4- product is the thermodynamic product (more stable) Volcanization (Formation of sulfur

bridges between different chaines)

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