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Chapter 16.1 – Conjugated Pi System
Chapter 16 - Conjugated Pi Systems and Pericyclic
Reactions
Class of Dienes Conjugated diene are more stable than cumulated isolated
diene
Dienes are compounds containing two double bonds. There are three types of dienes
Cumulated diene – the π bonds are adjacent (aka allene)
Conjugated diene – the two π bonds are separated by one σ bond
Isolated diene - the two π bonds are separated by more than one s bonds
The locations of π bonds makes them have different structure and reactivity, but do not memorize the
definition of these types of diene, rather recognize their structures. We will focus more on the conjugated
diene. Note that allene is not stable but it is reactive and is popular in organic synthesis. allenes have
shown distinct reactivity owing to the presence of orthogonal cumulative π bonds.
A C=C π bond can also be conjugated with other types of π bond, such as C=O bond
Concept-test Find all conjugated dienes
Me waiting for class to start.
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Chapter 16.1 – Conjugated Pi System
Preparations of Conjugated Dienes
We need to make two double bond next to each other (separated by 1
single bond). Here are starting materials that work
Conjugated dienes can be synthesized from allylic halide via an elimination
reaction with a strong base. t-BuOK is used to prevent a competing SN2
reaction. Conjugated dienes can also be formed from dihalides via two
successive eliminations. Note that t-BuOK is not strong enough to produce
an alkyne. The second elimination requires the use of a much stronger
base NaNH2 (1020 times stronger base).
Allenes are so reactive that cannot be made from an elimination reaction. Therefore, the use of t-BuOK
produces only conjugated dienes.
Practice Suggest reagents to complete the following synthesis.
Me waiting for class to start.
Chapter 16.1 – Conjugated Pi System
Chapter 16 - Conjugated Pi Systems and Pericyclic
Reactions
Class of Dienes Conjugated diene are more stable than cumulated isolated
diene
Dienes are compounds containing two double bonds. There are three types of dienes
Cumulated diene – the π bonds are adjacent (aka allene)
Conjugated diene – the two π bonds are separated by one σ bond
Isolated diene - the two π bonds are separated by more than one s bonds
The locations of π bonds makes them have different structure and reactivity, but do not memorize the
definition of these types of diene, rather recognize their structures. We will focus more on the conjugated
diene. Note that allene is not stable but it is reactive and is popular in organic synthesis. allenes have
shown distinct reactivity owing to the presence of orthogonal cumulative π bonds.
A C=C π bond can also be conjugated with other types of π bond, such as C=O bond
Concept-test Find all conjugated dienes
Me waiting for class to start.
, 2
Chapter 16.1 – Conjugated Pi System
Preparations of Conjugated Dienes
We need to make two double bond next to each other (separated by 1
single bond). Here are starting materials that work
Conjugated dienes can be synthesized from allylic halide via an elimination
reaction with a strong base. t-BuOK is used to prevent a competing SN2
reaction. Conjugated dienes can also be formed from dihalides via two
successive eliminations. Note that t-BuOK is not strong enough to produce
an alkyne. The second elimination requires the use of a much stronger
base NaNH2 (1020 times stronger base).
Allenes are so reactive that cannot be made from an elimination reaction. Therefore, the use of t-BuOK
produces only conjugated dienes.
Practice Suggest reagents to complete the following synthesis.
Me waiting for class to start.
