ALLYLIC SYSTEMS THEORY

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ALLYLIC SYSTEMS THEORY
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Positive, negative or radical charges in positions close to double bonds are delocalized by resonance, being especially stable.

 
conjugation 01
Allylic carbocation
 
The carbocation formed in neighboring positions of double bonds is known as an allylic carbocation. It is stabilized by delocalizing the charge between two carbons, and is therefore more stable than a normal secondary carbocation.

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ALLYLIC SYSTEMS THEORY
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The group CH2=CHCH2- is called allyl. Some compounds that derive from the allyl group are:

 
allyl system 01
 
[1] Allyl bromide
[2] Allyl alcohol
[3] Allyl chloride

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ALLYLIC SYSTEMS THEORY
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Carbocations formed on carbons located in allylic positions are called allylic cations.

cationes-alilo

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ALLYLIC SYSTEMS THEORY
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Allylic systems can act as substrates in nucleophilic substitution reactions. Thus, the reaction of 3-chloro-1-butene with water produces two allylic alcohols.

 

sn1-sistemas-alilicos
 
Since water is a bad nucleophile, the substitution mechanism will be unimolecular (S N 1), with the particularity of finding the leaving group (chloro) on an allylic position.

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ALLYLIC SYSTEMS THEORY
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The formation of the allylic carbocation, stabilized by resonance, allows the reaction to evolve in two ways that lead to the kinetic and thermodynamic products.

kinetic and thermodynamic control 01

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ALLYLIC SYSTEMS THEORY
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SN2 reactions with leaving groups in allylic positions proceed more rapidly than those for the corresponding saturated haloalkanes.

sn2 in allylic systems 01

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ALLYLIC SYSTEMS THEORY
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With tertiary allylic substrates and under SN2 conditions (good nucleophile and aprotic solvent), a concerted reaction is produced by the nucleophile attacking the carbon of the double bond with loss of the leaving group.

sn2 cousin 01

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ALLYLIC SYSTEMS THEORY
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Strong and hindered bases mostly give E2 with allylic systems, deprotonating the carbon located in position 4 with respect to the leaving group.

elimination 14 allylics

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ALLYLIC SYSTEMS THEORY
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Halogens can add to alkenes giving vicinal dihaloalkanes, but when this reaction is carried out at low halogen concentrations, radical mechanisms are favored. A widely used reagent in allylic brominations is NBS (N-bromosuccinimide).

radicals in allylic systems 01

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ALLYLIC SYSTEMS THEORY
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The conjugated dienes add the acids of the halogens forming kinetic and thermodynamic products, whose ratio can be controlled with the reaction conditions (temperature and time).

hx addition to dienos 01

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ALLYLIC SYSTEMS THEORY
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Unlike the addition of bromine to alkenes, conjugated dienes do not form halonium ions due to the high stability of the carbocation formed.

bromine addition conjugated dienes 01

 

[1] Product of addition 1,2

[2] Addition product 1.4