Chemistry Assignment Help With Substitution Reactions At Allylic Position

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5. Substitution Reactions At Allylic Position

Substitution reactions at the allylic position refer to chemical reactions that take place at the carbon atom adjacent to a carbon-carbon double bond in an organic molecule, which is commonly referred to as the allylic carbon. Allylic substitution reactions are important in organic chemistry and can occur through various mechanisms, including radical, nucleophilic, and electrophilic substitutions. Here, I'll provide an overview of some common types of substitution reactions at the allylic position:

1. Electrophilic Allylic Substitution: In this type of reaction, an electrophile (a species that is electron-deficient) reacts with the electron-rich allylic carbon. This can lead to the formation of a new bond at the allylic position. For example, the addition of a halogen such as bromine to an alkene with an allylic carbon can result in the formation of a halogen-substituted allylic compound.

   Example:

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   CH2=CH-CH2-Br + Br2 → CH2=CH-CH2-Br2

2. Nucleophilic Allylic Substitution: In this type of reaction, a nucleophile (a species that donates electrons) attacks the allylic carbon, displacing another group. One common reaction is the nucleophilic substitution of a leaving group with a nucleophile.

   Example:

   makefile

   CH2=CH-CH2-Cl + NaOH → CH2=CH-CH2-OH + NaCl

3. Radical Allylic Substitution: In radical reactions, a radical species (a molecule with an unpaired electron) reacts with the allylic carbon to replace another atom or group. This is often seen in radical halogenation reactions, where a halogen radical (e.g., Cl•) replaces a hydrogen atom at the allylic position.

   Example:

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   CH2=CH-CH2-H + Cl• → CH2=CH-CH2-Cl + H•

4. Conjugate Addition: In conjugate addition reactions, nucleophiles attack the allylic position to form a new bond. This is often seen in the Michael addition, where a nucleophile adds to the β-carbon (allylic carbon) of an α,β-unsaturated carbonyl compound.

   Example:

   mathematica

   CH2=CH-CH2-C(O)CH3+NaOH → CH2=CH-CH2-C(O)CH2OH

5. Metal-Catalyzed Allylic Substitution: Transition metal catalysts can facilitate various allylic substitution reactions, including allylic halogenation, allylic amination, and allylic alkylation. These reactions often proceed with high regioselectivity and can be used to synthesize complex molecules.

Allylic substitution reactions are important in the synthesis of various organic compounds, including pharmaceuticals and agrochemicals. The choice of reaction conditions, reagents, and catalysts can greatly influence the outcome and selectivity of these reactions.

Substitution Reactions At Allylic Position

Cl2 + H2C = CHCH3 H2C = CHCH2Cl + HCI

Br2 + HH2C = CH – CH3 CH2 = CH – CH2Br + HBr

These halogenations are like free radical substitution of alkanes. The order of reactivity of H-abstraction is allyl > 3° > 2° > 1° vinyl.

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