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Understanding Organic Synthesis Reactions
May 1, 2025
Synthesis Reactions in Organic Chemistry
Overview of Synthesis Reactions
Focus on synthesis reactions involving alkenes, Grignard reagents, and various examples.
Reaction Mechanisms
Acetylene to Acetal Ion
Reagents
: Sodium amide (NaNH2)
Mechanism
:
NH2- ion as a strong base removes hydrogen from acetylene (alkyne).
Produces acetal ion (nucleophilic carbon with a negative charge).
Formation of Carbon-Carbon Bonds
Mechanism
:
Nucleophilic acetal ion attacks electrophilic carbon in alkyl halide.
Formation via SN2 reaction.
Synthesis Examples
Example 1
Objective
: Convert alkyne to alkene with additional carbon chains.
Steps
:
Use sodium amide to generate acetal ion.
Add alkyl halide (e.g., 1-bromobutane) for chain extension.
Reduce alkyne to alkene using sodium in liquid ammonia.
Example 2
Objective
: Add different R groups to an alkyne.
Steps
:
Remove hydrogen using sodium amide.
Add alkyl bromides sequentially for different R groups.
Convert alkyne to cis-alkene using Lindlar’s catalyst (hydrogen gas with poisoned palladium).
Grignard Reagents
Mechanism
Reagent
: Methyl magnesium bromide.
Reaction
: Nucleophilic carbon from Grignard reagent attacks electrophilic carbon in carbonyl compounds.
Product
: Formation of alcohols (e.g., tertiary alcohol from ketone).
Synthesis Examples
Objective
: Add carbon atoms to a ketone or aldehyde.
Reagents
:
Grignard reagents (e.g., phenyl magnesium bromide) and acid (H2O+).
Special Cases
Acid Chlorides & Esters
:
Grignard reagents add two R groups.
Use Gilman reagents (organocopper compounds) to stop at ketone stage.
Reaction Mechanisms for Specific Conversions
Conversion of Acid Chloride to Ketone
Reagent
: Gilman reagent (e.g., R2CuLi).
Mechanism
: Attacks carbonyl carbon, stops at ketone.
Conversion of Esters
Reagent
: DIBAL (diisobutylaluminium hydride).
Mechanism
: Reduces ester to aldehyde.
Reagent Preparation
Grignard Reagents
Preparation
: Add magnesium to alkyl halide.
Mechanism
: Magnesium inserts into carbon-halogen bond.
Organolithium Reagents
Preparation
: Use lithium with alkyl halide.
Mechanism
: Lithium replaces halogen, forms R-Li.
Advanced Synthesis Techniques
Coupling Reactions
Gilman Reagents
: Used for combining different alkyl groups.
Witting Reaction
Objective
: Convert ketone to alkene.
Reagent
: Phosphonium ylides.
Summary
Synthesis reactions involve strategic use of reagents to transform functional groups.
Techniques involve nucleophilic substitutions, reductions, and carbon-carbon bond formations.
Understanding mechanisms is critical for predicting reaction outcomes and planning synthetic routes.
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