Organic reactions are the building blocks of chemical transformations. They come in four main types: addition, elimination, substitution, and rearrangement. Each type has unique characteristics and plays a crucial role in synthesizing new compounds.
Understanding these reactions helps predict products and design synthetic routes. By mastering the mechanisms and kinetics behind them, chemists can control reactions and create complex molecules. This knowledge is essential for developing new drugs, materials, and industrial processes.
Types of Organic Reactions
Differentiate between addition, elimination, substitution, and rearrangement reactions in organic chemistry.
- Addition reactions
- Atoms or groups of atoms are added to a molecule
- Occur across a double or triple bond
- Form a single product with no leaving group
- Examples: hydrogenation, hydration, hydrohalogenation
- Elimination reactions
- Atoms or groups of atoms are removed from a molecule
- Molecule loses a small molecule (water, hydrogen halide) to form a double or triple bond
- Form a single product with a double or triple bond
- Examples: dehydration, dehydrohalogenation
- Substitution reactions
- An atom or group of atoms is replaced by another atom or group
- A leaving group is replaced by a nucleophile
- Form a single product with a new bond and a leaving group
- Examples: nucleophilic substitution (SN1, SN2), electrophilic aromatic substitution
- Rearrangement reactions
- Atoms or groups are redistributed within a molecule
- Molecule undergoes a structural change without gaining or losing atoms
- Form a single product with a different connectivity of atoms
- Examples: hydride shifts, alkyl shifts, ring expansions/contractions
Examples in processes and pathways
- Addition reactions
- Hydrogenation of unsaturated fats produces saturated fats
- Hydration of alkenes forms alcohols (ethylene to ethanol)
- Addition of hydrogen cyanide to aldehydes or ketones forms cyanohydrins
- Elimination reactions
- Dehydration of alcohols forms alkenes (ethanol to ethylene)
- Dehydrohalogenation of alkyl halides forms alkenes (2-bromobutane to 2-butene)
- Decarboxylation of β-keto acids forms ketones (acetoacetic acid to acetone)
- Substitution reactions
- Nucleophilic substitution synthesizes ethers (Williamson ether synthesis)
- Electrophilic aromatic substitution synthesizes substituted benzenes (nitration, sulfonation, halogenation)
- Transamination reactions biosynthesize amino acids
- Rearrangement reactions
- Claisen rearrangement synthesizes allyl phenyl ethers
- Beckmann rearrangement synthesizes amides from oximes
- Pinacol rearrangement converts 1,2-diols to aldehydes or ketones
Predicting organic reaction products
- Addition reactions
- Alkene + $H_2$ (with catalyst) → Alkane
- Alkene + $H_2O$ (with acid catalyst) → Alcohol
- Alkene + $HX$ (X = halogen) → Alkyl halide
- Elimination reactions
- Alcohol + heat (with acid catalyst) → Alkene + $H_2O$
- Alkyl halide + strong base → Alkene + $HX$
- β-Keto acid + heat → Ketone + $CO_2$
- Substitution reactions
- Alkyl halide + nucleophile ($OH^-$, $CN^-$, $NH_3$) → Substituted product + leaving group
- Benzene + electrophile ($NO_2^+$, $SO_3$, $Br_2$) → Substituted benzene + proton
- Amino acid + α-keto acid → New amino acid + New α-keto acid
- Rearrangement reactions
- Allyl phenyl ether + heat → o-Allylphenol
- Oxime + acid catalyst → Amide
- 1,2-Diol + acid catalyst → Aldehyde or ketone
Understanding Reaction Mechanisms and Kinetics
- Reaction mechanisms
- Step-by-step description of how bonds are broken and formed during a reaction
- Involve the formation and breakdown of reactive intermediates
- Reaction kinetics
- Study of reaction rates and factors affecting them
- Involves analysis of transition states and energy barriers
- Stereochemistry
- Consideration of the three-dimensional arrangement of atoms in molecules and how it affects reactions