BackOrganic Chemistry: Substitution, Oxidation, Alkyl Halide Reactivity, and Acid-Base Strength
Study Guide - Smart Notes
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Substitution Reactions
Introduction to Substitution
Substitution reactions are fundamental in organic chemistry, involving the replacement of an atom or group in a molecule with another atom or group. These reactions are common with aromatic and alkyl halide compounds.
Definition: A substitution reaction replaces a hydrogen or another group with a new group.
Example: In aromatic substitution, a group replaces a hydrogen atom on a benzene ring.
Key Point: The new group attaches where the hydrogen was originally located.
Oxidation of Alcohols
Oxidizing Agents
Alcohols can be oxidized to aldehydes, ketones, or carboxylic acids depending on the oxidizing agent used.
PCC (Pyridinium chlorochromate): A mild oxidizing agent, typically converts primary alcohols to aldehydes and secondary alcohols to ketones.
H2CrO4 (Chromic acid): A strong oxidizing agent, can further oxidize aldehydes to carboxylic acids.
Reactivity of Alkyl Halides
Factors Affecting Reactivity
Alkyl halides react with nucleophiles via SN1 or SN2 mechanisms. The rate depends on the structure and the reaction conditions.
SN2 Mechanism: Favored by strong nucleophiles and less hindered (less crowded) substrates.
SN1 Mechanism: Favored by strong electrophiles and more substituted (more stable carbocation) substrates.
Reactivity Order: Lower energy transition states react faster; more crowded (sterically hindered) alkyl halides react slower in SN2 reactions.
Example Table: Reactivity of Alkyl Halides (SN2)
Alkyl Halide Structure | Relative Reactivity (SN2) |
|---|---|
Methyl | Fastest |
Primary | Faster |
Secondary | Moderate |
Tertiary | Slowest |
Additional info: SN1 reactions are favored by tertiary alkyl halides due to carbocation stability.
Nucleophilicity and Steric Effects
Strong Nucleophiles
Strong nucleophiles prefer to attack less crowded (less sterically hindered) positions.
In SN2 reactions, nucleophiles react faster with primary or methyl halides than with secondary or tertiary halides.
Acid-Base Strength and Deprotonation
Deprotonation Reactions
Deprotonation involves removing a proton (H+) from a molecule, typically using a base. The strength of acids and bases determines the direction and extent of these reactions.
Stronger acids have weaker conjugate bases.
Deprotonation is favored when a strong base removes a proton from a weaker acid, forming a weaker base as the conjugate.
Example Reaction:
Equation:
Here, sodium amide (NaNH2) deprotonates a terminal alkyne, forming an acetylide ion.
Key Point: The reaction favors the formation of the weaker base (ammonia, NH3).
Additional info: The relative acidities of alkynes, alkenes, and alkanes are important for predicting deprotonation outcomes. Terminal alkynes are more acidic than alkenes and alkanes.
