CHM235IN – Overview of Organic Reactions & Mechanisms

Vocabulary flashcards covering definitions and key terms related to organic reaction mechanisms, electrophiles, nucleophiles, reaction types, intermediates, and specific alkene addition processes.

Organic Reaction Mechanism

A step-by-step description of how reactants convert to products, showing bond-making and bond-breaking events.

Curved Arrow Formalism

Notation that tracks electron pair movement during reactions; arrows start at an electron source and point to an electron sink.

Addition Reaction

Two reactants combine to give one product; a π bond is converted to two new σ bonds.

Elimination Reaction

Two σ bonds are removed from a molecule to form a π bond, producing multiple products.

Substitution Reaction

One σ bond in a molecule is broken and replaced by a new σ bond.

Homolytic Bond Cleavage

Symmetrical bond breaking in which each atom retains one electron, producing radicals.

Heterolytic Bond Cleavage

Unsymmetrical bond breaking in which both electrons move to one atom, forming ions.

Electrophile

An electron-poor (Lewis acid) species that seeks electron density.

Nucleophile

An electron-rich (Lewis base) species that donates an electron pair to an electrophile.

Radical Reaction

A reaction that proceeds through species with unpaired electrons and fish-hook arrows.

Polar Reaction

A reaction involving heterolytic steps where electron-rich and electron-poor sites interact.

Polarizability

The tendency of a molecule’s electron cloud to distort in response to its environment.

Transition State

Highest-energy structure along a reaction path; exists at an energy maximum on the reaction coordinate.

Activation Energy (ΔG‡)

Energy required to reach the transition state from reactants.

Concerted Step

Multiple bond-making and bond-breaking events that occur in a single kinetic step.

Reactive Intermediate

A short-lived, high-energy species (e.g., carbocation, radical, carbanion) formed during a mechanism.

Carbocation

Positively charged carbon center (C⁺) that is electrophilic and electron-deficient.

Carbocation Stability Trend

Tertiary > secondary > primary > methyl due to hyperconjugation and inductive effects.

Hyperconjugation

Stabilization of an electron-deficient center by donation of electron density from adjacent σ bonds.

Allylic Carbocation

A carbocation adjacent to a π bond; stabilized by resonance delocalization.

Free Radical

Neutral species with an unpaired electron; stability parallels carbocations (3° > 2° > 1° > methyl).

Allylic Radical

Radical next to a π bond; stabilized by resonance of the odd electron.

Carbanion

Negatively charged carbon center (C:⁻); nucleophilic and strongly basic.

Carbanion Stability Trend

Methyl > primary > secondary > tertiary (opposite of carbocations) because alkyl groups destabilize negative charge.

Markovnikov Addition

In asymmetric additions, the proton bonds to the double-bond carbon bearing more hydrogens; explained by carbocation stability.

Hydrogenation

Addition of H₂ across C=C or C≡C bonds, converting unsaturated hydrocarbons to alkanes; catalyzed by metals (e.g., Pd/C).

Hydrohalogenation

Addition of HX (X = Cl, Br, I) to alkenes/alkynes to form haloalkanes.

Halogenation

Addition of X₂ (Cl₂, Br₂, I₂) across a π bond, yielding dihalo products; basis of the Br₂ unsaturation test.

Br₂ Test for Unsaturation

Loss of red Br₂ color indicates addition to an alkene/alkyne, confirming presence of a double or triple bond.

Hydration

Acid-catalyzed addition of H–OH to a C=C bond to form alcohols, following Markovnikov orientation.

Enol–Keto Tautomerization

Alkyne hydration initially gives an enol that rearranges to its more stable ketone form.

Electrophile–Nucleophile Arrow Rule

Curved arrows always originate at the nucleophile and point toward the electrophilic center.

Homolytic Arrow (Fish-Hook)

Single-barbed arrow indicating movement of one electron; used in radical mechanisms.

Heterolytic Arrow (Full Head)

Double-barbed arrow showing movement of an electron pair in polar reactions.

Addition to Alkenes (Summary)

Common reactions include hydrogenation, hydrohalogenation, hydration, dihalogenation, halohydrin formation, and others.

Hydrogenation of Oils

Industrial reduction of unsaturated vegetable oils to increase melting point and produce margarine/shortening.