Chapter 10 — Organohalides: Notes, Flashcards, and Practice Test Flashcards

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Organohalide

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An organic molecule that contains one or more halogen atoms (F, Cl, Br, I) bonded to carbon. Organohalides are often lipophilic and can be reactive due to halogen leaving-group ability.

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Leaving group

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An atom or group that departs with an electron pair in a substitution or elimination, stabilizing the transition state. Halides (Cl-, Br-, I-) are common good leaving groups in organic reactions.

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Radical initiation

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The step that generates radical species, typically by homolytic bond cleavage induced by light ($h\nu$) or heat ($\Delta$). Initiators such as peroxides or UV promote the first radical formation.

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Propagation

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A sequence of reactions where radicals react to form stable products and new radicals, allowing the chain to continue. Propagation steps determine product formation and selectivity in radical halogenation.

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Termination

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The process where two radicals combine or are trapped, stopping the radical chain. Radical inhibitors and scavengers accelerate termination or prevent initiation.

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Hyperconjugation

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Stabilization of radicals or carbocations by donation of electron density from adjacent C–H sigma bonds into the empty or half-filled orbital. More alkyl substitution increases hyperconjugative stabilization.

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Allylic radical

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A radical located on a carbon adjacent to a carbon–carbon double bond and stabilized by resonance. Allylic radicals often lead to regioisomeric product mixtures unless controlled.

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Vinylic radical

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A radical centered on a carbon that is part of a carbon–carbon double bond. Vinylic radicals are typically less stable and less reactive in hydrogen abstraction.

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NBS

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N-Bromosuccinimide, a reagent that slowly generates $Br\cdot$ and maintains low $Br_2$ concentration. NBS is used to favor allylic bromination while minimizing ionic addition across double bonds.

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Grignard reagent

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An organomagnesium halide with formula $R\!–\!MgX$ that behaves like a carbanion (strong base/nucleophile). Grignards must be prepared and used under dry, aprotic conditions and are quenched by protic functional groups.

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Gilman reagent

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An organocuprate reagent (e.g., $R_2CuLi$) used for coupling and substitution reactions with alkyl halides. Gilman reagents are useful for forming C–C bonds with controlled reactivity.

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Suzuki coupling

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A palladium-catalyzed cross-coupling between an organoboron reagent ($R\!–\!B(OH)_2$) and an organohalide to form a new C–C bond. Suzuki reactions are tolerant of many functional groups and use catalytic Pd.

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Protecting group

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A functional group temporarily installed to mask a reactive site during a multi-step synthesis. Silyl ethers (TMS, TBS) are common protecting groups for alcohols in Grignard chemistry.

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Bond dissociation energy

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The energy required to homolytically cleave a bond into radicals. BDEs determine the thermodynamics of radical H-abstraction and influence selectivity of halogenation.

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Selectivity (bromination)

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Bromination favors the most stable radical because its hydrogen abstraction step is endothermic and has a larger difference in transition state energies. This leads to higher regioselectivity compared to chlorination.

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Autooxidation

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A radical chain reaction between organic molecules and molecular oxygen to form peroxides/hydroperoxides, often occurring under mild conditions. Autooxidation causes rancidity and can be hazardous via peroxide accumulation.

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Radical inhibitor

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A compound that prevents radical chain propagation by reacting with radical intermediates to form non-propagating species. Examples include BHT (butylated hydroxytoluene) and certain oxygen species.

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Oxidation state (carbon)

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A formal bookkeeping number approximated by counting bonds: each bond to a more electronegative element (e.g., halogen) increases carbon OS by +1, each bond to H decreases OS by -1. Tracking OS changes shows oxidation/reduction processes.

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Polyhalogenation

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Successive halogenation that occurs when the initially halogenated product is more reactive toward further halogenation. Using limiting halogen equivalents and controlling radical concentration helps avoid polyhalogenation.

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Initiator

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A substance that decomposes to form radicals and start a radical chain (e.g., peroxides, AIBN, or UV light). Appropriate initiators allow radical reactions to proceed under controlled temperatures.