Organic Chemistry: Reactions of Alkenes, Alkynes, Radicals, Haloalkanes, and Alcohols

Comprehensive practice flashcards defining key reaction types, mechanism pathways, reagents, regiochemical outcomes, and stereochemical requirements across alkene, alkyne, radical, substitution/elimination, alcohol, and epoxide chemistry.

Markovnikov's Rule (HX Addition to Alkenes)

A regiochemical rule in the electrophilic addition of $HX$ to an alkene where the proton adds to the less substituted carbon to generate the more stable carbocation intermediate, directing the halide ($X$) to bond to the more substituted carbon with mixed stereochemistry.

Acid-Catalyzed Hydration of Alkenes

An addition reaction using catalytic $H_2SO_4$ and $H_2O$ that proceeds through a carbocation intermediate (susceptible to rearrangement) to add an $OH$ group to the more substituted carbon and an $H$ atom to the less substituted carbon with mixed stereochemistry.

Halonium Ion Intermediate

A strained, three-membered cyclic intermediate formed during the halogenation of alkenes with $X_2$; its ring opening occurs exclusively via backside attack by a halide ion to produce vicinal dihaloalkanes with anti stereochemistry.

Halohydrin Formation

The reaction of an alkene with $X_2$ in $H_2O$ proceeding through a three-membered halonium ion; water regioselectively attacks the more substituted carbon possessing greater partial positive charge, yielding a product with $OH$ on the more substituted carbon and $X$ on the less substituted carbon with anti stereochemistry.

Oxymercuration-Reduction of Alkenes

A two-step synthetic sequence using $1.\,Hg(OAc)_2 / H_2O$ followed by $2.\,NaBH_4$ to convert alkenes to alcohols with Markovnikov regiochemistry and mixed stereochemistry, notably preventing carbocation rearrangements.

Hydroboration-Oxidation of Alkenes

A two-step reaction using $1.\,BH_3$ followed by $2.\,H_2O_2 / HO^-$ that proceeds through a concerted four-membered ring transition state where $H$ and $B$ add to the same face, yielding an alcohol with non-Markovnikov regiochemistry and syn stereochemistry.

Vicinal Dihydroxylation (Ozzy Osbourne Reaction)

The oxidation of an alkene using $1.\,OsO_4$ followed by $2.\,NaHSO_3 / H_2O$, which passes through a cyclic osmate ester intermediate to deliver two $OH$ groups to the exact same face of the double bond (syn stereochemistry).

Ozonolysis of Alkenes

The oxidative cleavage of an alkene using $1.\,O_3$ followed by $2.\,(CH_3)_2S$, forming a molozonide that rearranges into an ozonide intermediate to cleave the $C=C$ double bond completely into two carbonyl groups ($C=O$), producing aldehydes and/or ketones.

Catalytic Hydrogenation of Alkenes

The reduction of an alkene to an alkane using $H_2$ over a metal catalyst ($Pt^0$, $Pd^0$, or $Ni^0$), where the alkene and hydrogen adsorb onto the metal surface and add both hydrogen atoms simultaneously to the same face with syn stereochemistry.

Synthesis of Alkynes via Double $E2$ Elimination

The dehydrohalogenation of vicinal dihalides using strong base ($NaNH_2$) to install two pi bonds; when synthesizing terminal alkynes, $3$ equivalents of $NaNH_2$ are required due to terminal proton deprotonation, followed by workup with mild $H_3O^+$ to reprotonate the alkyne.

Alkylation of Terminal Alkynes

A two-step reaction where a terminal alkyne is deprotonated by $NaNH_2$ to create an acetylide nucleophile, which then displaces a primary haloalkane via an $S_N2$ mechanism to forge a new $C-C$ bond without competing $E2$ elimination.

Lindlar's Catalyst Hydrogenation

A selective reduction of alkynes using $H_2$ over a palladium catalyst poisoned with $Pb$ and quinoline, which stops after one addition to exclusively yield a $Z$ (cis) alkene via syn stereochemistry.

Dissolving Metal Reduction of Alkynes

The reduction of an alkyne using sodium metal in liquid ammonia ($Na^0 / NH_3$) via a radical pathway involving two single-electron transfers and two proton transfers, yielding predominantly the more stable $E$ (trans) alkene with anti stereochemistry.

Hydroboration-Oxidation of Terminal Alkynes

A reaction sequence using a hindered dialkylborane "antler" reagent ($(sia)_2BH$) followed by $H_2O_2 / HO^-$ to achieve non-Markovnikov addition across a terminal alkyne, yielding an enol intermediate that undergoes keto-enol tautomerization to form an aldehyde.

Oxymercuration-Hydration of Alkynes

The Markovnikov addition of water across an alkyne catalyzed by $HgSO_4$ and $H_2SO_4 / H_2O$, placing the oxygen on the more substituted internal carbon to generate an enol intermediate that tautomerizes into a ketone.

Hammond's Postulate (Alkane Bromination Selectivity)

The principle stating that because the hydrogen abstraction propagation step in free-radical bromination ($Br_2$) is endothermic, the transition state closely resembles the high-energy radical intermediate, imparting significant radical character and high selectivity for the most substituted carbon.

Allylic Halogenation

A free-radical chain reaction using $N$-bromosuccinimide (NBS) and light ($h\nu$) to selectively abstract a hydrogen atom from the carbon adjacent to a $C=C$ bond, forming resonance-stabilized allylic radicals that capture bromine to give the most substituted (most stable) alkene product.

Peroxide Effect (Radical Addition of $HBr$)

The reaction of an alkene with $HBr$ in the presence of peroxides ($ROOR$) and heat or light, proceeding via a free-radical chain mechanism that yields a haloalkane with non-Markovnikov regiochemistry and mixed stereochemistry.

$S_N2$ Mechanism

A concerted, bimolecular substitution pathway where a nucleophile attacks the backside of the carbon-leaving group bond simultaneously as the leaving group departs, resulting in complete inversion of stereochemical configuration.

$E2$ Mechanism

A concerted, bimolecular elimination pathway where a base removes a beta-proton as the leaving group departs, requiring an antiperiplanar geometry between the departing hydrogen and the leaving group, and regioselectively yielding the Zaitsev (most substituted) alkene.

$S_N1$ Mechanism

A stepwise, unimolecular substitution pathway where the leaving group departs first to generate a planar carbocation intermediate, followed by nucleophilic attack from either face to give scrambled (not quite racemic) stereochemistry.

Phosphorus Tribromide ($PBr_3$)

A reagent that selectively reacts with primary and secondary alcohols to form an intermediate with an $O-P$ bond (a good leaving group), which undergoes an $S_N2$ displacement by bromide ion to yield bromoalkanes with inversion of configuration.

Thionyl Chloride ($SOCl_2$)

A reagent used with primary and secondary alcohols to construct an intermediate containing an $O-S$ bond, enabling an $S_N2$ displacement by chloride ion to yield chloroalkanes with inversion of stereochemistry.

Acid-Catalyzed Dehydration of Alcohols

A reversible elimination reaction using $H_2SO_4$ that protonates an alcohol to form a carbocation intermediate via water loss, ultimately eliminating an adjacent proton to yield the Zaitsev (most substituted) alkene.

Sulfonyl Chloride Esterification

The nucleophilic attack of an alcohol on a sulfonyl chloride ($RSO_2Cl$) to form a sulfonyl ester ($OSO_2R$), converting a poor leaving group into an excellent leaving group with complete retention of configuration at the alcohol carbon.

Jones Reagent (Chromic Acid, $H_2CrO_4$)

A strong oxidizing reagent that converts primary alcohols through an aldehyde and hydrate intermediate into carboxylic acids, and converts secondary alcohols into ketones.

Pyridinium Chlorochromate (PCC)

An anhydrous chromium-based oxidizing agent that oxidizes secondary alcohols to ketones and selectively oxidizes primary alcohols to aldehydes, halting further oxidation due to the absence of water.

Williamson Ether Synthesis

An $S_N2$ reaction between a metal alkoxide nucleophile and an unhindered primary haloalkane to synthesize an ether while avoiding competing $E2$ elimination.

Peracid Epoxidation

A single-step concerted reaction between an alkene and a peroxy acid ($RCO_3H$) to deliver an oxygen atom across the double bond, yielding an epoxide while preserving the original cis/trans stereochemical relationship of the alkene substituents.

Halohydrin Epoxidation

An intramolecular $S_N2$ reaction where base deprotonates the hydroxyl group of a halohydrin to produce an alkoxide, which attacks the adjacent carbon from an antiperiplanar geometry relative to the halogen, expelling the halide to form an epoxide.

Epoxide Ring-Opening (Acid vs. Base Catalysis)

Nucleophilic ring cleavage of epoxides yielding vicinal diols with anti stereochemistry: base directs nucleophilic attack to the less hindered carbon via $S_N2$, whereas acid protonates the epoxide oxygen, causing water to attack the more substituted carbon bearing greater partial positive charge.