Alkene and Alkyne Reaction Mechanisms and Patterns

Comprehensive vocabulary flashcards covering the reactions, mechanisms, reagents, regiochemistry, and stereochemistry of alkenes and alkynes.

Markovnikov Addition

A regiochemical outcome in addition reactions where the $C-H$ bond forms at the least substituted carbon (the carbon with fewer bonds to carbon and more bonds to hydrogen) and the other substituent attaches to the most substituted carbon.

Anti-Markovnikov Addition

A regiochemical outcome where the $C-H$ bond forms at the most substituted carbon of the pi bond and the other substituent attaches to the least substituted carbon.

Syn Addition

A stereochemical addition outcome in which both new bonds are formed on the same face of the alkene or alkyne.

Anti Addition

A stereochemical addition outcome in which the two new bonds are formed on opposite faces of the alkene or alkyne.

Carbocation Pathway (Pattern 1)

A reaction pathway proceeding via electrophilic attack of an alkene pi bond on an acid to form a planar, $sp^2$-hybridized carbocation; attack of a nucleophile can occur from either face (producing a mixture of syn and anti addition), and rearrangements via hydride or alkyl shifts are possible.

3-Membered Ring Pathway (Pattern 2)

A reaction pathway proceeding through the formation of a 3-membered, positively charged cyclic intermediate (such as a halonium or mercurinium ion), followed by backside nucleophilic attack at the most substituted carbon to yield anti addition.

Concerted Pathway (Pattern 3)

A reaction pathway where two new bonds form simultaneously on the same face of the alkene in a single concerted transition state, yielding exclusively syn stereochemistry without carbocation intermediates.

Alkene Hydrohalogenation

Addition of $HX$ ($X = Cl$, $Br$, $I$) across an alkene via a planar carbocation intermediate to give an alkyl halide with Markovnikov regiochemistry, a mixture of syn and anti products, and the potential for carbocation rearrangements.

Acid-Catalyzed Alkene Hydration

Addition of $H_2O$ across an alkene catalyzed by $H_2SO_4$ (or $H_3O^+$) via a carbocation intermediate to yield an alcohol with Markovnikov regiochemistry, a mixture of syn and anti stereochemistry, and susceptibility to hydride or alkyl shifts.

Oxymercuration-Demercuration (Alkenes)

A two-step hydration using 1) $Hg(OAc)_2$, $H_2O$ and 2) $NaBH_4$ that proceeds through a 3-membered mercurinium ion intermediate to give Markovnikov alcohols without rearrangements; using an alcohol ($ROH$) produces an ether.

Hydroboration-Oxidation (Alkenes)

A two-step reaction using 1) $BH_3$ and 2) $NaOH$, $H_2O_2$ that adds across an alkene in a concerted transition state to yield an alcohol with anti-Markovnikov regiochemistry and syn stereochemistry, converting the $C-B$ bond to $C-OH$ with retention of configuration.

Alkene Hydrogenation

The addition of $H_2$ across an alkene using metal catalysts such as $Pd/C$, $Pt$, or $Ni$, resulting in syn addition of two $C-H$ bonds across the double bond.

Epoxidation

Reaction of an alkene with a peroxyacid ($RCO_3H$, such as $m$-CPBA) containing a weak $O-OH$ bond, proceeding through a concerted transition state to form a three-membered cyclic ether (epoxide) with syn stereochemistry.

Syn Dihydroxylation

Concerted addition of two hydroxyl groups to an alkene using either 1) $OsO_4$ followed by $NaHSO_3/H_2O$, or cold, dilute $KMnO_4$ with $NaOH$, producing vicinal (1,2-) diols with syn stereochemistry.

Cyclopropanation

The concerted addition of a carbene or carbenoid across an alkene double bond using $CH_2I_2$ and $Zn/Cu$ (or $CHCl_3$ with strong base $NaOH$ to yield dichlorocyclopropanes), resulting in syn stereochemistry.

Alkene Halogenation

Addition of elemental halogen ($X_2$, where $X = Cl$, $Br$, $I$) across an alkene via a 3-membered halonium ion intermediate attacked from the opposite face by halide, yielding vicinal dihalides with anti stereochemistry.

Halohydrin Formation

Reaction of an alkene with $X_2$ ($X = Cl$, $Br$, $I$) in the presence of $H_2O$, where water attacks the halonium ion at the more substituted, electrophilic carbon to produce a halohydrin with Markovnikov regiochemistry for the $C-O$ bond and anti stereochemistry.

Haloether Formation

Reaction of an alkene with $X_2$ ($X = Cl$, $Br$, $I$) in an alcohol ($ROH$) solvent, where the alcohol attacks the halonium ion intermediate at the most substituted carbon to yield a haloether with anti stereochemistry.

Anti Dihydroxylation

A two-step transformation where an alkene is first treated with a peroxyacid ($RCO_3H$) to form an epoxide, followed by ring opening with aqueous acid ($H_3O^+$) or base to give vicinal diols with anti stereochemistry.

Radical Addition of HBr

Addition of $HBr$ to an alkene in the presence of peroxides ($RO-OR$) and heat or light ($h\nu$), where $Br\bullet$ adds to generate the radical on the most substituted carbon, yielding anti-Markovnikov regiochemistry and a mixture of syn and anti addition.

Alkene Ozonolysis (Reductive Workup)

Cleavage of the alkene $C=C$ bond using ozone ($O_3$) followed by reduction with dimethyl sulfide ($CH_3SCH_3$) or $Zn/H^+$, preserving $C-H$ bonds to yield aldehydes and/or ketones.

Alkene Ozonolysis (Oxidative Workup)

Cleavage of the alkene $C=C$ bond using ozone ($O_3$) followed by hydrogen peroxide ($H_2O_2$) or acidic $KMnO_4$, which oxidizes any aldehyde $C-H$ bonds into carboxylic acids while leaving ketones unaffected.

Alkyne Double Elimination

Preparation of alkynes from geminal or vicinal dihalides using the strong base $NaNH_2$ ($2$ equivalents for internal alkynes, $3$ equivalents for terminal alkynes) to eliminate two equivalents of $HBr$ according to Zaitsev's rule.

Acetylide Ion

A strongly nucleophilic carbanion formed by deprotonating a relatively acidic terminal alkyne ($pK_a = 25$) using a strong base such as $NaNH_2$.

Alkyne Alkylation

A carbon-carbon bond-forming reaction where an acetylide ion attacks a primary alkyl halide to lengthen the carbon chain; secondary alkyl halides undergo elimination instead.

Complete Alkyne Hydrogenation

Reduction of an alkyne directly to an alkane using excess $H_2$ with metal catalysts such as $Pd/C$, $Pt$, or $Ni$.

Lindlar Catalyst

A poisoned palladium catalyst containing $Pb$ and quinoline (or nickel boride) that selectively reduces alkynes with $H_2$ to cis-alkenes without reducing alkenes to alkanes.

Dissolving Metal Reduction

Reduction of alkynes selectively to trans-alkenes using sodium ($Na$) or lithium ($Li$) in liquid ammonia ($NH_3$), which stops at the alkene stage.

Alkyne Hydrohalogenation

Addition of $HX$ across an alkyne with Markovnikov selectivity; $1$ equivalent yields a vinyl halide (alkenyl halide), while excess ($2$ equivalents) yields a geminal dihalide.

Alkyne Halogenation

Addition of halogens across an alkyne; $1$ equivalent of $X_2$ gives a trans-alkenyl dihalide, while $2$ equivalents give a tetrahalide.

Acid-Catalyzed Alkyne Hydration

Hydration of alkynes using strong acid ($H_2SO_4$, $H_2O$) with Markovnikov selectivity to generate an enol that tautomerizes into a ketone.

Alkyne Oxymercuration

Hydration of alkynes where $HgSO_4$ coordinates to the pi bond to form a cyclic intermediate attacked by $H_2O$ at the most substituted carbon; mercury is removed with aqueous acid to yield an enol that tautomerizes to a ketone.

Hydroboration-Oxidation (Alkynes)

Reaction of an alkyne with $BH_3$ or bulky boranes ($9\text{-BBN}$, $\text{Sia}_2BH$, or $R_2BH$) followed by $NaOH$ and $H_2O_2$ via anti-Markovnikov syn addition, producing an enol that tautomerizes to an aldehyde (from terminal alkynes) or a ketone (from internal alkynes).

Alkyne Ozonolysis

Cleavage of an alkyne using ozone ($O_3$) and $H_2O$, which oxidizes both sp carbons to carboxylic acids, with terminal alkynes producing $CO_2$ as one of the oxidation products.