Alkene and Alkyne Reactions

• These notes cover the reactions of alkenes and alkynes.

Preparation of Alkenes: Elimination Reactions

• Alkenes can be formed through elimination reactions.
• Elimination reactions involve removing groups from adjacent carbons to form a double bond.
• One common method is dehydrohalogenation.
• Dehydrohalogenation involves eliminating HX from an alkyl halide.
• Another method is dehydration.
• Dehydration involves eliminating H-OH from an alcohol.
• Bromocyclohexane plus KOH yields Cyclohexene (81%)
• 1-Methylcyclohexanol plus H2SO4, H2O, THF at 50 °C yields 1-Methylcyclohexene (91%)

Electrophilic Addition Reactions of Alkenes

• Electrophilic addition reactions involve the addition of electrophiles to alkenes.
• The pi bond in alkenes is electron-rich.
• Alkenes act as nucleophiles.
• Alkenes attract electrophiles to form pi bonds.
• The mechanism involves two steps, protonation and halide addition.
• First, protonation occurs.
• Second, a halide ion adds to the carbocation intermediate.

Diverse Reactions of Alkenes

• Alkenes react with electrophiles.
• Electrophiles include halohydrin, alcohol, alkane, 1,2-diol, carbonyl compound, 1,2-dihalide, alkene, halide, epoxide, and cyclopropane.
• These produce useful products by addition often through special reagents.

Electrophilic Addition Reactions of Alkenes

• Common electrophilic addition reactions include hydrohalogenation.
• Hydrohalogenation is the addition of H-X (X = halogen).
• Another reaction is hydration.
• Hydration is the addition of H₂O.
• A third reaction is halogenation.
• Halogenation is the addition of halogens.
• A fourth reaction is hydrogenation.
• Hydrogenation is the addition of H₂.

Hydrohalogenation: Markovnikov Addition

• Hydrohalogenation follows Markovnikov's rule.
• A hydrogen atom on HBr is attacked by pi electrons from the nucleophilic double bonds forming a C-H bond.
• Bromide donates an electron pair to the positively charged carbon atom, creating a C-Br bond and a neutral addition product.

Markovnikov's Rule

• In the addition of HX to an alkene, the H attaches to the carbon with more hydrogen substituents.
• The X attaches to the carbon with a greater number of alkyl groups.
• If both double-bond carbons have the same substitution degree, a mixture of addition products results.
• The more highly substituted carbocation forms as the intermediate rather than the less substituted one.

Carbocation Structure and Stability

• Alkyl groups stabilize carbocations by donating electrons via inductive effect.
• Alkyl groups stabilize carbocations by delocalizing sigma-electrons into the empty p orbital via hyperconjugation.
• Highly substituted carbocations are more stable than less highly substituted ones.

Predicting Hydrohalogenation Products

• Predict hydrohalogenation reaction products.
• Consider Markovnikov's rule.
• Remember that the H will bond to the alkene C with the most H's.
• Identify the reactant that could yield the given hydrohalogenation reaction product.

Halogenation of Alkenes: Addition of X₂

• Bromine and chlorine are added to alkenes, yielding 1,2-dihalides via halogenation.
• Halogenation of cycloalkenes forms only the trans dihalide addition product.
• Halogenation reactions occur with anti-stereochemistry.
• Halogens are added to opposite faces of the double bond.
• The large Br atom shields one face of the alkene.
• Br adds to the other, via anti addition.
• This gives a trans product.

Halohydrins From Alkenes

• Alkenes react with hypohalous acids (HO-Cl or HO-Br) to yield 1,2-halo alcohols called halohydrins.
• Addition occurs indirectly with Br₂ or Cl₂ in water.
• Regiochemistry follows Markovnikov's rule.
• X goes to the less substituted carbon.
• OH goes to the more substituted carbon.
• Stereochemistry is anti, like halogenation, due to the halonium intermediate.

Biological Halohydrin Reactions

• Halohydrin formation, common in marine organisms, is catalyzed by haloperoxidases.
• Haloperoxidases use hydrogen peroxide to incorporate halogens into organic molecules.
• Halohydrin formation contributes to halogenated metabolites and chemical defense.

Hydration of Alkenes: Acid-Catalyzed

• Alkenes undergo acid-catalyzed addition with water to yield alcohols.
• Follows Markovnikov's rule where the H (from H₂O) will bond to the alkene C with the most H's.
• The OH attaches to the more highly substituted carbon atom, and the -H attaches to the less highly substituted carbon.

Hydration of Alkenes: Oxymercuration

• Involves electrophilic addition of Hg2+ to the alkene on treatment with mercury(II) acetate [(CH3COO)2Hg, or Hg(OAc)2] in aqueous tetrahydrofuran (THF) solvent.
• Follows the Markovnikov's rule

Hydration of Alkenes: Hydroboration

• If non-Markovnikov addition of H₂O across a double bond is desired, hydroboration-oxidation is used.
• Hydroboration/oxidation involves the addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2.
• Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives the alcohol.
• Non-Markovnikov regiochemistry occurs as boron attaches to the less crowded carbon of the alkene.
• C-H and C-B bonds form at the same time and from the same face of the alkene product.
• Syn stereochemistry results.

Reduction of Alkenes: Hydrogenation

• Reduction results in an increase in electron density.
• Reduction occurs by forming a C-H bond or breaking a C-O, C-N, or C-X bond.
• Catalytic hydrogenation occurs when H₂ is added across the same face of the double bond using catalytic Pt or Pd on carbon.
• Stereochemistry is syn because the hydrogens are added in the same direction.
• This will make cis- substituted cycloalkanes.
• Alkenes are much more reactive than most other unsaturated functional groups in terms of catalytic hydrogenation.
• Aldehydes, ketones, and esters remain unchanged in mild alkene hydrogenation
• They may react under harsher conditions.

Catalytic Hydrogenation in Food Industry

• Food industry uses catalytic hydrogenation to convert vegetable oils into solid cooking fats like margarine.
• These have longer shelf life.
• Complete hydrogenation produces saturated fatty acids,
• Incomplete hydrogenation can cause partial cis-trans isomerization of the remaining double bond.
• Trans fats are edible but raise cholesterol and triacylglycerol levels.
• Trans fats increase the risk of artery disease.

Biological Reduction Reactions

• Biological reductions occur in two steps with the double bond adjacent to a carbonyl group.
• NADPH transfers a hydride ion (H:-) to the double bond.
• The process forms an anion which is then protonated.
• Thisprocess is key in fatty acid and steroid metabolism.

Oxidation of Alkenes: Syn Epoxidation

• Oxidation is the Loss of electron density by carbon via C-O, C-N, or C-X bond formation or C-H bond cleavage.
• Alkenes react with peroxy acids to form epoxides via syn oxygen transfer.

Oxidation of Alkenes: Epoxidation

• When halohydrins are treated with base, HX is eliminated and an epoxide is produced.

Biological Epoxidation Reactions

• Squalene undergoes biological epoxidation during steroid biosynthesis.
• O₂ reacts with the reduced coenzyme flavin adenine dinucleotide, FADH2.
• This drives the formation of a flavin hydroperoxide.

Oxidation of Alkenes: Anti Hydroxylation

• Acid-catalyzed hydrolysis of epoxides gives corresponding 1,2-dialcohol or diol.
• These are also called a glycol.
• During anti-hydroxylation the O is protonated first.
• This increases the electrophilicity of the carbons
• Water attacks a carbon from the opposite face of the epoxide creating an anti addition overall

Oxidation of Alkenes: Syn Hydroxylation

• Syn hydroxylation involves treating an alkene with osmium tetroxide (OsO₄).
• Osmium tetroxide delivers oxygen on the same faces.
• NaHSO3- cleaves the cyclic osmium intermediate, leaving a cis-1,2-diol.
• Syn stereochemistry results.
• This does not involve a carbocation intermediate.

Biological Hydroxylation Reactions

• Benzo[a]pyrene, a carcinogen found in cigarette smoke, soot and grilled meat, is detoxified in the liver.
• It's converted to a diol epoxide, then hydrolyzed to a soluble tetrol.

Oxidation of Alkenes (O3): Cleavage to Carbonyl Compounds

• Ozonolysis involves breaking carbon bonds using Ozone, and reduction with Zn.
• This also known as ozone cleaving double bonds, which forms carbonyl bonds.
• A tetrasubstituted alkene gives two ketones
• A trisubstituted alkene yields one ketone and one aldehyde.
• A disubstituted alkene produces two aldehydes.

Permanganate Oxidation of Alkenes

• Potassium permanganate (KMnO4) cleaves alkenes.
• Oxidizes carbons with hydrogens to carboxylic acids or CO2.
• Two hydrogens on one carbon of the double bond form CO2.
• One hydrogen on one carbon of the double bond forms carboxylic acids.
• No hydrogens (tetrasubstituted) form ketones.

Periodate Oxidation of 1,2-diols

• Alkene cleavage by hydroxylation forms a 1,2-diol.
• When treated with periodic acid (HIO4), different products are made.
• A ring with -OH groups gives a single dicarbonyl compound.
• An open-chain diol yields two carbonyl compounds.

Addition of Carbenes to Alkenes: Cyclopropane Synthesis

• Cyclopropanes can be formed by adding a carbene to a double bond.
• One way to make carbenes is to react chloroform (CHCl3) with a strong base like KOH.
• The addition is stereospecific.
• A cis alkene produces only cis-disubstituted cyclopropane.

Alkene Polymers

• Polymerization builds large molecules by repetitively bonding smaller molecules (monomers) together
• Polymerization can result in substances such as, polyethylene, polypropylene and polystyrene
• Heating benzoyl peroxide breaks its weak O-O bond generating benzoyloxy radicals
• Polymerization forms new radicals to extend the polymer chain and create a polymer.
• When a reaction consumes the radicals the chain is broken.

Diels Alder Cycloaddition Reaction

• The Diels-Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene (dienophile).
• The Diels Alder results in a substituted cyclohexene derivative after the diene donates 2 electrons to the dienophile and creates sigma bonds.

Reactions of Alkynes

• Contain at least one C=C triple bond with sp-hybridized C atoms
• Involve same addition reactions as alkenes
• Use 2 equivalents of addition reagent (i.e., + 2HBr)
• Reactions follow Markovnikov's rule in the same manner
• The resulting alkene has cis stereochemistry

HBr Addition

• First the Alkyne is reacted with HBr and CH3CO₂H (Acetic acid) to yield 2-Bromohex-1-ene with the addition being Markovnikov
• Second the 2-Bromohex-1-ene is then reacted again with HBr and CH3CO₂H (Acetic acid) to yield 2,2-Dibromohexane with the addition being Markovnikov

Other Additions

• HCI addition results in chloroalkenes and dichloroalkanes
• Br₂ addition results in dibromoalkenes and tetrabromoalkanes
• H₂ addition results in alkenes with cis configuration, as well as alkanes

Conversion into Acetylide Anions

• Terminal alkynes (RC≡CH) are acidic.
• A strong base like NaNH₂ removes the terminal hydrogen.
• This forms an acetylide anion.

Reaction of Acetylide Anions With Alkyl Halides

• Acetylide anion is strongly nucleophilic and will react with many different kinds of electrophiles.
• Alkyl halides yield a new alkyne product.

Notes covering the reactions of alkenes and alkynes, including alkene preparation via elimination reactions like dehydrohalogenation and dehydration. Also covers electrophilic addition reactions, detailing mechanisms such as protonation and halide addition. Examples include bromocyclohexane plus KOH yielding cyclohexene.