Key Concepts of Alkenes and Alkynes

Alkenes and alkynes are the reactive workhorses of organic chemistry, and understanding them unlocks your ability to predict products, design syntheses, and tackle mechanism problems. You're being tested on more than just memorizing reactions—exams want you to understand why double and triple bonds behave the way they do, how electron density, carbocation stability, and stereochemistry drive reaction outcomes, and when to apply rules like Markovnikov's versus anti-Markovnikov addition.

These unsaturated hydrocarbons connect to nearly every major topic you'll encounter: reaction mechanisms, stereoisomerism, polymer chemistry, and synthetic strategy. The key is recognizing patterns—electrophilic addition reactions all follow similar logic, but the specific reagents determine regiochemistry and stereochemistry. Don't just memorize that $\text{HBr}$ adds to alkenes; know why the hydrogen goes where it does and what intermediate forms along the way.

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Structure and Bonding Fundamentals

Before diving into reactions, you need a solid grasp of what makes alkenes and alkynes structurally unique. The restricted rotation around double bonds and the linear geometry of triple bonds create distinct properties that drive both isomerism and reactivity.

Carbon-Carbon Double and Triple Bonds

• Alkenes contain a C=C double bond—one sigma bond plus one pi bond, with $sp^2$ hybridized carbons and ~120° bond angles
• Alkynes contain a C≡C triple bond—one sigma bond plus two pi bonds, with $sp$ hybridized carbons and 180° linear geometry
• Pi electrons are the reactive site—the electron-rich pi cloud above and below the bond plane attracts electrophiles in addition reactions

Geometric Isomerism in Alkenes

• Restricted rotation around C=C bonds—the pi bond prevents free rotation, locking substituents in place and creating distinct isomers
• Cis isomers have substituents on the same side of the double bond; trans isomers have them on opposite sides
• E/Z nomenclature uses Cahn-Ingold-Prelog priority rules—E (entgegen) means high-priority groups are opposite; Z (zusammen) means they're together

Nomenclature Essentials

• Parent chain must include the double/triple bond—number the chain to give the unsaturation the lowest possible locant
• Suffix changes indicate bond type—"-ene" for alkenes (butene), "-yne" for alkynes (butyne), with position numbers preceding the suffix
• Geometric prefixes (cis/trans or E/Z) appear at the beginning of the name to specify stereochemistry when applicable

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Electrophilic Addition Reactions

The defining reactivity of alkenes and alkynes is electrophilic addition—the pi electrons attack an electrophile, breaking the pi bond and forming new sigma bonds. Understanding this mechanism is essential because it explains regiochemistry (where atoms add) and stereochemistry (how they add spatially).

General Mechanism of Electrophilic Addition

• Pi bond acts as nucleophile—electron-rich double/triple bond attacks electron-poor electrophiles like $\text{H}^+$, $\text{Br}^+$, or carbocations
• Carbocation intermediate forms—after initial attack, a positively charged carbon intermediate determines the reaction's regiochemistry
• Nucleophile completes addition—the second atom or group attacks the carbocation, converting the unsaturated compound to a more saturated product

Markovnikov's Rule

• Hydrogen adds to the carbon with more hydrogens already attached—this places the positive charge on the more substituted carbon during the mechanism
• Carbocation stability drives regioselectivity—tertiary carbocations > secondary > primary, due to hyperconjugation and inductive electron donation
• Predicts major product in additions of $\text{HX}$, $\text{H}_2\text{O}$ (acid-catalyzed), and similar unsymmetrical reagents

Hydrohalogenation

• Addition of HX (HCl, HBr, HI) across the double bond—follows Markovnikov's rule to produce alkyl halides
• Mechanism proceeds through carbocation—protonation of the alkene forms the more stable carbocation, then halide attacks
• Rearrangements possible—if a more stable carbocation can form via hydride or methyl shift, expect rearranged products

Halogenation

• Addition of $\text{X}_2$ ($\text{Cl}_2$, $\text{Br}_2$) produces vicinal dihalides—both halogens add across the double bond
• Anti addition stereochemistry—halogens add to opposite faces of the double bond via a cyclic halonium ion intermediate
• Bromine test for unsaturation—$\text{Br}_2$ decolorizes (orange to colorless) in the presence of alkenes/alkynes, a classic qualitative test

Hydration (Acid-Catalyzed)

• Addition of water requires acid catalyst—$\text{H}_2\text{SO}_4$ or $\text{H}_3\text{PO}_4$ protonates the alkene to initiate the reaction
• Follows Markovnikov's rule—OH ends up on the more substituted carbon, producing alcohols from alkenes
• Alkynes hydrate to carbonyls—initial enol product tautomerizes to ketone (internal alkynes) or aldehyde (terminal alkynes, with special catalysts)

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Anti-Markovnikov and Stereoselective Additions

Not all additions follow Markovnikov's rule. Certain reagents and conditions reverse the regiochemistry or control stereochemistry through different mechanistic pathways.

Hydroboration-Oxidation

• Two-step anti-Markovnikov hydration—borane ($\text{BH}_3$) adds to the alkene, then oxidation with $\text{H}_2\text{O}_2$/$\text{NaOH}$ replaces boron with OH
• Syn addition stereochemistry—both H and OH add to the same face of the double bond (concerted mechanism, no carbocation)
• OH ends up on less substituted carbon—the opposite of acid-catalyzed hydration, giving you synthetic flexibility

Hydrogenation

• Addition of $\text{H}_2$ reduces pi bonds to sigma bonds—requires metal catalyst (Pd, Pt, or Ni) and converts alkenes to alkanes
• Syn addition—both hydrogens add to the same face of the double bond (catalyst surface delivers both H atoms simultaneously)
• Alkynes can be partially or fully reduced—full hydrogenation gives alkanes; controlled conditions give alkenes

Reduction of Alkynes to Cis-Alkenes

• Lindlar's catalyst enables partial hydrogenation—"poisoned" palladium catalyst stops reduction at the alkene stage
• Produces cis (Z) alkenes exclusively—syn addition of $\text{H}_2$ from the catalyst surface ensures both hydrogens are on the same side
• Dissolving metal reduction gives trans alkenes—Na or Li in $\text{NH}_3$ produces (E)-alkenes through a different radical mechanism

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Oxidation and Cleavage Reactions

Oxidation reactions either add oxygen atoms to alkenes or cleave the carbon-carbon double bond entirely. These reactions are powerful tools for synthesis and for determining unknown structures.

Dihydroxylation

• Adds two OH groups across the double bond—produces 1,2-diols (vicinal diols or glycols)
• $\text{OsO}_4$ gives syn addition—both hydroxyl groups add to the same face via a cyclic osmate ester intermediate
• $\text{KMnO}_4$ (cold, dilute) also works—syn dihydroxylation, though harsher conditions can lead to over-oxidation

Ozonolysis

• Cleaves the double bond completely—$\text{O}_3$ followed by reductive workup ($\text{Zn}$ or $\text{DMS}$) breaks C=C into two carbonyl fragments
• Products depend on substitution—disubstituted carbons become ketones; monosubstituted carbons become aldehydes; terminal =$\text{CH}_2$ becomes formaldehyde
• Useful for structure determination—working backward from ozonolysis products reveals the original alkene structure

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Alkyne-Specific Reactivity

Alkynes have unique properties beyond just "more reactive double bonds." The linear geometry, increased electron density, and surprising acidity of terminal alkynes open up distinct reaction pathways.

Acidity of Terminal Alkynes

• Terminal alkynes have $pK_a \approx 25$—much more acidic than alkenes (~44) or alkanes (~50) due to $sp$ hybridization
• $sp$ orbitals have more s-character (50%)—electrons in orbitals closer to the nucleus are held more tightly, stabilizing the conjugate base
• Strong bases deprotonate terminal alkynes—$\text{NaNH}_2$ or $n\text{-BuLi}$ forms acetylide anions ($\text{RC≡C}^-$), powerful nucleophiles for synthesis

Acetylide Chemistry

• Acetylide anions are carbon nucleophiles—react with alkyl halides ($\text{S}_\text{N}2$) to extend carbon chains
• Only works with primary alkyl halides—secondary and tertiary substrates undergo elimination instead
• Key for carbon-carbon bond formation—acetylide alkylation is a fundamental synthetic strategy for building molecular complexity

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Polymerization and Ring-Forming Reactions

Alkenes participate in reactions that build complex structures—either long polymer chains or new ring systems. These reactions demonstrate how simple unsaturated monomers become materials with real-world applications.

Addition Polymerization

• Repeated addition of alkene monomers forms long chains—the pi bond opens, and carbons link together in a chain-growth process
• Radical, cationic, or anionic mechanisms possible—initiator type determines mechanism and polymer properties
• Produces major industrial polymers—polyethylene (from ethene), polypropylene (from propene), PVC (from vinyl chloride)

Diels-Alder Reaction

• [4+2] cycloaddition between diene and dienophile—conjugated diene (4 pi electrons) reacts with electron-poor alkene (2 pi electrons)
• Forms six-membered rings in one step—concerted, pericyclic mechanism with no intermediates
• Stereospecific and regioselective—substituent geometry in reactants is preserved in the product; endo products often favored kinetically

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Stability and Reactivity Trends

Understanding why certain alkenes and alkynes are more stable or reactive helps you predict outcomes and compare structures. Stability trends connect to thermodynamics (which isomer is lower energy), while reactivity trends connect to kinetics (which reacts faster).

Alkene Stability

• More substituted alkenes are more stable—tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted
• Hyperconjugation stabilizes the double bond—overlap between C-H sigma bonds and the pi system delocalizes electron density
• Trans isomers are more stable than cis—reduced steric strain between substituents on opposite sides of the double bond

Alkyne Reactivity

• Alkynes are more electron-rich than alkenes—two pi bonds mean greater nucleophilicity and reactivity toward electrophiles
• Internal alkynes are more stable than terminal—similar substitution effect as alkenes; more substitution = more stability
• Triple bonds can undergo two sequential additions—first addition gives substituted alkene; second addition gives fully saturated product

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Quick Reference Table

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Self-Check Questions

1. Which two reactions both add water across a double bond but give alcohols with opposite regiochemistry? What mechanistic difference accounts for this?

2. You need to convert 1-butyne to cis-2-butene. What reagent/catalyst would you use, and why does it give the cis product specifically?

3. Compare the products of treating 2-butene with (a) $\text{Br}_2$ and (b) $\text{OsO}_4$ followed by $\text{NaHSO}_3$. How do the stereochemistries differ?

4. Why are terminal alkynes significantly more acidic than alkenes, and what synthetically useful species can you generate by deprotonating them?

5. An unknown alkene undergoes ozonolysis to yield acetone and formaldehyde as the only products. Draw the structure of the original alkene and explain your reasoning.