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. Exams test more than memorization: they 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. The three $sp^2$ orbitals lie in a plane, and the unhybridized p orbital sticks out above and below to form the pi bond.
• Alkynes contain a C≡C triple bond: one sigma bond plus two pi bonds, with $sp$ hybridized carbons and 180° linear geometry. The two unhybridized p orbitals form two mutually perpendicular pi bonds.
• Pi electrons are the reactive site: the electron-rich pi cloud above and below the bond plane attracts electrophiles, making these bonds nucleophilic 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 that can't interconvert at room temperature.
• 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 and works for any alkene, not just those with two identical substituents. E (entgegen) means the two higher-priority groups (one on each carbon) are on opposite sides; Z (zusammen) means they're on the same side.

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 (e.g., but-2-ene), "-yne" for alkynes (e.g., but-1-yne), 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. Use E/Z when cis/trans is ambiguous (i.e., when the four substituents are all different).

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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

The mechanism follows three key steps:

1. Pi bond acts as nucleophile: the electron-rich double/triple bond donates its pi electrons to an electrophile like $\text{H}^+$ or $\text{Br}^+$.
2. Carbocation intermediate forms: after the initial attack, a positively charged carbon intermediate is generated. Which carbon bears the positive charge determines the reaction's regiochemistry.
3. 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 in stability, due to hyperconjugation (overlap of adjacent C-H sigma bonds with the empty p orbital) and inductive electron donation from alkyl groups.
• Predicts the major product in additions of $\text{HX}$, $\text{H}_2\text{O}$ (acid-catalyzed), and similar unsymmetrical reagents to unsymmetrical alkenes.

Hydrohalogenation

• Addition of HX (HCl, HBr, HI) across the double bond follows Markovnikov's rule to produce alkyl halides.
• Mechanism proceeds through a carbocation: protonation of the alkene forms the more stable carbocation, then the halide ion attacks as the nucleophile.
• Rearrangements are possible: if a more stable carbocation can form via a 1,2-hydride shift or 1,2-methyl shift, expect rearranged products. Always check whether the initially formed carbocation can rearrange.

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: the reaction proceeds through a cyclic halonium ion intermediate (a three-membered ring with a positive halogen bridging both carbons). The nucleophilic halide then attacks from the opposite face, forcing anti addition.
• Bromine test for unsaturation: $\text{Br}_2$ in $\text{CCl}_4$ decolorizes (orange to colorless) in the presence of alkenes or alkynes. This is a classic qualitative test for pi bonds.

Hydration (Acid-Catalyzed)

• Addition of water requires an 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: the initial enol product tautomerizes to a ketone (internal alkynes) or aldehyde (terminal alkynes with mercury(II) sulfate/$\text{H}_2\text{SO}_4$ catalysis). Markovnikov selectivity means internal alkynes give ketones, while terminal alkynes give methyl ketones under standard acid-catalyzed conditions.

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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

This is a two-step sequence that achieves anti-Markovnikov hydration:

1. Hydroboration: borane (\text{BH}_3}) adds across the double bond in a single concerted step (no carbocation). Boron attaches to the less substituted carbon because it's the larger atom and avoids steric crowding.
2. Oxidation: treatment with $\text{H}_2\text{O}_2$/$\text{NaOH}$ replaces the boron with OH, retaining the same stereochemistry.

• Syn addition stereochemistry: both H and B (later replaced by OH) add to the same face of the double bond.
• OH ends up on the less substituted carbon: the opposite regiochemistry of acid-catalyzed hydration, giving you synthetic flexibility to place the hydroxyl group where you need it.

Hydrogenation

• Addition of $\text{H}_2$ reduces pi bonds to sigma bonds: requires a metal catalyst (Pd, Pt, or Ni on a solid support) and converts alkenes to alkanes.
• Syn addition: both hydrogens add to the same face of the double bond because the catalyst surface delivers both H atoms simultaneously.
• Alkynes can be partially or fully reduced: full hydrogenation with excess $\text{H}_2$ gives alkanes; controlled conditions (specific catalysts or stoichiometry) can stop at the alkene stage.

Reduction of Alkynes to Cis- or Trans-Alkenes

• Lindlar's catalyst enables partial hydrogenation: this "poisoned" palladium catalyst (Pd on $\text{CaCO}_3$ with lead acetate and quinoline) stops reduction at the alkene stage by deactivating the catalyst surface.
• Produces cis (Z) alkenes exclusively: syn addition of $\text{H}_2$ from the catalyst surface ensures both hydrogens end up on the same side.
• Dissolving metal reduction gives trans (E) alkenes: Na or Li in liquid $\text{NH}_3$ proceeds through a radical anion mechanism, and the thermodynamically more stable trans product forms preferentially.

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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 to produce 1,2-diols (vicinal diols, also called glycols).
• $\text{OsO}_4$ gives syn addition: both hydroxyl groups add to the same face via a cyclic osmate ester intermediate. $\text{OsO}_4$ is typically used in catalytic amounts with a co-oxidant like NMO (N-methylmorpholine N-oxide).
• Cold, dilute $\text{KMnO}_4$ also gives syn dihydroxylation, though harsher conditions (heat, concentrated, or acidic $\text{KMnO}_4$) will cleave the double bond entirely.

Ozonolysis

Ozonolysis cleaves the C=C bond completely and is performed in two steps:

1. Ozone ($\text{O}_3$) reacts with the alkene to form an ozonide intermediate.
2. Reductive workup with $\text{Zn}$/$\text{CH}_3\text{COOH}$ or $\text{(CH}_3\text{)}_2\text{S}$ (DMS) breaks the ozonide into carbonyl fragments.

• Products depend on substitution: disubstituted carbons become ketones; monosubstituted carbons become aldehydes; a terminal $\text{=CH}_2$ becomes formaldehyde ($\text{CH}_2\text{O}$).
• Useful for structure determination: working backward from ozonolysis products reveals the original alkene structure. Just connect the carbonyl carbons with a double bond.

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Alkyne-Specific Reactivity

Alkynes have unique properties beyond just being "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).
• The reason is hybridization: $sp$ orbitals have 50% s-character, meaning the electrons in the C-H bond are held closer to the carbon nucleus. This stabilizes the conjugate base (the acetylide anion) because the negative charge sits in an orbital closer to the nucleus.
• Strong bases deprotonate terminal alkynes: $\text{NaNH}_2$ (sodium amide, $pK_a$ of $\text{NH}_3$ ≈ 38) or $n\text{-BuLi}$ converts terminal alkynes to acetylide anions ($\text{RC≡C}^-$), which are powerful carbon nucleophiles.

Acetylide Chemistry

• Acetylide anions are carbon nucleophiles: they react with primary alkyl halides via $\text{S}_\text{N}2$ to extend carbon chains, forming new C-C bonds.
• Only works with primary (and methyl) alkyl halides: secondary and tertiary substrates undergo elimination (E2) instead because the acetylide is also a strong base.
• Key for carbon-carbon bond formation: acetylide alkylation is one of the most important synthetic strategies for building molecular complexity in introductory organic chemistry.

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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 are possible: the type of initiator determines the mechanism and influences polymer properties like molecular weight and branching.
• Produces major industrial polymers: polyethylene (from ethene), polypropylene (from propene), and PVC (from vinyl chloride, $\text{CH}_2\text{=CHCl}$).

Diels-Alder Reaction

• [4+2] cycloaddition between a diene and a dienophile: a conjugated diene (4 pi electrons) reacts with an electron-poor alkene or alkyne (2 pi electrons) called the dienophile.
• Forms six-membered rings in one step: this is a concerted, pericyclic mechanism with no intermediates and no catalyst required.
• Stereospecific and regioselective: substituent geometry in the reactants is preserved in the product (cis dienophile substituents stay cis in the product). The endo product is often favored kinetically due to secondary orbital interactions.
• Diene must be in the s-cis conformation to react. If the diene is locked in s-trans (e.g., by a ring), the Diels-Alder reaction won't proceed.

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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. You can confirm this experimentally by comparing heats of hydrogenation: more stable alkenes release less heat.
• Hyperconjugation stabilizes the double bond: overlap between adjacent C-H (or C-C) sigma bonds and the pi system delocalizes electron density onto the double bond.
• Trans isomers are generally more stable than cis: reduced steric strain between substituents on opposite sides of the double bond lowers the energy.

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: the same substitution effect seen in alkenes applies here. More alkyl substitution = more stability.
• Triple bonds can undergo two sequential additions: the first addition converts the alkyne to a substituted alkene; the second addition gives a fully saturated product. You can often stop at the first addition with careful control of stoichiometry.

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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 two steps would you use, and why does the second step give the cis product specifically?

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