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Organic Chemistry Unit 4: Alkenes and alkynes

Alkenes and alkynes react by addition across their π bonds, with regiochemistry and stereochemistry set by the mechanism.

Covers: Alkene stability and nomenclature, electrophilic addition, Markovnikov and anti-Markovnikov additions, hydration and hydroboration, halogenation, hydrogenation, oxidation, alkyne reactions

Study guide

Alkenes contain a C=C double bond. More substituted alkenes are more stable, and trans alkenes are generally more stable than cis because of reduced steric strain. E/Z naming uses CIP priorities: Z (zusammen) means the higher-priority groups are on the same side, E (entgegen) means opposite sides. Degrees of unsaturation, calculated from the molecular formula, count rings plus π bonds.

The π bond acts as a nucleophile in electrophilic addition. Adding HX follows Markovnikov's rule: H adds to the carbon with more hydrogens, and X adds to the more substituted carbon, because the reaction goes through the more stable carbocation (rearrangements are possible). With peroxides (ROOR), HBr adds anti-Markovnikov by a radical mechanism. Acid-catalyzed hydration (H₂O, H₂SO₄) gives Markovnikov alcohols; oxymercuration–demercuration gives Markovnikov alcohols without rearrangement; hydroboration–oxidation (BH₃ then H₂O₂, NaOH) gives anti-Markovnikov alcohols with syn addition. Halogenation (Br₂) goes through a bromonium ion and gives anti addition; with water it forms a halohydrin.

Catalytic hydrogenation (H₂ with Pt, Pd or Ni) adds H₂ in a syn fashion. Oxidation reactions include epoxidation with peroxyacids like mCPBA, syn dihydroxylation with OsO₄, anti dihydroxylation through epoxide opening, and ozonolysis (O₃ then a reducing agent like DMS or Zn), which cleaves the double bond into carbonyl compounds. Alkynes undergo similar additions. Hydrogenation with Lindlar's catalyst stops at the cis alkene, while sodium in liquid ammonia gives the trans alkene. Terminal alkynes are relatively acidic (pKa about 25), so they can be deprotonated by strong bases like NaNH₂ to form acetylide ions, which act as nucleophiles in SN2 reactions to build longer carbon chains.

Conjugated dienes are extra stable because their π electrons are delocalized. Adding HX or X₂ goes through a resonance-stabilized allylic cation and gives 1,2- and 1,4-addition products. At low temperature the faster-forming 1,2-product dominates (kinetic control). At higher temperature, where the reaction can reverse, the more stable, usually more substituted 1,4-product dominates (thermodynamic control). In the Diels–Alder reaction, a conjugated diene in the s-cis conformation and a dienophile (an alkene or alkyne, best with electron-withdrawing groups) combine in one concerted step to form a cyclohexene. The reaction is stereospecific: groups that are cis on the dienophile stay cis in the product. With cyclic dienes, the endo product is usually favored.

Combining p orbitals in a conjugated system makes π molecular orbitals of increasing energy, each with one more node. For 1,3-butadiene, the four π electrons fill ψ₁ and ψ₂, so ψ₂ is the HOMO and ψ₃ is the LUMO. Reactions such as cycloadditions happen when the HOMO of one molecule overlaps in phase with the LUMO of another. Electrocyclic reactions open or close a ring through a cyclic shift of π electrons. Under heat, systems with 4n π electrons react conrotatory and systems with 4n + 2 react disrotatory, and light reverses these rules. Thermal [4 + 2] cycloadditions like the Diels–Alder are allowed, while thermal [2 + 2] cycloadditions are not.

Alkenes such as ethylene, styrene and vinyl chloride can join into long-chain polymers by radical chain-growth polymerization. A radical initiator adds to the double bond, and the resulting radical keeps adding monomers until two radicals combine.

Key ideas

• More substituted alkenes are more stable.
• HX adds by Markovnikov's rule through the more stable carbocation.
• Hydroboration–oxidation gives anti-Markovnikov, syn addition.
• Br₂ adds anti through a bromonium ion; H₂ adds syn.
• Lindlar's catalyst gives cis alkenes; Na/NH₃ gives trans.

Formulas and rules

Common mistakes

• Forgetting carbocation rearrangements in HX additions and acid-catalyzed hydration.
• Mixing up Lindlar (cis) and Na/NH₃ (trans) alkyne reductions.

Words to know

Alkene

A hydrocarbon with a C=C double bond.

Alkyne

A hydrocarbon with a C≡C triple bond.

E/Z notation

Naming alkenes by the positions of higher-priority groups.

Degrees of unsaturation

The number of rings plus π bonds.

Electrophilic addition

Adding an electrophile and nucleophile across a π bond.

Markovnikov's rule

H adds to the carbon with more hydrogens.

Anti-Markovnikov addition

The nucleophile adds to the less substituted carbon.

Regiochemistry

Which carbon a group attaches to.

Syn addition

Two groups add to the same face of a double bond.

Anti addition

Two groups add to opposite faces.

Hydroboration–oxidation

Anti-Markovnikov, syn addition of water.

Oxymercuration–demercuration

Markovnikov hydration without rearrangement.

Bromonium ion

A three-membered ring intermediate in halogenation.

Halohydrin

A compound with a halogen and OH on adjacent carbons.

Catalytic hydrogenation

Adding H₂ with a metal catalyst.

Epoxide

A three-membered cyclic ether.

Ozonolysis

Cleaving a C=C bond with ozone to form carbonyls.

Lindlar's catalyst

A poisoned catalyst that reduces alkynes to cis alkenes.

Acetylide ion

A deprotonated terminal alkyne.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.