Study guides › C/P › Organic Chemistry
Alkenes & Alkynes
Alkenes and alkynes react by adding across their π bonds. You will rank alkene stability, then predict the regiochemistry and stereochemistry (syn or anti) of hydrohalogenation, hydration, halogenation, epoxidation, ozonolysis, and alkyne reactions.
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What this chapter covers
- Structure, Stability & Hydrogenation. Before any addition reaction, know what the π bond looks like and which alkene is more stable.
- Hydrohalogenation & Carbocation Rearrangement. HX addition is the model electrophilic addition; Markovnikov's rule and hydride shifts appear constantly.
- Hydration of Alkenes. Three routes add H and OH across a C=C. The MCAT asks which one gives which alcohol.
- Halogenation, Epoxidation, Dihydroxylation & Ozonolysis. Each reagent here has a fixed stereochemical outcome (syn or anti) that MCAT questions test directly.
- Alkynes: Acidity, Addition & Reduction. Alkynes add twice, form ketones by hydration, and can be reduced to either cis or trans alkenes.
Key points to know
- Alkene C is sp², planar, 120°; alkyne C is sp, linear, 180°
- Stability: tetra > tri > di > mono; trans > cis
- HX addition goes through a carbocation; Markovnikov product
- Hydride and methyl shifts occur to reach a 3° cation
- Acid hydration: Markovnikov, rearrangements possible
- Oxymercuration-demercuration: Markovnikov, no rearrangement
- Br₂ addition is anti (bromonium ion)
- mCPBA epoxidation and OsO₄ dihydroxylation are syn
Try a practice question
What are the hybridization and geometry of the carbons in an alkyne?
- sp, linear (180°)
- sp², trigonal planar (120°)
- sp³, tetrahedral (109.5°)
- sp, bent (120°)
Show the answer
A. Each alkyne carbon uses one s and one p orbital for its σ framework, giving a linear 180° geometry. The two remaining p orbitals form the two π bonds.
Common questions
What does this chapter cover?
Alkenes & Alkynes covers Structure, Stability & Hydrogenation, Hydrohalogenation & Carbocation Rearrangement, Hydration of Alkenes, Halogenation, Epoxidation, Dihydroxylation & Ozonolysis, Alkynes: Acidity, Addition & Reduction.
What should I know first?
Alkene C is sp², planar, 120°; alkyne C is sp, linear, 180° Stability: tetra > tri > di > mono; trans > cis HX addition goes through a carbocation; Markovnikov product
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