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Alkene structures and addition-product formulas

Organic chemistry • Reactions of alkenes and alcohols

Key concepts

What you'll likely be quizzed about

  • Alkene molecules contain one carbon–carbon double bond and follow the general formula CnH2n for non-cyclic alkenes.
  • The double bond serves as the reactive site; breaking the pi bond leads to the formation of new single bonds.
  • This reaction transforms an unsaturated molecule into a saturated or more saturated product.

Flashcards

Test your knowledge with interactive flashcards

Describe stereochemical outcome for addition to a cis-alkene with bromine.

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Addition of bromine to a cis-alkene results in enantiomeric or meso products depending on substituents, as the bromonium ion facilitates anti addition across the former double bond.

Key notes

Important points to keep in mind

Alkenes follow CnH2n; count carbons to check formula correctness.

Draw double bond clearly as C=C; show all H or use condensed formulae (e.g., CH2=CHCH3).

Addition reactions break the pi bond; show new single bonds to the added atoms.

Hydration of unsymmetrical alkenes follows Markovnikov's rule: OH attaches to the more substituted carbon.

Halogen addition (Cl2, Br2, I2) yields vicinal dihalides (1,2-dihaloalkanes).

Bromine water decolourises in the presence of an alkene; that indicates addition has occurred.

In hydrogenation include a catalyst (Pt, Pd, Ni); show H2 as reagent and alkane as product.

Show cis/trans for but-2-ene where required; structural isomerism impacts physical properties.

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