Alkene structures and addition-product formulas

Organic chemistry • Reactions of alkenes and alcohols

Flashcards

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What is the general formula for non-cyclic alkenes?

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The general formula is CnH2n.

Key concepts

What you'll likely be quizzed about

Definition and general formula

Alkene molecules contain one carbon–carbon double bond and follow the general formula CnH2n for non-cyclic alkenes. The double bond is the reactive site because breaking the pi bond allows two new single bonds to form. Reaction at the double bond converts an unsaturated molecule into a saturated or more-saturated product.

First four alkenes - structure and formula

Ethene (C2H4): fully-displayed structural formula shows two carbons joined by a double bond and two hydrogens on each carbon. Condensed: CH2=CH2. Propene (C3H6): one double bond between C1 and C2; condensed: CH2=CHCH3. But-1-ene (C4H8): double bond at the end of the chain; condensed: CH2=CHCH2CH3. But-2-ene (C4H8): double bond between C2 and C3; condensed: CH3CH=CHCH3. But-2-ene exists as cis and trans isomers when different groups are attached to the carbons of the double bond.

Cause → effect: why addition reactions occur

High electron density in the pi bond attracts electrophiles (positive or electron-poor species). Electrophile attack causes the pi bond to break (effect) and creates positively charged intermediates that are stabilised by the addition of nucleophiles or hydrogen. The overall effect is the addition of atoms across the former double bond, converting it into a single bond.

Hydrogenation (addition of H2)

Hydrogen molecules add across the double bond in the presence of a metal catalyst (e.g., Pt, Pd or Ni). The pi bond breaks and each carbon gains one hydrogen atom. Example: CH2=CH2 + H2 → CH3CH3 (ethane). The product is an alkane with the same carbon skeleton.

Hydration (addition of H2O)

Water adds across the double bond under acidic conditions or using catalysts, forming an alcohol. The OH group attaches to the more substituted carbon in unsymmetrical alkenes (Markovnikov addition) because the more substituted carbocation intermediate is more stable. Example: CH2=CHCH3 + H2O → CH3CH(OH)CH3 (propan-2-ol) as the major product.

Halogenation (addition of Cl2, Br2, I2)

Diatomic halogens add across the double bond to give vicinal (1,2-) dihalogenoalkanes. The pi bond induces a polarised halogen approach and forms a cyclic halonium ion intermediate or a bridged intermediate for bromine and chlorine. Example: CH2=CH2 + Br2 → BrCH2CH2Br (1,2-dibromoethane). Addition of iodine gives diiodo products analogously, though I2 is less reactive.

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 rule: OH attaches to the more substituted carbon.

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

Bromine water decolourises when an alkene is present; 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 affects physical properties.