Alkene addition reactions and conditions

Organic chemistry • Reactions of alkenes and alcohols

Flashcards

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What limits the rate of industrial steam hydration of ethene?

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Catalyst activity, steam pressure, temperature and contact time limit conversion efficiency.

Key concepts

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Hydrogenation of alkenes (addition of hydrogen)

Hydrogenation converts an alkene to an alkane by adding H2 across the C=C double bond. The π bond breaks and two new C–H σ bonds form. The process requires molecular hydrogen and a metal catalyst that adsorbs hydrogen and the alkene, allowing hydrogen atoms to add to the carbons. Nickel, palladium or platinum catalysts are common. Laboratory hydrogenation uses Pt or Pd at mild temperatures; industrial hydrogenation typically uses a nickel catalyst with elevated temperature to increase rate. Pressure of hydrogen increases the reaction rate. The reaction proceeds by surface-catalysed addition rather than by free radical pathways, so a catalyst and H2 are limiting factors.

Hydration of alkenes (addition of water)

Hydration forms an alcohol by adding H and OH across the C=C bond. Acid-catalysed electrophilic addition operates in two main contexts: laboratory acid hydration and industrial steam hydration. Laboratory hydration uses a strong acid (for example, concentrated sulfuric acid) to add H+ across the double bond to form a carbocation intermediate, followed by attack by water and deprotonation to give an alcohol. Industrial hydration uses ethene plus steam in the presence of an acid catalyst such as phosphoric acid on a solid support at high temperature and pressure to produce ethanol. Regiochemistry follows Markovnikov orientation for asymmetric alkenes: the hydrogen attaches to the carbon with more hydrogens already, and the OH attaches to the more substituted carbon because the more stable carbocation intermediate forms preferentially.

Halogenation of alkenes (addition of halogens)

Halogenation adds a halogen molecule (Br2 or Cl2) across a double bond to give a vicinal dihalide. The reaction proceeds by electrophilic addition: the π electrons polarise the halogen molecule, forming a cyclic halonium ion intermediate rather than a free carbocation, and then a halide ion attacks the more substituted carbon from the opposite side, resulting in anti addition. Conditions are mild: room temperature in an inert solvent such as carbon tetrachloride or an aqueous test mixture for bromine. No catalyst is required. The reaction of bromine with an alkene decolourises orange bromine solution to colourless, providing a practical test for unsaturation. Reaction rate depends on double-bond substitution pattern and the nucleophilicity of the halide.

Key notes

Important points to keep in mind

Electrophilic addition occurs because the π bond is electron-rich and easily attacked by electrophiles.

Hydrogenation requires H2 and a metal catalyst (Ni, Pt or Pd); pressure and temperature influence rate.

Hydration follows Markovnikov orientation; acid catalysts form carbocations that determine product placement.

Steam hydration uses phosphoric acid catalysts at high temperature and pressure industrially.

Halogenation uses X2 (Br2, Cl2) and proceeds through a cyclic halonium ion, giving anti addition.

Bromine water decolourisation provides a quick test for unsaturation (alkenes).

Carbocation intermediates allow rearrangements; monitor conditions to minimise undesired shifts.

Catalyst choice affects temperature requirements and reaction selectivity.

More substituted alkenes generally react faster in electrophilic addition due to greater carbocation stability.

Solvent and safety considerations influence reagent choice; avoid toxic solvents in practical work.

Regiochemistry depends on intermediate stability; consider both electronic and steric factors.

Industrial conditions (temperature, pressure, catalyst) differ from laboratory methods to optimise yield.