Describe the properties of alkenes in terms of addition reactions with: (a) bromine or aqueous bromine (b) hydrogen in the presence of a nickel catalyst (c) steam in the presence of an acid catalyst and draw the structural or displayed formulae of th

Organic Chemistry – Alkenes

Objective

Describe the properties of alkenes in terms of addition reactions with: (a) bromine or aqueous bromine, (b) hydrogen in the presence of a nickel catalyst, and (c) steam in the presence of an acid catalyst. Draw the structural or displayed formulae of the products.

(a) Bromine (Br₂) or Aqueous Bromine (Br₂/H₂O)

Alkenes act like a double bridge that can be split by adding two new “poles” (bromine atoms). The double bond is broken and each carbon gets one bromine atom.

  1. Alkene (C=C) approaches Br₂.

  2. Br₂ forms a cyclic bromonium ion intermediate.

  3. Br⁻ attacks the more substituted carbon, opening the ring.

  4. Result: a vicinal dibromide.

🔬 Example with ethene:

\$\ce{CH2=CH2 + Br2 -> CH2Br-CH2Br}\$

🧪 In aqueous Br₂, water adds across the double bond (Markovnikov addition) giving a bromo‑alcohol:

\$\ce{CH2=CH2 + Br2 + H2O -> CH2Br-CH(OH)Br}\$

(b) Hydrogen (H₂) with Nickel Catalyst (Hydrogenation)

Think of the alkene as a bridge that needs a new road. Nickel acts like a construction crew that helps lay down a straight, single bond.

  1. H₂ gas is adsorbed onto the nickel surface.

  2. Alkene also adsorbs onto the surface.

  3. Hydrogen atoms add across the double bond.

  4. Result: a saturated alkane.

🚧 Example with ethene:

\$\ce{CH2=CH2 + H2 -> CH3-CH3}\$

(c) Steam (H₂O) with Acid Catalyst (Hydration)

Imagine the alkene as a bridge that can be flooded. Acid catalysis helps water “jump” onto the bridge, forming an alcohol.

  1. Proton (H⁺) from the acid adds to the less substituted carbon.

  2. Water attacks the more substituted carbon, forming a carbocation intermediate.

  3. Water then deprotonates to give the alcohol.

🌊 Example with ethene:

\$\ce{CH2=CH2 + H2O + H+ -> CH3-CH2OH}\$

Markovnikov’s rule: the H attaches to the carbon with more hydrogens; the OH attaches to the more substituted carbon.

Summary Table

ReactionConditionsProductAnalogy
Alkene + Br₂Br₂ (dry)\$\ce{R-CH=CH-R' + Br2 -> R-CHBr-CHBr-R'}\$“Cutting the double bridge with two new poles”
Alkene + Br₂ + H₂OAqueous Br₂ (acidic)\$\ce{R-CH=CH-R' + Br2 + H2O -> R-CHBr-CH(OH)Br-R'}\$“Adding a water bridge to the split”
Alkene + H₂ (Ni catalyst)Nickel catalyst, heat\$\ce{R-CH=CH-R' + H2 -> R-CH2-CH2-R'}\$“Building a straight road across the bridge”
Alkene + H₂O (Acid catalyst)Acid (e.g., H₂SO₄), steam\$\ce{R-CH=CH-R' + H2O + H+ -> R-CH(OH)-CH2-R'}\$“Flooding the bridge to create an alcohol”