Topic 5 · page 6 of 15

Adding hydrogen halides and steam

A hydrogen halide or steam adds H to one carbon of the C=C and X or OH to the other. A symmetrical alkene gives one product; an unsymmetrical alkene gives a major and a minor product.

14 min read5.45.4(iii)5.4(iv)

Read first: Adding hydrogen and halogens

Hydrogen halides 5.45.4(iii)

Hydrogen chloride, hydrogen bromide and hydrogen iodide all add across a C=C at room temperature. One hydrogen atom adds to one carbon and the halogen atom adds to the other, so the product is a mono-substituted halogenoalkane: it has one halogen atom.

CHX2=CHX2+HBr→CHX3CHX2Br \ce{CH2=CH2 + HBr -> CH3CH2Br}

Ethene and HBr give bromoethane. HCl gives chloroethane, and HI gives iodoethane.

Symmetrical and unsymmetrical alkenes

An alkene is symmetrical if both carbons of the C=C carry the same pair of groups, so the molecule looks the same from both ends. Ethene, but-2-ene and cyclohexene are symmetrical. It is unsymmetrical if the two carbons carry different groups: propene, but-1-ene and 2-methylpropene.

Skeletal formulae. Propene, an unsymmetrical alkene, reacts with hydrogen bromide to give two possible products: 2-bromopropane, the major product, and 1-bromopropane, the minor product. The bromine atom is drawn in orange.
Propene is unsymmetrical, so HBr can give two products. 2-bromopropane is made in larger amounts.

Propene and HBr give both 2-bromopropane and 1-bromopropane, but they are not made in equal amounts. 2-bromopropane is the major product and 1-bromopropane the minor product. The reason is the stability of the carbocation intermediate in the mechanism (Carbocation stability and Propene and hydrogen bromide).

Worked examplePredicting the products

  1. But-2-ene + HBr. Symmetrical: one product, CH₃CH₂CHBrCH₃, 2-bromobutane.
  2. But-1-ene + HBr. Unsymmetrical: two products. Br on C2 gives 2-bromobutane (major); Br on C1 gives 1-bromobutane (minor).
  3. 2-methylpropene + HBr. Unsymmetrical: Br on the middle carbon gives (CH₃)₃CBr, 2-bromo-2-methylpropane (major); Br on the end carbon gives (CH₃)₂CHCH₂Br, 1-bromo-2-methylpropane (minor).

Steam 5.45.4(iv)

Steam adds across the C=C to make an alcohol. This is called hydration.

CHX2=CHX2(g)+HX2O(g)⇌CHX3CHX2OH(g) \ce{CH2=CH2(g) + H2O(g) <=> CH3CH2OH(g)}

This is how most ethanol for industry is made. It is the other route to ethanol alongside fermentation of sugar crops, which you met as bioethanol in Topic 4: hydration of ethene uses crude oil as the raw material, while fermentation uses plants.

Skeletal formulae. Propene reacts with steam, with an acid catalyst, to give mainly propan-2-ol and a little propan-1-ol. The OH group is drawn in orange.
Propene + steam gives mostly propan-2-ol, for the same reason as with HBr.

Propene is unsymmetrical, so steam gives two alcohols: propan-2-ol (major) and propan-1-ol (minor). The reason for the major product is the same as in the reaction with HBr.

Check yourself

  1. Write an equation for ethene reacting with hydrogen chloride and name the product.
  2. Explain why but-2-ene gives only one product with HBr, but but-1-ene gives two.
  3. Name the major product of the reaction between propene and steam, and give the conditions.
  4. Write an equation for the reaction of ethene with steam and name the catalyst. Why does the equation use ⇌?
  5. Give the structural formula and name of the major product when 2-methylpropene reacts with HBr.
  6. A student writes "ethene + water → ethanol, using a nickel catalyst". Give the two errors.

Answers cover these until you have tried the questions

  1. CH₂=CH₂ + HCl → CH₃CH₂Cl, chloroethane.
  2. But-2-ene is symmetrical: each carbon of the C=C has one CH₃ and one H, so H adding to either carbon gives the same compound, 2-bromobutane. But-1-ene is unsymmetrical (CH₂= at one end, CH(C₂H₅)= at the other), so Br can end up on C1 or on C2, giving 1-bromobutane and 2-bromobutane.
  3. Propan-2-ol (CH₃CH(OH)CH₃). Conditions: steam, a phosphoric acid catalyst, about 300 °C and 60 atm.
  4. CH₂=CH₂(g) + H₂O(g) ⇌ CH₃CH₂OH(g); the catalyst is phosphoric acid. It uses ⇌ because the reaction is reversible: ethanol can break down again into ethene and steam.
  5. H adds to the CH₂ carbon and Br to the carbon with no H: (CH₃)₃CBr, 2-bromo-2-methylpropane.
  6. (1) The reagent should be steam (water vapour), not liquid water. (2) The catalyst is phosphoric acid, not nickel; nickel is used with hydrogen. (The equation also needs ⇌, and the conditions are about 300 °C and 60 atm.)