Hydrogenation of Alkenes Chemistry Tutorial

Key Concepts

• Alkenes are unsaturated hydrocarbons.
• The double bond (C=C) is the active site on the molecule.
• Hydrogenation is a reaction in which hydrogen, H₂, adds across the double bond.
• Hydrogenation of alkenes is a reduction reaction.
• Hydrogenation of alkenes requires a catalyst, so it is also referred to as "catalytic hydrogenation".
• In general, the hydrogenation of an alkene can represented as shown below:

  alkene	+	hydrogen gas	catalyst →	alkane
  R           R"'   \       /       C = C       /       \   R'           R"	+	H2(g)	catalyst →	H |   H |   R− C − C −R"'   | R'   | R"
  R, R', R", and R"' can be hydrogen atoms or alkyl groups.

• Hydrogenation is a chemical reaction which can convert an unsaturated hydrocarbon into a saturated hydrocarbon.

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Catalytic Hydrogenation of Alkenes

If you mix an alkene with hydrogen gas at room temperature and pressure no reaction will be observed:

The activation energy for the reaction between an alkene and hydrogen gas is said to be very high.
However, we can use a suitable catalyst to lower the activation energy so that the alkene molecules will react with the hydrogen gas molecules.
This reaction between an alkene and hydrogen gas is referred to as a hydrogenation reaction.
In a hydrogenation reaction hydrogen (H₂) adds across the double bond in the alkene (C=C):

Because a suitable catalyst is required for the hydrogenation reaction it is also referred to as a catalytic hydrogenation.

Suitable catalysts for the hydrogenation of alkenes are usually metals⁽¹⁾: nickel (Ni), or palladium (Pd), or platinum (Pt).

Note that the alkene is an unsaturated hydrocarbon, but with the addition of hydrogen across the double bond it is converted to a saturated hydrocarbon, an alkane.

Examples of Catalytic Hydrogenation of Alkenes

Ethylene (ethene)⁽²⁾, H₂C=CH₂, is an alkene, an unsaturated hydrocarbon. It will undergo catalytic hydrogenation in the presence of a nickel catalyst, Ni. The product of this reaction is ethane, CH₃−CH₃ which is an alkane, a saturated hydrocarbon.

But-1-ene (1-butene)⁽³⁾, CH₃CH₂CH=CH₂, is an alkene, an unsaturated hydrocarbon. It will react with hydrogen gas, H₂(g), in the presence of a palladium catalyst (Pd).
The product of this hydrogenation reaction is butane, CH₃CH₂CH₂CH₃, which is an alkane, a saturated hydrocabon.

But-2-ene (2-butene), CH₃CH=CHCH₃, is an alkene, an unsaturated hydrocarbon. It will react with hydrogen gas, H₂(g), in the presence of a palladium catalyst (Pd).
The product of this hydrogenation reaction is also butane, CH₃CH₂CH₂CH₃, which is an alkane, a saturated hydrocarbon.

Summary Table for Reactions of Alkenes with Hydrogen

When an alkene (unsaturated hydrocarbon) reacts with hydrogen gas in the presence of a suitable catalyst, an alkane (saturated hydrocarbon) is produced.

The table below lists the catalytic hydrogenations for some straight-chain alkenes.