Hydrogenation is the addition of hydrogen across a C=C double bond. The alkene is converted into the corresponding alkane.CH2=CH2 + H2 → CH3CH3 The reaction needs a nickel catalyst. The alkene and hydrogen gases are passed over finely divided nickel at about 150°C. Finely divided nickel has a large surface area, which increases the rate. (Platinum or palladium catalysts work at room temperature but are far more expensive.)Every C=C bond in the molecule adds one H2. So one mole of a compound with two C=C bonds needs two moles of hydrogen to become fully saturated.Worked example: What volume of hydrogen at RTP is needed to fully hydrogenate 0.10 mol of buta-1,3-diene, CH2=CHCH=CH2?
Buta-1,3-diene has two C=C bonds, so it needs 2 × 0.10 = 0.20 mol H2.
Volume = 0.20 × 24 dm3 mol−1 = 4.8 dm3. The product is butane.Use in industry: vegetable oils contain many C=C bonds, which make them liquid at room temperature. Hydrogenating some of these bonds straightens the chains, so they pack more closely and the melting point rises. This 'hardening' turns oils into solid spreads such as margarine.
Buta-1,3-diene has two C=C bonds, so it needs 2 × 0.10 = 0.20 mol H2.
Volume = 0.20 × 24 dm3 mol−1 = 4.8 dm3. The product is butane.Use in industry: vegetable oils contain many C=C bonds, which make them liquid at room temperature. Hydrogenating some of these bonds straightens the chains, so they pack more closely and the melting point rises. This 'hardening' turns oils into solid spreads such as margarine.