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Difficulty: MediumAlkanoic Acids, Esterification, Saponification, Fats, and Oils

Which of the following chemical changes occurs during the industrial conversion of a liquid vegetable oil into solid margarine via catalytic hydrogenation?

  1. A
    The ester linkages are hydrolyzed to yield glycerol and free fatty acids
  2. The degree of unsaturation decreases as carbon-carbon double bonds are converted to single bondsAnswer
  3. C
    The alkanoic acid chains undergo dehydration to produce gaseous alkynes
  4. D
    The triacylglycerol molecules are oxidized to form short-chain dicarboxylic acids

Answer

The degree of unsaturation decreases as carbon-carbon double bonds are converted to single bonds.
Vegetable oils are naturally liquid at room temperature due to their high degree of unsaturation (presence of multiple C=CC=C double bonds in the fatty acid tails). During industrial hydrogenation, hydrogen gas (H2H_2) is reacted with the oil using a nickel catalyst at elevated temperatures. This addition reaction converts unsaturated C=CC=C bonds into saturated C−CC-C single bonds, thereby decreasing the degree of unsaturation, increasing the packing efficiency of the molecules, and raising the melting point to produce solid margarine.

Step-by-Step Solution

1
Identify the chemical structure of vegetable oils
Vegetable oils are liquid triacylglycerols (esters of glycerol and long-chain fatty acids) containing a high proportion of unsaturated carbon-carbon double bonds (C=CC=C).
Liquid fats have lower melting points due to kinks created by double bonds in the hydrocarbon chains.
2
Determine the reaction mechanism of catalytic hydrogenation
Addition of hydrogen gas (H2H_2) in the presence of a finely divided nickel catalyst converts C=CC=C double bonds into C−CC-C single bonds.
Adding hydrogen saturates the hydrocarbon chains, causing them to pack more tightly and raising their melting point to solidify the oil into margarine.

Key Concept

Hardening of Oils and Hydrogenation of Unsaturated Lipids
Estimated Time:1m 0s
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Topics

Alkanoic Acids, Esterification, Saponification, Fats, and Oils

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