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25.18: Condensation Reactions

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    54011
  • Vegetable oils are being explored for a variety of uses in which they could replace petroleum products. One such application is in the field of lubricants. Every moving part in machinery (such as engine pistons) needs lubrication to decrease friction and prolong the life of the equipment. Petroleum products serve this purpose now, but are not good for the environment. New techniques for making specialized esters from vegetable oil are being explored that will make the compounds more stable and more useful as lubricants.

    Condensation Reactions

    condensation reaction is a reaction in which two molecules combine to form a single molecule. A small molecule, often water, is usually removed during a condensation reaction. Amino acids are important biological molecules that have an amine functional group on one end of the molecule and a carboxylic acid functional group on the other end. When two amino acids combine in a condensation reaction, a covalent bond forms between the amine nitrogen of one amino acid and the carboxyl carbon of the second amino acid. A molecule of water is then removed as a second product.

    Structure of a generic condensation reaction

    Figure 15.18.1: Amino acids join together to form a molecule called a dipeptide. The \(\ce{-OH}\) from the carboxyl group of one amino acid combines with a hydrogen atom from the amine group of the other amino acid to produce water (blue).

    This reaction forms a molecule called a dipeptide and the carbon-nitrogen covalent bond is called a peptide bond. When repeated numerous times, a lengthy molecule called a protein is eventually produced.

    Esterification

    An esterification is a condensation reaction in which an ester is formed from an alcohol and a carboxylic acid. Esterification is a subcategory of condensation reactions because a water molecule is produced in the reaction. The reaction is catalyzed by a strong acid, usually sulfuric acid. When the carboxylic acid butanoic acid is heated with an excess of methanol and a few drops of sulfuric acid, the ester methyl butanoate is produced. Methyl butanoate has the scent of pineapples. The reaction is shown below with both molecular and structural formulas.

    Structure of an esterification reaction

    The esterification reaction is reversible. When an ester is heated in the presence of a strong base such as sodium hydroxide, the ester breaks down. The products are an alcohol and the conjugate base of the carboxylic acid as a salt.

    \[\begin{array}{ccccccc} \ce{CH_3COOCH_2CH_3} & + & \ce{NaOH} & \rightarrow & \ce{CH_3COO^- Na^+} & + & \ce{CH_3CH_2OH} \\ \text{ethyl ethanoate} & & & & \text{sodium acetate} & & \text{ethanol} \end{array}\]

    The sodium hydroxide is not acting as a catalyst, but is consumed in the reaction.

    Saponification describes the alkaline hydrolysis reaction of an ester. The term saponification originally described the hydrolysis of long-chain esters called fatty acid esters to produce soap molecules, which are the salts of fatty acids. One such soap molecule is sodium stearate, formed from the hydrolysis of ethyl stearate.

    \[\begin{array}{ccccccc} \ce{C_{17}H_{35}COOC_2H_5} & + & \ce{NaOH} & \rightarrow & \ce{C_{17}H_{35}COO^- Na^+} & + & \ce{C_2H_5OH} \\ \text{ethyl stearate} & & & & \text{sodium stearate (soap)} & & \end{array}\]

    Contributors

    • CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.