Calculate the number of carbon carbon double bonds in an unsaturated fat / triglyceride


A C=C double bond in a unsaturated fat can be classified as either being a cis or trans structural isomers. A structural isomer is where the atoms are arranged in different positions. In the cis isomer below both H atoms are on the same side of the double bond. In the trans isomer the H atoms are on opposite sides of the double bond.

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Addition reactions

C=C double bonds (612 kJ/mol) are more reactive than C-C single bonds (348 kJ/mol). This is evident in the average bond enthalpies. The reactive double bond undergoes addition reactions where reactive molecules like iodine add across the double bond. The reaction below shows the addition of iodine across the double bond of an alkene molecule.
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Determining the number of C=C bonds
Addition of iodine to the C=C double bond in unsaturated fats can be used to experimentally determine the number of C=C bonds. The iodine number is defined as the number of grams of iodine that reacts with 100g of a triglyceride/fat in an addition reaction. The larger the iodine number the more unsaturated the fat.

The amount of unsaturatation can be found by determining the number of moles of iodine that react with one mole (or a stated mass, in which case you would convert to moles) of fat. Iodine adds across the double bond in a 1:1 ratio. In other words 1 mole of iodine (I2) molecules will react with 1 mole of double bonds between C atom. Iodine is colored and the purple color of the iodine will disappear when the reaction has occurred.

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