Fats and oils are names for the same class of compounds, called triglycerides, which contain three ester groups in which \(\mathrm{R}, \mathrm{R}^{\prime},\) and \(\mathrm{R}^{\prime \prime}\) represent long hydrocarbon chains. (a) Suggest a reaction that leads to the formation of a triglyceride molecule, starting with glycerol and carboxylic acids (see p. 398 for structure of glycerol). (b) In the old days, soaps were made by hydrolyzing animal fat with lye (a sodium hydroxide solution). Write an equation for this reaction. (c) The difference between fats and oils is that at room temperature, the former are solid and the latter are liquids. Fats are usually produced by animals, whereas oils are commonly found in plants. The melting points of these substances are determined by the number of \(\mathrm{C}=\mathrm{C}\) bonds (or the extent of unsaturation) present- -the larger the number of \(\mathrm{C}=\mathrm{C}\) bonds, the lower the melting point and the more likely the substance is a liquid. Explain. (d) One way to convert liquid oil to solid fat is to hydrogenate the oil, a process by which some or all of the \(\mathrm{C}=\mathrm{C}\) bonds are converted to \(\mathrm{C}-\mathrm{C}\) bonds. This procedure prolongs shelf life of the oil by removing the more reactive \(\mathrm{C}=\mathrm{C}\) group and facilitates packaging. How would you carry out such a process (that is, what reagents and catalyst would you employ)? (e) The degree of unsaturation of oil can be determined by reacting the oil with iodine, which reacts with the \(\mathrm{C}=\mathrm{C}\) as follows: The procedure is to add a known amount of iodine to the oil and allow the reaction to go to completion. The amount of excess (unreacted) iodine is determined by titrating the remaining iodine with a standard sodium thiosulfate \(\left(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\right)\) solution: $$\mathrm{I}_{2}+2 \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3} \longrightarrow \mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}+2 \mathrm{NaI}$$ The number of grams of iodine that reacts with \(100 \mathrm{~g}\) of oil is called the iodine number. In one case, \(43.8 \mathrm{~g}\) of \(\mathrm{I}_{2}\) were treated with \(35.3 \mathrm{~g}\) of corn oil. The excess iodine required \(20.6 \mathrm{~mL}\) of \(0.142 \mathrm{M} \mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) for neutralization. Calculate the iodine number of the corn oil.

Step by step solution

01

Formation of a triglyceride

Triglycerides are esters derived from glycerol and three fatty acids. The chemical reaction for the formation of a triglyceride molecule from glycerol (C3H8O3) and three carboxylic acids (RCOOH) is: \[ \)C3H5(OH)3 + 3 RCOOH → RCOOC3H5O3 + 3 H2O\( \] The hydroxyl groups of the glycerol molecule react with the carboxylic acid groups to release water, thus forming an ester bond.

02

Saponification

Saponification or soap making involves the hydrolysis of animal fat with lye (a sodium hydroxide solution). The reaction is:\[ \)3 NaOH + RCOOC3H5O3 → C3H5(OH)3 + 3 RCOONa\( \] This reaction leads to the formation of glycerol and three molecules of a soap (sodium salt of the fatty acid).

03

Liquid vs Solid explanation

At room temperature, fats are solid and oils are liquid. This is because fats usually contain more saturated fatty acids which pack tightly together, making the substance solid. Oils, on the other hand, contain a higher amount of unsaturated fatty acids, which have kinks in their structure due to the double bonds. This prevents the fatty acid chains from packing tightly together, resulting in a liquid substance. Hence, the higher the number of C=C bonds (indicating unsaturation), the lower the melting point and the more likely the substance is to be a liquid.

04

Oil to fat transformation

Transforming oil to fat is achieved through hydrogenation. This is accomplished by reacting the oil with hydrogen gas, usually in the presence of a catalyst such as nickel. This process is capable of converting C=C bonds to C-C bonds, essentially reducing the degree of unsaturation and turning the liquid oil into a hardened fat. The reaction can be represented as:\[ \)RCOOC3H5 + H2 → R'COOC3H5 + H2O\( \] where R' represents a hydrogenated fatty acid chain.

05

Degree of unsaturation and iodine number calculation

The degree of unsaturation of an oil can be determined by its reaction with iodine. Each molecule of iodine reacts with one C=C bond in the oil. The oil's degree of unsaturation can then be measured by calculating the 'Iodine Number', which represents the amount of iodine (in grams) that reacts with 100g of oil. In the given case, 43.8g of I2 is treated with 35.3g of corn oil. The unreacted iodine is then titrated with a standard sodium thiosulfate solution. To calculate the Iodine number:First, calculate the millimoles of I2 reacted with Na2S2O3:(0.142 mol/L Na2S2O3) × (0.0206 L Na2S2O3) × (1 mol I2/2 mol Na2S2O3) = 1.47 mmol I2Then, subtract this from the total millimoles of I2 added to find the millimoles of I2 reacted with the oil:(43.8 g I2) × (1 mol I2/253.8 g I2) × 1000 = 173 mmol I2173 mmol I2 - 1.47 mmol I2 = 171.53 mmol I2Multiply by the molar mass of I2 to get the mass of I2 that reacted with the oil:171.53 mmol I2 × (253.8 g I2/1000 mmol I2) = 43.5 g I2Finally, divide this by the mass of the oil and multiply by 100 to get the Iodine number:(43.5 g I2)/(35.3 g oil) × 100 = 123.2 g I2/100g oil

06

Final Answer

The iodine number of the corn oil is approximately 123.2 g I2/100g oil.

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