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.
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