Chemistry 9701 · AS & A Level · Atoms, molecules and stoichiometry

Atoms, molecules and stoichiometry — practice question

Animal fats and vegetable oils are generally glyceryl esters, meaning esters formed from glycerol, propane-1,2,3-triol. In many animal fats, the ester groups derive from stearic acid, $\text{CH}_3(\text{CH}_2)_{16}\text{CO}_2\text{H}$. By contrast, vegetable oils commonly contain esters of oleic acid, $\text{CH}_3(\text{CH}_2)_7\text{CH}=\text{CH}(\text{CH}_2)_7\text{CO}_2\text{H}$.
(a)[1]

Draw the structural formula of the glyceryl ester produced when one molecule of glycerol is fully esterified with stearic acid.

(b)[1]

In a school or college laboratory, which reagent(s) would you use to obtain a small sample of oleic acid, $\text{C}_{17}\text{H}_{33}\text{CO}_2\text{H}$, from the glyceryl ester in a vegetable oil?

(c)[1]

Using this formula, draw the structural formula of elaidic acid, and show its stereochemistry clearly.

(d(i))

Suggest what the term polyunsaturated fatty acid means.

(d(ii))[3]

What reagent and what condition(s) are needed for the hydrogenation of an unsaturated fatty acid?

(e(i))

Give the structural formulae for the two aldehydes obtained when the unsaturated fat below undergoes partial oxidation. In the structure, $X$ stands for the rest of the fat molecule: $\text{CH}_3(\text{CH}_2)_7\text{CH}=\text{CH}(\text{CH}_2)_7X$.

(e(ii))

Name the reagent you would use to show that the product contained either an aldehyde or a ketone. What change would be observed?

(e(iii))[6]

What reagent would you use to confirm the presence of an aldehyde? What change would be observed?

(f(i))

How many chiral carbon atoms are in one molecule of ascorbic acid? If there are none, write 'none'.

(f(ii))[2]

Ascorbic acid contains three functional groups. Two of them are alcohol (primary and secondary) and alkene. What is the name of the third functional group?

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: Triglyceride structure with all three alcohol groups esterified

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