Organic compounds which contain oxygen may contain alcohol, aldehyde, carboxylic acid, ester or ketone functional groups - CIE - A-Level Chemistry - Question 5 - 2012 - Paper 1

The reaction of X with sodium shows the presence of an alcohol functional group (-OH), as hydrogen is released when sodium reacts with alcohol.

To find the number of moles of hydrogen produced:

1. Calculate the number of moles of X: $n(X) = \frac{0.600 \text{ g}}{90 \text{ g/mol}} = 0.00667 \text{ mol}$
2. Given that 160 cm³ of H₂ is produced and at room temperature and pressure, this is equivalent to 0.0072 moles of H₂: $n(H₂) = \frac{160 \text{ cm³}}{24000 \text{ cm³/mol}} = 6.67 \times 10^{-3} \text{ mol}$
3. There are 2 hydrogen atoms per molecule of X for each -OH group: $n(H) = 2 \times n(X) = 2 \times 0.00667 \text{ mol} = 1.33 \times 10^{-2} \text{ mol}$

From the calculation in (ii), we found that the total moles of hydrogen atoms produced from 0.600 g of X is approximately 1.33 x 10^{-2} mol. This corresponds to two -OH groups, confirming that each molecule of X contains two functional groups since two -OH groups release two hydrogen atoms.

The reactions with Fehling's reagent and 2,4-dinitrophenylhydrazine indicate the presence of an aldehyde functional group (-CHO). The displayed formula is:

[ \text{H}_2C=O ]

Using the information gathered:

1. X contains an alcohol functional group (-OH) and an aldehyde functional group (-CHO).
2. The empirical formula is CH₂O, leading us to conclude that X must be glycolaldehyde: [ \text{HOCH}_2\text{CHO} ]

When treated with Fehling's reagent, the aldehyde group in X is oxidized to a carboxylic acid. Therefore, the structural formula of the organic product would be: [ \text{HOCH}_2\text{COOH} ]

The reduction of X using NaBH₄ will convert the aldehyde group into an alcohol, producing: [ \text{HOCH}_2\text{CH}_2\text{OH} ]

The oxidation of X under reflux conditions with acidified K₂Cr₂O₇ will enhance the conversion of the aldehyde to a carboxylic acid, yielding: [ \text{HOCH}_2\text{COOH} ]