(a) Name: (i) one structural isomer of glucose. (ii) the process by which starch is converted to glucose. (b) The open-chain structure of glucose is shown b...

Answer Details

(a)(i) Structural isomer of glucose.

Fructose. Both glucose and fructose share the molecular formula \(C_6H_{12}O_6\) but differ in structure: glucose is an aldohexose (carries an aldehyde, \(-CHO\), group) while fructose is a ketohexose (carries a ketone, \(>C=O\), group). Having the same molecular formula but a different arrangement of atoms makes them structural isomers.

(a)(ii) Process converting starch to glucose.

Hydrolysis (either acid hydrolysis using dilute acid, or enzymatic hydrolysis using the enzyme amylase/diastase). Water molecules split the long starch chain at its glycosidic linkages to release individual glucose units.

(b)(i) Functional groups present in the structure.

Reading the open-chain structure shown, the top carbon carries \(H-C=O\), and each of the remaining carbons carries an \(-O-H\) group (four secondary \(-OH\) groups plus a terminal \(-CH_2OH\)). Therefore the functional groups are:

• the aldehyde group, \(-CHO\) (one, at the top of the chain);
• the hydroxyl group, \(-OH\) (alcohol groups, several along the chain).

(b)(ii) Group that reacts with warm Fehling's solution.

The aldehyde group, \(-CHO\). Warm Fehling's solution (a blue \(Cu^{2+}\) complex) is reduced by the aldehyde group; the copper is reduced from \(Cu^{2+}\) to \(Cu^{+}\), depositing a brick-red precipitate of copper(I) oxide, \(Cu_2O\), while the aldehyde is oxidised to a carboxylic acid group. This is why glucose is classed as a reducing sugar. The hydroxyl groups do not give this test.