Biomolecules · Study notes

CHECarbohydrates

Carbohydrates: Classification & Definitions

Carbohydrates are classified by how many sugar units they release on hydrolysis and by whether they reduce Fehling's/Tollens'. This subtopic tests crisp definitions, the empirical formula, and the reducing-vs-non-reducing distinction with named examples.

General carbohydrate formula

C_x(H_2O)_y \qquad \text{e.g. glucose } C_6H_{12}O_6 = C_6(H_2O)_6

Many carbohydrates fit a 'hydrate of carbon' formula, though deoxy sugars (e.g. deoxyribose) are exceptions.

Sucrose hydrolysis (inversion)

C_{12}H_{22}O_{11} + H_2O \xrightarrow{H^+/\text{invertase}} \underset{\text{glucose}}{C_6H_{12}O_6} + \underset{\text{fructose}}{C_6H_{12}O_6}

Equimolar glucose + fructose = invert sugar.

Optical rotation on inversion

[\alpha]_{\text{sucrose}} = +66.5^\circ \rightarrow [\alpha]_{\text{mixture}} \approx -20^\circ

Strongly levorotatory fructose (

-92.4^\circ

) outweighs dextrorotatory glucose (

+52.7^\circ

), so net rotation becomes negative.

Carbohydrates are polyhydroxy aldehydes or ketones (or compounds that yield them on hydrolysis); many fit $C_x(H_2O)_y$ , e.g. glucose $C_6H_{12}O_6 = C_6(H_2O)_6$ .
Classes: monosaccharides (cannot be hydrolysed, e.g. glucose, fructose), oligosaccharides (2–10 units, e.g. sucrose, maltose, lactose), polysaccharides (e.g. starch, cellulose).
Aldoses contain a $\text{-}CHO$ group (glucose, galactose); ketoses contain a $>C=O$ ketone group (fructose).
Reducing sugars have a free anomeric carbon (free hemiacetal $\text{-}OH$ ) and reduce Tollens'/Fehling's — all monosaccharides, plus maltose and lactose; the common non-reducing sugar is sucrose.
Sucrose is non-reducing because both anomeric carbons (glucose C1 and fructose C2) are tied up in the glycosidic bond; maltose is reducing because one glucose retains a free anomeric $\text{-}OH$ .
Invert sugar: the equimolar glucose + fructose mixture from sucrose hydrolysis; the rotation inverts from $+66.5$ degrees (sucrose) to a net negative value because fructose is strongly levorotatory.
Glycosidic linkages: sucrose $\alpha,\beta\text{-}(1\rightarrow2)$ ; maltose $\alpha\text{-}(1\rightarrow4)$ between two glucose units; lactose $\beta\text{-}(1\rightarrow4)$ between galactose and glucose.
Sub-classification by carbon number: trioses (3C), tetroses (4C), pentoses (5C, e.g. ribose), hexoses (6C, e.g. glucose); names combine carbon count with aldo-/keto-.
Sugars vs non-sugars: mono- and oligosaccharides are crystalline, sweet and water-soluble ('sugars'); polysaccharides are amorphous, tasteless and largely insoluble ('non-sugars').
On complete hydrolysis: sucrose gives glucose + fructose; maltose gives 2 glucose; lactose gives glucose + galactose; starch and cellulose give many glucose units.
A glycosidic linkage is the C-O-C ether (acetal) bond formed when the anomeric $\text{-}OH$ of one sugar condenses with an $\text{-}OH$ of another, releasing water.
The configurational label D/L refers to the chiral carbon farthest from the carbonyl (compared with glyceraldehyde) and is independent of the sign of optical rotation (+/-).

Where the marks go

Confusing the D/L configuration label with the (+)/(-) sign of rotation — D-glucose happens to be dextrorotatory, but D and (+) are unrelated by definition.
Calling sucrose reducing — both anomeric carbons are engaged in the glycosidic bond, so it is the standard non-reducing disaccharide.
Treating 'invert sugar' as a new compound rather than the equimolar glucose+fructose mixture, and forgetting why the rotation sign flips (fructose is strongly levorotatory).
Stating cellulose or starch is hydrolysed to maltose only — complete hydrolysis gives glucose (maltose is an intermediate for starch).
Forgetting the linkage stereochemistry: maltose is $\alpha\text{-}1,4$ while lactose is $\beta\text{-}1,4$ ; getting $\alpha$ / $\beta$ wrong loses marks in digestibility questions.

How the board asks it

Define / statethe empirical formula and class definitions
Define the term 'carbohydrate' and explain why glucose can be written as $C_6(H_2O)_6$ . Name one example each of a monosaccharide, a disaccharide and a polysaccharide.
Give reasonsfree anomeric carbon and the glycosidic bond
Account for the fact that sucrose is a non-reducing sugar whereas maltose is a reducing sugar.
Distinguishreducing vs non-reducing sugars (Tollens'/Fehling's)
How will you chemically distinguish between glucose and sucrose? State the reagent used and the observation.
Predict the productcomplete hydrolysis of disaccharides
Name the products formed when (i) sucrose, (ii) lactose and (iii) maltose are completely hydrolysed.
Give reasonsinvert sugar and inversion of optical rotation
Why is the equimolar mixture of glucose and fructose obtained on hydrolysis of sucrose called 'invert sugar'? Give reasons.
Assertion–ReasonD/L configuration label vs the sign of rotation
Assertion: D-glucose is dextrorotatory. Reason: The letter D in D-glucose denotes its $(+)$ optical rotation. State whether each statement is true and whether the Reason correctly explains the Assertion.

Practise this topic

NextGlucose: Structure, Evidence & Tests

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.