CHEM20018 · Chemistry: Reactions And Synthesis
Carbohydrate Structure & Reactions
Carbohydrates apply the whole carbonyl toolkit to polyhydroxy aldehydes and ketones. You must move fluently between Fischer projections (open chain, with D/L assigned by the highest-numbered stereocentre) and Haworth projections (the cyclic hemiacetal). Key transformations are keto–enol epimerisation through a common ene-diol, reduction to an alditol, mild oxidation to an aldonic acid and strong oxidation to an aldaric acid, and the Kiliani–Fischer chain ascent. Cyclisation creates an anomeric centre (α/β anomers), and the ring-opening/closing equilibrium between them is mutarotation; locking the anomeric OH as an acetal makes a glycoside.
What this chapter covers
- 01Aldose vs ketose; triose → tetrose → pentose → hexose nomenclature
- 02Fischer projections and D/L assignment by the highest-numbered stereocentre OH (right = D)
- 03Epimerisation and aldose/ketose interconversion via the common ene-diol (keto–enol tautomerism)
- 04Redox of sugars: NaBH₄ → alditol; mild (Br₂/Tollens) → aldonic acid; strong (HNO₃) → aldaric acid
- 05Kiliani–Fischer ascent: add HCN, then hydrolyse/reduce → one-carbon-longer epimeric pair of aldoses
- 06Cyclic sugars: intramolecular hemiacetal → pyranose/furanose; the anomeric centre and α/β anomers
- 07Mutarotation: ring-opening/closing equilibrates the α and β anomers
- 08Glycoside (acetal) formation and 1,4-glycosidic links in di-/oligo-/polysaccharides
Ketose epimerisation, cyclisation and a disaccharide link
- 2 marks — ene-diol intermediate + D-glucose as the C2 epimer of D-mannose(a) Mild base enolises D-fructose at C1–C2 to a C1–C2 ene-diol. This single ene-diol can re-ketonise in three ways: back to D-fructose, or to either C2 epimer of the aldohexose. The two aldohexoses related as C2 epimers are D-glucose and D-mannose, so the other aldohexose is D-glucose.
- 2 marks — correct pyranose ring + β anomeric configuration(b) Cyclise by attacking the C1 aldehyde with the C5 hydroxyl to form a six-membered (pyranose) hemiacetal. The new stereocentre at C1 is the anomeric centre; for the β-anomer the anomeric OH points up (cis to the C6 CH₂OH) in the standard Haworth drawing of β-D-glucopyranose.
- 2 marks — name the acetal/glycosidic bond + the 1→4 regiochemistry(c) A β-1,4 link joins the anomeric carbon (C1) of one β-D-glucopyranose to the C4 hydroxyl of the next via an acetal (glycosidic) bond, with β stereochemistry at the linking anomeric centre.
- 1 mark — identify the cellobiose-type productThe resulting disaccharide is a cellobiose-type unit (the β-1,4-linked diglucose); both ring stereochemistry and the 1→4 regiochemistry must be shown explicitly.
Key terms
- Fischer projection
- A 2-D convention for drawing open-chain sugars with the carbonyl at top; horizontal bonds point toward the viewer, vertical bonds away.
- Anomeric centre
- The new stereocentre created at the former carbonyl carbon when a sugar cyclises to a hemiacetal; its configuration defines the α or β anomer.
- Mutarotation
- The equilibration of α and β anomers in solution via reversible ring-opening to the open-chain form, observed as a change in optical rotation toward an equilibrium value.
- Epimer
- Two diastereomeric sugars differing in configuration at exactly one stereocentre (e.g. D-glucose and D-mannose differ only at C2).
- Glycoside
- An acetal formed when the anomeric hydroxyl of a sugar is replaced by an –OR group, locking the anomeric configuration and preventing mutarotation.
Carbohydrate Structure & Reactions FAQ
How do I assign D or L to a sugar?
Find the highest-numbered stereocentre (the chiral carbon furthest from the carbonyl) in the Fischer projection. If its hydroxyl points to the right it is the D series; to the left, the L series. Almost all natural sugars are D.
Why do glucose and mannose interconvert in base but a glycoside does not mutarotate?
Base epimerisation needs an acidic α-H next to the carbonyl to form the ene-diol; that requires a free (hemiacetal/open-chain) carbonyl. A glycoside has its anomeric centre locked as an acetal with no free carbonyl, so it can neither open to the chain nor mutarotate.
What's the difference between an aldonic and an aldaric acid?
Mild oxidation (Br₂ water, Tollens) oxidises only the aldehyde end (C1) to a carboxylic acid → an aldonic acid. Strong oxidation (hot dilute HNO₃) oxidises both the C1 aldehyde and the terminal C6 primary alcohol → a diacid, the aldaric acid.
Exam move
The exam tests fluency in switching representations, so practise the conversions cold: Fischer → Haworth → chair, and back. Build the D-aldose family tree once by hand (it is supplied in the exam, but drawing it teaches the C2-epimer relationships). Group the reactions by what they touch: redox at the ends (alditol, aldonic, aldaric), epimerisation/interconversion through the ene-diol, chain ascent by Kiliani–Fischer, and cyclisation chemistry (anomers, mutarotation, glycosides, glycosidic links). For every cyclic-sugar question, fix ring size, then anomeric configuration, then regiochemistry of any glycosidic bond. This section connects directly back to the carbonyl chapters — treat sugars as polyfunctional aldehydes/ketones, not a separate topic.
