A sugar $X$ is a reducing sugar, has a molecular formula $C_6H_{12}O_6$, and gives a positive Seliwanoff's test. Sugar $X$ is:

Carbohydrates (saccharides) are polyhydroxy aldehydes or ketones, or compounds that yield these on hydrolysis. Classification by carbon number: trioses (C₃), tetroses (C₄), pentoses (C₅: ribose, deoxyribose), hexoses (C₆: glucose, fructose, galactose). By functional group: Aldoses (aldehyde group: -CHO at C-1). Ketoses (ketone group: C=O, usually at C-2). Aldoses: D-glucose (C-1 CHO), D-galactose, D-mannose, D-ribose, D-deoxyribose. Ketoses: D-fructose (C-2 keto), ribulose (C-2), xylulose. By degree of polymerisation: Monosaccharides: cannot be hydrolysed further. Disaccharides: 2 monosaccharides linked by glycosidic bond. Oligosaccharides: 3-10 units. Polysaccharides: many units (starch, cellulose, glycogen).

Reducing sugar: has a free aldehyde or ketone group (in open chain form or hemiacetal form that can open). Reduces: Fehling's reagent (Cu²⁺ → Cu₂O, brick-red precipitate). Benedict's reagent (Cu²⁺ → Cu₂O, brick-red). Tollens' reagent (Ag⁺ → Ag, silver mirror). All monosaccharides are reducing sugars (both aldoses and ketoses — ketoses undergo tautomerisation to aldose in alkaline conditions). Disaccharides: Reducing: maltose (glucose-glucose, one free anomeric OH), lactose (galactose-glucose, one free anomeric OH). Non-reducing: sucrose (glucose-fructose glycosidic bond between BOTH anomeric carbons, no free anomeric OH → cannot open ring → non-reducing). Trehalose: also non-reducing (both anomeric). Polysaccharides: starch and cellulose are technically reducing (one reducing end per chain) but effectively non-reducing due to tiny proportion.

Fehling's test: alkaline CuSO₄ + sodium potassium tartrate. Reducing sugar → brick-red Cu₂O precipitate. Positive: glucose, fructose, maltose, lactose. Negative: sucrose (non-reducing). Benedict's test: milder version (CuSO₄ + sodium carbonate + sodium citrate). Used in clinical labs (urine glucose testing). Tollens' test: [Ag(NH₃)₂]⁺ reduced to Ag (silver mirror). All reducing sugars positive. Seliwanoff's test: resorcinol + dilute HCl. Ketoses (fructose): cherry-red within 1-2 min. Aldoses (glucose): slow pale pink (10+ min). Mechanism: HCl converts ketohexose to HMF (5-hydroxymethylfurfural) rapidly; aldohexose converts slowly. Iodine test: Starch → deep blue-black with I₂/KI solution. Amylose (helical structure traps I₂ molecules). Glycogen → reddish-brown. Cellulose → no colour. Molisch test: any carbohydrate + H₂SO₄ (conc.) + α-naphthol → purple ring (general test for all carbohydrates).

D-Glucose: aldohexose, molecular formula C₆H₁₂O₆. Open chain (Fischer projection): CHO at C-1. OH groups: C-2(R), C-3(S), C-4(R), C-5(R). CH₂OH at C-6. The D-configuration determined by orientation of OH at C-5 (same side as D-glyceraldehyde). Ring form (pyranose): C-1 aldehyde attacks C-5 OH → forms 6-membered ring. Two anomers: α-D-glucose (OH at C-1 below plane in Haworth, axial in chair) and β-D-glucose (OH at C-1 above plane, equatorial). β-D-glucose is more stable (equatorial OH). Mutarotation: interconversion of α and β forms in solution via open-chain form. Equilibrium: 36% α, 64% β. Specific rotation: α = +112.2°, β = +18.7°, equilibrium = +52.5°. Glucose is the most abundant sugar in nature, primary fuel for cells (glycolysis).

D-Fructose: ketohexose, formula C₆H₁₂O₆ (isomer of glucose). Open chain: HOCH₂ at C-1, C=O at C-2, OH at C-3,4,5, CH₂OH at C-6. Ring form: C-2 ketone attacks C-5 OH → 5-membered furanose ring (fructofuranose). Also exists as 6-membered pyranose in free form but furanose predominates in disaccharides. In sucrose: fructose exists as β-D-fructofuranose linked through anomeric C-2. Fructose is the sweetest of all common sugars (1.73× sweeter than sucrose). Found in: fruits (fruit sugar), honey (50% fructose), HFCS (high fructose corn syrup). Metabolism: fructose bypasses the key regulatory step of glycolysis (phosphofructokinase) → rapidly converted to fat → role in obesity and metabolic syndrome when consumed in excess.

Maltose (malt sugar): glucose-α(1→4)-glucose. One free anomeric OH (C-1 of second glucose) → reducing sugar. Forms from starch hydrolysis (amylase). Used in brewing. Cellobiose: glucose-β(1→4)-glucose. From cellulose hydrolysis. Reducing sugar. Lactose (milk sugar): galactose-β(1→4)-glucose. Reducing sugar. Found in milk (4-5%). Lactase enzyme digests it (lactase deficiency → lactose intolerance → gas, bloating). Sucrose (table sugar): glucose-α(1→2β)-fructose. NON-reducing (anomeric carbons of both sugars involved in glycosidic bond). Hydrolysis (invertase/sucrase) → glucose + fructose = "invert sugar" (sweeter than sucrose since fructose > sucrose sweetness). Trehalose: glucose-α(1→1α)-glucose. Non-reducing. Found in insects, fungi, yeast. Stabilises proteins in anhydrous conditions.

Starch: reserve polysaccharide in plants. Two components: Amylose: linear, α(1→4) linkages, helical structure, MW ~50,000. Gives blue-black with iodine. Amylopectin: branched, α(1→4) main chain + α(1→6) at branch points every 24-30 glucose units, MW ~10⁶. Gives reddish-purple with iodine. Glycogen: animal reserve polysaccharide (liver, muscles). Like amylopectin but more branched (branch every 8-12 glucose units). Faster mobilisation during exercise. Red-brown with iodine. Cellulose: structural polysaccharide in plant cell walls. β(1→4) linkages → extended chain → H-bonds between adjacent chains → microfibrils → rigid structure. Cannot be digested by humans (no cellulase). Dietary fiber. Chitin: β(1→4)-N-acetylglucosamine. Second most abundant natural polymer. Insect exoskeleton, fungal cell walls. Inulin: polymer of fructose. Dissolved in food → prebiotic fiber. Hyaluronic acid: glycosaminoglycan in connective tissue, synovial fluid, vitreous humour. Heparin: anticoagulant glycosaminoglycan.

Optical isomers (enantiomers): non-superimposable mirror images. One has +R, other has -S configuration at all chiral centres. D/L notation: based on configuration at highest-numbered asymmetric carbon (reference: D and L glyceraldehyde). D-sugars: OH at highest asymmetric C on right in Fischer projection. All naturally occurring sugars are D-form. (+)/(-) notation: refers to direction of rotation of plane-polarised light. (+) = dextrorotatory. (-) = laevorotatory. D and (+) are NOT the same! D-glucose is D(+)-glucose (dextrorotatory). D-fructose is D(-)-fructose (laevorotatory despite D-configuration). Mutarotation: change in optical rotation on dissolving a pure anomer in water → equilibrium mixture of α and β forms. D-Glucose: α = +112.2°, β = +18.7°, equilibrium = +52.5°. Measured with polarimeter. Important for studying: enzyme kinetics, structural chemistry, pharmaceutical analysis of optically active drugs.