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Every idea in the Carbohydrates & Biomolecules chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 21 and download the PDF.

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Carbohydrates: classification and monosaccharides

Classification of carbohydrates
4× asked, last 2026
monosaccharide: no hydrolysisoligosaccharide: 2–10 unitspolysaccharide: many units\text{monosaccharide: no hydrolysis}\qquad \text{oligosaccharide: 2–10 units}\qquad \text{polysaccharide: many units}
Polyhydroxy aldehydes or ketones, or what hydrolyses to them. Oligosaccharides may give unlike units: sucrose, lactose.
Aldoses, ketoses and D/L configuration
9× asked, last 2026
glucose (aldohexose): OHC−(CHOH)X4−CHX2OH,fructose (ketohexose): HOCHX2−CO−(CHOH)X3−CHX2OH\text{glucose (aldohexose): }\ce{OHC-(CHOH)4-CH2OH},\qquad \text{fructose (ketohexose): }\ce{HOCH2-CO-(CHOH)3-CH2OH}
D or L: −OH\ce{-OH} on the lowest chiral carbon to the right or left in Fischer. D-glucose C2–C5: right, left, right, right.
AldotetroseOH at C2, C3+ HNO₃ → tartaric acidOptically active?
D-erythrosesame sidemeso✗
D-threoseopposite sides(2S,3S) or (2R,3R)✓
Built from each tetrose (both ends → COOH) and tested against its mirror image.
HOHROHHSHOHRHOHRCHOCH₂OHD-glucoseOHHSHOHROHHSOHHSCHOCH₂OHL-glucose=OOHHSHOHRHOHRCH₂OHCH₂OHD-fructosemirror
Fischer projections, R/S from CIP ranks (checked against the structures); C5 (shaded) sets D/L. Enumerating every R/S choice gives 16 aldohexoses and 8 ketohexoses, half D and half L
  • Open chain: glucose has 4 chiral C (C2–C5), so 24=162^4=16 stereoisomers, the aldohexoses (8 D, 8 L); fructose has 3 (C3–C5), so 23=82^3=8 (4 D, 4 L). Ring closure adds the anomeric C.
  • Not D = dextro. D/L is configuration, (+)/(−) is measured rotation: D-(−)-fructose. L-glucose is the full mirror image, so flip every OH, not just C5. D-fructose shares C3–C5 with D-glucose.
  • Quick method. Aldotetroses (CX4HX8OX4\ce{C4H8O4}, two chiral C) give tartaric acid with HNOX3\ce{HNO3}. Both OH on one side (erythrose) gives meso, inactive acid; opposite sides (threose) an active one. The lower OH then sets D or L.
Open-chain evidence for glucose
6× asked, last 2026
glucose→HI, Δn-hexane,→NHX2OHoxime,→HCNcyanohydrin→BrX2 waterHOOC−(CHOH)X4−CHX2OH (gluconic),→HNOX3HOOC−(CHOH)X4−COOH (saccharic)→(CHX3CO)X2Opentaacetate\begin{gathered}\text{glucose}\xrightarrow{\ce{HI},\ \Delta}n\text{-hexane},\qquad \xrightarrow{\ce{NH2OH}}\text{oxime},\qquad \xrightarrow{\ce{HCN}}\text{cyanohydrin}\\ \xrightarrow{\ce{Br2}\ \text{water}}\ce{HOOC-(CHOH)4-CH2OH}\ (\text{gluconic}),\qquad \xrightarrow{\ce{HNO3}}\ce{HOOC-(CHOH)4-COOH}\ (\text{saccharic})\\ \xrightarrow{\ce{(CH3CO)2O}}\text{pentaacetate}\end{gathered}
HI: six C in a straight chain. Oxime, cyanohydrin: a C=O. Gluconic acid: the C=O is CHO. Pentaacetate: five OH on different C.
ReagentProduct (formula from the structure)Shows
HI, Δn-hexane (C₆H₁₄)six C in an unbranched chain
NH₂OHoxime, C₆H₁₃NO₆a C=O is present
HCNcyanohydrin, C₇H₁₃NO₆a C=O is present
Br₂ watergluconic acid, C₆H₁₂O₇the C=O is a –CHO (one COOH)
HNO₃saccharic acid, C₆H₁₀O₈CH₂OH end also oxidised (two COOH)
(CH₃CO)₂Opentaacetate, C₁₆H₂₂O₁₁5 OH on different carbons
Every product formula worked out on the open-chain structure.
  • Not HNOX3\ce{HNO3} for gluconic acid. Mild BrX2\ce{Br2} water oxidises only the −CHO\ce{-CHO}; strong HNOX3\ce{HNO3} also oxidises the CHX2OH\ce{CH2OH} end (saccharic diacid). NaHCOX3\ce{NaHCO3} needs a COOH, so no reaction.
  • Quick method. Decode an unknown sugar: acetate count = number of OH; an acid with BrX2\ce{Br2} water = aldose; the HI product = carbon skeleton. A CX5HX10OX5\ce{C5H10O5} giving isopentane (HI) and an acid (BrX2\ce{Br2} water) is a branched aldopentose.
What the open chain cannot explain
4× asked, last 2026
No Schiff's or 2,4-DNP test, no NaHSOX3\ce{NaHSO3} adduct; pentaacetate gives no oxime. It dissolves via its five OH, not the CHO.
  • Not 'no aldehyde at all'. A little open chain stays in equilibrium, so glucose still reduces Tollens' and gives an oxime. But no 2,4-DNP, Schiff's or NaHSOX3\ce{NaHSO3} product; the ring-locked pentaacetate gives no oxime.
Cyclic glucose: anomers and mutarotation
7× asked, last 2026
α (+112∘) ⇌ open chain ⇌ β (+19∘),equilibrium mixture +52.5∘\alpha\ (+112^\circ)\ \rightleftharpoons\ \text{open chain}\ \rightleftharpoons\ \beta\ (+19^\circ),\qquad \text{equilibrium mixture}\ +52.5^\circ
C5–OH adds to the C1 CHO: six-membered pyranose hemiacetal. C1 becomes chiral; α and β differ only there (anomers).
O H O O H O H O H O H
α-D-glucopyranose
O H O O H O H O H O H
β-D-glucopyranose
AnomerC1 (from the structure)[α]D
α-D-glucopyranoseS+112°
β-D-glucopyranoseR+19°
Same constitution; the two structures differ only at C1 (CIP label read off each).
α 36%β 64%at +52.5°0.36 × 112 + 0.64 × 19 = 52.5
Equilibrium mixture worked out from the sheet’s rotations (open chain neglected)
  • α crystallises from a concentrated solution at 303 K, β from a hot one at 371 K; β melts higher (423 K vs 419 K). In water either changes to the +52.5° mixture: mutarotation.
  • Not anomers for glucose and galactose. They differ at C4 (epimers). Anomers differ only at C1.
  • Quick method. Haworth of a D-sugar: CHX2OH\ce{CH2OH} above the ring; each Fischer right-hand OH points down. Anomeric OH below the ring (trans to CHX2OH\ce{CH2OH}) = α, above = β.
Fructose
5× asked, last 2026
D-(−)-fructose: C2 ketone; C5–OH closes on C2 ⇒ furanose (five-membered ring)\text{D-(−)-fructose: C2 ketone; C5–OH closes on C2}\ \Rightarrow\ \text{furanose (five-membered ring)}
Ketohexose CX6HX12OX6\ce{C6H12O6}, straight six-carbon chain; −92.4°. In sucrose it is β-D-fructofuranose.
O H O O H O H O H O H
β-D-fructofuranose
  • Not non-reducing for lack of CHO. Tollens' and Fehling's are basic, and base turns fructose into glucose and mannose via an enediol. So fructose, like every monosaccharide, reduces both.

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15 more concepts in 5 topics

Disaccharides and polysaccharides; Amino acids; Peptides, proteins and enzymes; Vitamins; and Nucleic acids. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Disaccharides and polysaccharides

  • Glycosidic link and reducing sugars8×
  • Sucrose and invert sugar10×
  • Maltose and lactose13×
  • Starch, cellulose and glycogen9×

Amino acids

  • α-Amino acids: essential and types10×
  • Side chains and one-letter codes13×
  • Zwitterion, isoelectric point, chirality6×

Peptides, proteins and enzymes

  • Peptide bond and peptide counting17×
  • Levels of protein structure7×
  • Fibrous vs globular; denaturation8×
  • Enzymes7×

Vitamins

  • Fat-soluble vs water-soluble5×
  • Vitamin names and deficiency diseases8×

Nucleic acids

  • Nucleotides, DNA and RNA8×
  • Bases and base pairing6×

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