Carbohydrates

• The definition of carbohydrates as polyhydroxy aldehydes/ketones or their polymers
• Open-chain structures of glucose (aldohexose) and fructose (ketohexose) with Fischer projections
• Haworth projections of α-D-glucose and β-D-glucose, and the concept of anomers
• Mutarotation — why optical rotation of glucose changes in solution
• Structures and glycosidic linkages in sucrose, maltose, and lactose
• Polysaccharides: starch (amylose + amylopectin), cellulose, glycogen
• How to distinguish reducing sugars from non-reducing sugars

What are Carbohydrates?

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that hydrolyse to give them. The empirical formula is often Cₙ(H₂O)ₙ, though this does not always hold (e.g., deoxyribose C₅H₁₀O₄).

Classification:

Sugars with an aldehyde group are aldoses; those with a keto group are ketoses. By carbon count: triose (C3), pentose (C5), hexose (C6).

Glucose — Structure and Properties

Molecular formula: C₆H₁₂O₆ (aldohexose, molecular mass = 180 g/mol)

Open-chain (Fischer projection) — key facts:

• C1: aldehyde (—CHO)
• C2–C5: chiral carbons with —OH groups
• C6: —CH₂OH (primary alcohol)
• Four chiral centres → 2⁴ = 16 possible stereoisomers

        CHO         ← C1 (aldehyde)
        |
   H — C — OH      ← C2
        |
   HO — C — H      ← C3
        |
   H — C — OH      ← C4
        |
   H — C — OH      ← C5
        |
       CH₂OH        ← C6

D-Configuration: —OH on C5 is on the right in Fischer projection (reference to D-glyceraldehyde).

Key chemical tests:

• Tollens' test: Glucose + [Ag(NH₃)₂]⁺ → silver mirror (glucose is a reducing sugar)
• Fehling's test: Glucose + Cu²⁺ (alkaline) → brick-red Cu₂O precipitate
• Glucose is a reducing sugar because it has a free aldehyde group (or free anomeric —OH in ring form).

Fructose — Structure

Molecular formula: C₆H₁₂O₆ (isomer of glucose; ketohexose)

• C2: keto group (C=O)
• C1: —CH₂OH
• C3–C5: chiral —OH groups
• C6: —CH₂OH
• Fructose is also a reducing sugar (forms enediol intermediate under alkaline Fehling/Tollens conditions)

Mnemonic: "Fructose = Found in Fruit, Forms a Five-membered ring (furanose)"

Haworth Projections — Cyclic Forms of Glucose

When glucose cyclises, C1 (aldehyde) reacts with the —OH on C5 to form a six-membered pyranose ring (glucopyranose). The new —OH at C1 is the anomeric hydroxyl.

• α-D-Glucose: —OH at C1 is below the ring plane (same side as CH₂OH is above in D-series)
• β-D-Glucose: —OH at C1 is above the ring plane (opposite to the D convention, trans to C6 —CH₂OH)

α-D-Glucopyranose:
         OH
          |
    6    1
  HOCH₂  C—OH  ← anomeric OH below (α)
      \  /
       O
      / \
    C5   C2
   H|    |OH
    C4   C3
   HO|    |H
    (OH)(OH)

Haworth rule of thumb: Groups on the right in Fischer projection go down in Haworth; groups on the left go up.

Mutarotation

When crystalline α-D-glucose (specific rotation +112°) is dissolved in water, the optical rotation gradually changes to an equilibrium value of +52.7°. Pure β-D-glucose (+19°) also reaches the same equilibrium.

Cause: In solution, the ring opens to the open-chain aldehyde form and re-closes randomly as α or β anomer. The equilibrium mixture is ~36% α and ~64% β.

α-D-Glucose (+112°) ⇌ Open-chain aldehyde ⇌ β-D-Glucose (+19°)
        ↑________________________equilibrium = +52.7°_______________↑

α and β forms are called anomers; they differ only in configuration at C1 (the anomeric carbon).

Disaccharides

Disaccharides form when two monosaccharides join via a glycosidic bond (—O— linkage between anomeric carbon of one sugar and —OH of another, with loss of water).

Why is sucrose non-reducing? Both anomeric carbons (C1 of glucose and C2 of fructose) are involved in the glycosidic bond — there is no free anomeric —OH to open the ring and reduce Cu²⁺ or Ag⁺.

Mnemonic for disaccharides: "Sucrose is Stubborn (non-reducing); Maltose and Lactose are Liberal (reducing)"

Polysaccharides

Why can't humans digest cellulose? Human enzymes can only cleave α-glycosidic bonds; cellulose has β-1,4 bonds.

Iodine test: Starch turns blue-black with I₂/KI solution (iodine trapped in amylose helix). Glycogen gives reddish-brown; cellulose gives no colour.

Reducing vs Non-Reducing Sugars Summary

• Reducing sugar: Has a free aldehyde group or free anomeric —OH (can open ring); gives positive Tollens/Fehling test.
• Non-reducing sugar: Anomeric —OH involved in glycosidic bond; cannot open ring; negative Tollens/Fehling.

Reducing sugars: All monosaccharides, maltose, lactose Non-reducing sugar: Sucrose

Optical Activity and Configuration

Glucose has 4 chiral centres (C2–C5), giving 2⁴ = 16 stereoisomers (8 pairs of enantiomers). The naturally occurring form is D-(+)-glucose — D refers to configuration at the penultimate carbon (C5 in hexose), and (+) refers to dextrorotatory optical activity. These two properties are independent.

• D/L = absolute configuration (compared to glyceraldehyde)
• (+)/(−) = direction of plane-polarised light rotation (must be measured)

Key distinction: D-fructose is laevorotatory (−) despite being a D-sugar. The hydrolysis of sucrose converts dextrorotatory (+66.5°) mixture to laevorotatory (−39.7°) mixture — called inversion and the product is invert sugar.

Conformational Analysis of Pyranose Rings

In aqueous solution, β-D-glucose is more stable than α-D-glucose because:

• In β form, all bulky groups (—OH and —CH₂OH) are in equatorial positions in chair conformation
• In α form, the anomeric —OH is axial → slightly less stable (steric strain)
• This explains the 64:36 (β:α) equilibrium in mutarotation

Glycosidic Bond Formation — Mechanism Insight

The glycosidic bond forms by:

1. Protonation of anomeric —OH
2. Formation of oxocarbenium ion (carbocation stabilised by ring oxygen)
3. Attack by —OH of second sugar

For sucrose, the bond between C1-α of glucose and C2-β of fructose is unique — it is designated α,β-1,2-glycosidic bond. This uses up both anomeric carbons, making sucrose non-reducing and resistant to most glycosidases.

Structural vs Storage Polysaccharides — Why the Bond Matters

The α-1,4 linkage creates a natural coiling that makes starch compact for storage. The β-1,4 linkage in cellulose creates linear chains that pack via intermolecular H-bonds into strong microfibrils — ideal for cell wall structural support.

Example 1: Identify the Glycosidic Bond and Reducing Nature of Maltose

Problem: Maltose is formed from two glucose units.
Identify the glycosidic bond and state whether maltose is a reducing sugar.

Step 1: In maltose, the C1 (anomeric carbon) of one glucose
        forms a bond with the C4 —OH of the second glucose.
        → This is an α-1,4-glycosidic bond.

Step 2: After the bond forms, the C1 of the FIRST glucose
        is locked in α configuration (used in bond).
        The C1 of the SECOND glucose is still FREE (anomeric —OH available).

Step 3: The free anomeric —OH at C1 of the second glucose
        can open the ring → aldehyde exposed → reduces Cu²⁺.

Conclusion: Maltose IS a reducing sugar.
           It gives a positive Fehling's test.
           Sucrose does NOT because both C1 (Glc) and C2 (Fru)
           are involved in the bond.

Key rule: If ANY anomeric —OH is free → reducing sugar.

Example 2: Mutarotation Calculation

Problem: A freshly prepared solution of α-D-glucose shows specific rotation
         +112°. On standing, it changes to +52.7°. What is the composition
         of α and β anomers at equilibrium if β-D-glucose has [α] = +19°?

Let fraction of α = x, fraction of β = (1 − x)

Equilibrium rotation = x(+112) + (1−x)(+19) = +52.7

112x + 19 − 19x = 52.7
93x = 33.7
x = 0.362 ≈ 36.2%

→ α anomer = ~36%, β anomer = ~64% at equilibrium.

This confirms β-D-glucose is more stable in solution
(equatorial anomeric —OH, lower steric strain in chair conformation).

1. Why does glucose give a positive Tollens' test even in its cyclic (Haworth) form?
2. Draw the open-chain Fischer projection of D-glucose and label C1 through C6.
3. Sucrose is hydrolysed to give "invert sugar." What are the products and why is it called "invert"?
4. Starch and cellulose are both polymers of glucose. What makes their properties so different?
5. (Stretch) A sugar X has molecular formula C₆H₁₂O₆, gives a positive Fehling's test, does not show mutarotation in solution, and forms a five-membered ring. Identify X and explain why it does not show mutarotation despite being a reducing sugar.

• NCERT Class 12 Chemistry, Chapter 14 — Biomolecules, Sections 14.1–14.3
• JEE Foundation: 1–2 questions per exam; common formats: identify reducing/non-reducing, name the glycosidic bond, predict Tollens test result
• NEET relevance: high — carbohydrate structure questions appear regularly

Study strategy: Learn the 3 disaccharides (sucrose/maltose/lactose) with their bonds and reducing nature as a table. Understand mutarotation mechanistically — don't just memorise the rotation values.

Practice MCQs on carbohydrate structures and reducing/non-reducing sugars in the Biomolecules — Carbohydrates topic bank. Start with Easy (identify monosaccharides), then Medium (glycosidic bonds in disaccharides).

Confused about why sucrose is non-reducing while maltose is reducing — even though both are disaccharides? Ask the Drishti AI tutor to walk you through the anomeric carbon concept with a diagram.

Complete the Quick Check questions and mark them in your Drishti progress tracker. Aim for 4/5 before moving to proteins.

• Read: Biomolecules — Proteins and Enzymes — amino acid structures, peptide bonds, protein folding levels
• Then: Biomolecules — Nucleic Acids — DNA, RNA, Watson-Crick model
• Practice: Mixed Biomolecules MCQs (Medium difficulty)