Unit 10.1 – Carbohydrates 🍚🍯

1. Big Picture 🌟

  • Plants build carbohydrates and pack them into food like sugarcane, honey, fruits, and grains. They fuel life and give structure to cells. :contentReference[oaicite:0]{index=0}
  • Most carbs fit the handy formula \(\mathrm{C}_x(\mathrm{H_2O})_y\), but chemists define them more precisely as optically active poly-hydroxy aldehydes or ketones, or compounds that release those units after hydrolysis. :contentReference[oaicite:1]{index=1}
  • Sweet-tasting carbs are “sugars” (🍬), while big, bland carbs like starch and cellulose serve as energy stores and building blocks. :contentReference[oaicite:2]{index=2}

2. How We Classify Carbs 📚

2.1 By Hydrolysis Behavior

  • Monosaccharides – one unit; they do not hydrolyze further. Example: glucose, fructose, ribose. :contentReference[oaicite:3]{index=3}
  • Oligosaccharides – 2 – 10 units.
    • Disaccharides (2 units) – sucrose, maltose, lactose
    • Tri-, tetra-… up to deca-saccharides follow the same idea.
  • Polysaccharides – long chains; hundreds to thousands of units. Example: starch, cellulose, glycogen. :contentReference[oaicite:4]{index=4}

2.2 Reducing vs Non-Reducing Sugars 🔬

Carbs that reduce Fehling’s or Tollens’ reagents are reducing sugars. All monosaccharides qualify; some disaccharides (e.g., maltose, lactose) do too. Sucrose does not, so we call it non-reducing. :contentReference[oaicite:5]{index=5}

2.3 By Carbon Count & Functional Group

CarbonsGeneral NameAldose ExampleKetose Example
3TrioseAldotrioseKetotriose
4TetroseAldotetroseKetotetrose
5PentoseAldopentoseKetopentose
6HexoseAldohexose (glucose)Ketohexose (fructose)
7HeptoseAldoheptoseKetoheptose

When the carbonyl group is an aldehyde we say aldose; when it’s a ketone we say ketose. :contentReference[oaicite:6]{index=6}

3. Monosaccharide Spotlight 🔎

3.1 Glucose (D-(+)) 🍇

  • Molecular formula: \(\mathrm{C_6H_{12}O_6}\) (an aldohexose). :contentReference[oaicite:7]{index=7}
  • Prep in the lab:
    1. Cane-sugar hydrolysis: \(\mathrm{C_{12}H_{22}O_{11}} + \mathrm{H_2O} \xrightarrow[\text{dil.\,HCl/H_2SO_4}]{\text{heat}} \mathrm{C_6H_{12}O_6} + \mathrm{C_6H_{12}O_6}\) (equal glucose + fructose). :contentReference[oaicite:8]{index=8}
    2. Starch hydrolysis: \((\mathrm{C_6H_{10}O_5})_n + n\,\mathrm{H_2O} \xrightarrow[\text{393 K, 2–3 atm}]{\text{dil.\,H_2SO_4}} n\,\mathrm{C_6H_{12}O_6}\). :contentReference[oaicite:9]{index=9}
  • Key tests & clues:
    • Heating with HI gives n-hexane ⇒ straight-chain six carbons.
    • Forms oxime with NH2OH and cyanohydrin with HCN ⇒ has a carbonyl group.
    • Mild Br2 water oxidizes it to gluconic acid ⇒ carbonyl is an aldehyde.
    • Acetylation yields penta-acetate ⇒ five –OH groups on different carbons.
    • Strong HNO3 oxidation gives saccharic acid ⇒ one –CH2OH primary alcohol end.
    :contentReference[oaicite:10]{index=10}
  • Cyclic forms: –OH on C-5 attacks the aldehyde, creating a six-membered pyranose ring. The anomeric C-1 can flip the –OH up or down, giving α- and β- anomers (🌪 mutarotation in solution!). :contentReference[oaicite:11]{index=11}

3.2 Fructose (D-(−)) 🍏

  • An important ketohexose with formula \(\mathrm{C_6H_{12}O_6}\). :contentReference[oaicite:12]{index=12}
  • Usually appears with glucose in fruit, honey, and on sucrose hydrolysis.
  • The keto group lies at C-2 in the open chain. –OH on C-5 attacks it to make a five-membered furanose ring, again in α and β anomeric flavors. :contentReference[oaicite:13]{index=13}

4. Disaccharides – Two Rings, One Link 🔗

The bond between two monosaccharides is an O-glycosidic linkage (loss of water). :contentReference[oaicite:14]{index=14}

DisaccharideUnits & LinkReducing?Fun Facts
Sucrose 🍬α-D-Glc (C1) ↔ β-D-Fru (C2)NoHydrolysis flips optical sign; the mix (invert sugar) tastes sweeter.
Maltose 🍺α-D-Glc (C1) ↔ α-D-Glc (C4)YesReleased when grains germinate — key to brewing!
Lactose 🥛β-D-Gal (C1) ↔ β-D-Glc (C4)YesMain sugar in milk; some people lack the enzyme to digest it.

Reducing disaccharides have a free hemi-acetal/ketal end that can open to the aldehyde (or keto) form. :contentReference[oaicite:15]{index=15}

5. Polysaccharides – Giant Energy Packs 🏋️‍♂️

5.1 Starch (Plants’ Pantry)

  • Made of α-glucose.
  • Amylose: straight chains (C1→C4) – 15-20 %, water-soluble.
  • Amylopectin: branched (C1→C4 main, C1→C6 branches) – 80-85 %, water-insoluble.
:contentReference[oaicite:16]{index=16}

5.2 Cellulose (Plant Walls)

  • Straight β-D-glucose chains (C1→C4).
  • Forms wood, cotton, and paper – we build furniture and wear clothes thanks to it! 🪑👕
:contentReference[oaicite:17]{index=17}

5.3 Glycogen (Animal Starch)

  • Even more highly branched than amylopectin.
  • Stores glucose in liver, muscles, and brain; enzymes snip it back to glucose when we need a quick energy boost. 💪⚡
:contentReference[oaicite:18]{index=18}

6. Why Carbs Matter to Us ❤️

  • Instant energy: athletes love honey for quick fuel. 🏃‍♀️
  • Long-term energy: plants stash starch; animals stash glycogen.
  • Structural strength: cellulose reinforces cell walls and lets trees stand tall. 🌳
  • Industrial uses: carbs feed textile, paper, lacquer, and brewing industries. :contentReference[oaicite:19]{index=19}

7. High-Yield NEET Nuggets 🎯

  1. Recognize α vs β anomers and mutarotation in glucose and fructose.
  2. Classify sugars as reducing or non-reducing and predict Tollens’/Fehling’s results.
  3. Know the glycosidic linkages in sucrose (α1→β2), maltose (α1→4), and lactose (β1→4).
  4. Compare structures of amylose, amylopectin, cellulose, and glycogen.
  5. Recall the prep and key reactions of D-glucose (e.g., Br2 water → gluconic acid, HNO3 → saccharic acid).