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Biomolecules — Class 12 Maharashtra HSC Chemistry revision notes

A 7-page visual revision sheet for Biomolecules, written for Class 12 Maharashtra HSC Chemistry. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Biomolecules in one look

carbohydrates, proteins and nucleic acids are all polymers of a small, repeating chemical unit

Classification of carbohydrates

  • carbohydrates: polyhydroxy aldehydes or ketones, or units that hydrolyse to give them
  • name comes from C_x(H_2O)_y, hydrates of carbon
  • monosaccharide: does NOT hydrolyse further, e.g. glucose, fructose, ribose
  • oligosaccharide: hydrolyses to 2-10 monosaccharide units
  • disaccharide (2 units): sucrose, maltose, lactose; trisaccharide (3): raffinose; tetrasaccharide (4): stachyose
  • polysaccharide: hydrolyses to many monosaccharide units, e.g. starch, cellulose, glycogen

Glucose: proof of its aldohexose structure

  • formula C_6H_12O_6, from elemental analysis and colligative properties
  • SIX carbons, ONE straight chain: gives n-hexane on prolonged heating with HI
  • ONE carbonyl: forms an oxime with NH_2OH, a cyanohydrin with HCN
  • carbonyl is an ALDEHYDE: bromine water oxidises it to gluconic acid, so it was terminal
  • FIVE -OH groups: reacts with 5 mol acetic anhydride to give glucose pentaacetate
  • ONE primary alcohol -CH_2OH at C6: glucose and gluconic acid both give saccharic acid with dilute HNO_3

Glucose from sucrose and from starch

  • acid-catalysed hydrolysis of sucrose C_12H_22O_11+H_2Oto C_6H_12O_6+C_6H_12O_6
  • acid-catalysed hydrolysis of starch, 393 K, 2-3 atm (C_6H_10O_5)_n+nH_2Oto nC_6H_12O_6

Fischer projection and D/L configuration

  • an aldohexose has 4 chiral carbons (C2-C5), so 2^4=16 optical isomers are possible
  • Fischer (Nobel 1902) fixed glucose's configuration at all four by chemical correlation
  • glucose is dextrorotatory, [alpha]_D^20=+52.7°, hence dextrose; D-(+)-glucose names configuration, not just sign
  • glyceraldehyde has ONE chiral carbon: -OH on the RIGHT defines D-, on the LEFT defines L-
  • a sugar's D/L label is read off its LOWEST (highest-numbered) chiral carbon

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