Chemistry · Ch 14 — Biomolecules
Structure and properties of glucose
Structure and properties of glucose
Glucose's open-chain structure -- an aldohexose, CHO-(CHOH)4-CH2OH, with one aldehydic carbonyl at C-1 and a hydroxyl group on each of the other five carbons -- was worked out entirely from chemical evidence, one structural feature at a time, well before any spectroscopic technique existed. (1) Its molecular formula, C6H12O6, was established from elemental composition analysis and colligative-property (molecular mass) measurements. (2) That its six carbons form one unbranched straight chain was shown by reducing glucose all the way with concentrated HI, which strips every oxygen function and gives n-hexane, CH3-(CH2)4-CH3 -- an unbranched product, proving the carbon skeleton itself was never branched. (3) That glucose contains exactly one carbonyl group was shown by its forming exactly one oxime on reaction with hydroxylamine (NH2OH) and exactly one cyanohydrin on reaction with HCN -- both are classic single-carbonyl addition reactions. (4) That this one carbonyl is specifically an aldehyde (not a ketone) was shown by mild oxidation: bromine water (a mild oxidant) converts glucose's -CHO selectively into a -COOH, giving the six-carbon monocarboxylic acid gluconic acid, while leaving the rest of the molecule (including the terminal -CH2OH) untouched -- a ketone would resist this mild an oxidant. (5) That glucose carries five hydroxyl groups, each on a different carbon, was shown by its reacting with five moles of acetic anhydride to give a fully acetylated glucose pentaacetate (compare the acetylation-counting logic worked through numerically in Problem 14.1); since glucose survives this reaction as one stable compound rather than fragmenting, the five -OH groups must all sit on five distinct carbons. (6) That one of those hydroxyls is specifically a PRIMARY alcohol (-CH2OH, at the chain terminus) was shown by oxidising both glucose and gluconic acid separately with dilute nitric acid: both give the SAME dicarboxylic acid, …
Worked out. Glucose, CHO-(CHOH)4-CH2OH, on prolonged heating with concentrated HI (a powerful reducing agent that also removes every oxygen function), gives n-hexane, CH3-(CH2)4-CH3. Since n-hexane has an unbranched six-carbon skeleton, this proves that all six carbons of glucose are joined in one continuous straight chain, with no branching. …
Worked out. Glucose, CHO-(CHOH)4-CH2OH, reacts with bromine water (Br2/H2O, a mild oxidising agent, symbolised (O)) to give gluconic acid, COOH-(CHOH)4-CH2OH -- a six-carbon monocarboxylic acid in which only the carbonyl carbon has been oxidised, while the terminal -CH2OH is untouched. A ketone could not be mildly oxidised this way (ketones resist mild oxidants), so this selective, mild oxidation confirms the carbonyl in glucose is specifically an aldehyde (-CHO), not a ke …
Worked out. Glucose (CHO-(CHOH)4-CH2OH) reacts with hydroxylamine (NH2OH) with loss of a water molecule to give glucose oxime (CH=N-OH-(CHOH)4-CH2OH), and separately reacts by nucleophilic addition of HCN to give glucose cyanohydrin (CH(OH)(CN)-(CHOH)4-CH2OH). Both are classic single-carbonyl reactions (each carbonyl group reacts with exactly one molecule of the reagent), and since glucose forms only one oxime and one cyanohydrin, it is inferred to contain exactly one carbonyl g …
Worked out. Worked problem: an alcoholic compound of molecular mass 90 u is acetylated (each -OH is converted to -O-CO-CH3, replacing one H atom by an acetyl group, -COCH3, a net mass increase of (12+16+12+3x1)-1 = 42 u per -OH group); the acetyl derivative has molecular mass 174 u. Solution: total increase in mass = 174 - 90 = 84 u; number of -OH groups = 84 u / 42 u per group = 2. This is the same acetylation logic used on glucose in the main text, where glucose (mass 180) forms a pentaacetate by reacting with five moles of acetic anhydride, each acetylation replacing one -OH hydrogen and confirming five hydroxyl groups are present, each on a different carbon (since glucose remains a …
Worked out. States the general rule that a structural formula with 'n' chiral carbons can have a maximum of 2^n possible stereostructures (optical isomers). Since an aldohexose such as glucose has four chiral carbons (C-2 through C-5), it can theoretically exist in 2^4 = 16 distinct optical-isomeric forms; D-(+)-glucose is just one specific member of this family of sixteen aldohexose stereoisom …