Reactions of Carbs

• Carbohydrates


  Undergo several chemical reactions: Acetylation, Oxidation, Reduction, Formation of glycosides

  
• Sugar functional groups


  Aldehydes, ketones and hydroxyls will undergo the same reactions which you have seen them take part in previously

  
• Acetylation


  1. Reaction of glucose with acetic anhydride gives a penta-acetate product showing that glucose has five OH groups

  2. Pyridine is used as a solvent

  3. An ester is formed and the hydroxyl groups are acetylated; an acetic ester acetate is formed with each of the hydroxyl groups and each of them will react

  4. They form very quickly and easily and the reaction is done at room temp

  
• Acetylation of natural products


  • Acetates are much easier to separate on a column than the hydroxyl groups, which interact very strongly with the silica gel on the column for column chromatography
  • Makes them very difficult to separate and wash off the column
  • By acetylating the hydroxyl groups first, they're easier to separate and purify
• Oxidation of sugars


  1. Metal ions oxidise aldehyde groups in aldoses to carboxylic acids (aldonic acids)

  2. The metal ions are in turn reduced

  
• Oxidation tests


  • Benedict's
  • Fehling's
  • Tollen's
• Reducing sugars


  • Sugars which are oxidised by Fehling's or Tollen's reagents
  • Any aldehyde-containing sugar which can exist in the open-chain form will be a reducing sugar, as the aldehyde group will be available to be oxidised
  • If a sugar remains in its cyclised form, it will not be a reducing sugar

  
• These oxidation reactions are not used preparatively since the conditions can eventually lead to the decomposition of the carbohydrate
• Preparative oxidation


  Aqueous bromine can be used to isolate carboxylic acids
• Some ketoses are also reducing sugars


  Due to a rearrangement in the basic conditions of the oxidising agent, e.g. D-fructose, where they form the aldehyde

  
• Mechanism of ketose-aldose rearrangement under basic conditions


  Squiggly lines shows that OH and H could be on either side, so there's no commitment to stereochemistry at C2

  
• Stronger oxidising agents can produce other products including diacids
• Reduction of monosaccharides


  1. Monosaccharides can be reduced by a variety of methods to give polyalcohols (alditols)

  2. The aldehyde is reduced by the sodium borohydride to form the alcohol functional group

  
• Reduced monosaccharides


  • D-glucitol (D-sorbitol), a natural product found in many fruits and used as an artificial sweetener
  • D-mannitol, another naturally occurring alditol found in sources such as olives, mushrooms, marine algae and onions
• Formation of glycosides


  1. When sugars are mixed with alcohols in the presence of acid, cyclic acetals known as glycosides are formed

  2. Mechanism: Protonation of the hydroxyl group, nucleophilic attack by the alcohol, loss of water

  
• Glycoside formation and hydrolysis


  • Glycoside formation is an equilibrium
  • If a glycoside is mixed with water in the presence of a catalytic amount of mineral acid then the equilibrium will favour the formation of a mixture of anomers of the parent sugar — the glycoside is hydrolysed
  • If a glycoside is dissolved in neutral aqueous solution then neither hydrolysis nor mutarotation occurs
  • Under neutral conditions glycosides are stable and exist only as rings
  • Hence glycosides are not reducing sugars and do not display mutarotation (unless they contain more than one sugar unit)
  • If a glycoside is mixed with water in the presence of a catalytic amount of mineral acid then it will be hydrolysed back to the original sugar and alcohol
  • Glycoside formation and hydrolysis can be compared to ester formation and hydrolysis
  • The hydrolysis of glycosides can be catalysed by certain enzymes which will selectively cleave either α- or β- glycosidic bonds

  
• Glycosides


  • β-maltose, a reducing sugar which displays mutarotation in solution
  • Sucrose, not a reducing sugar which does not display mutarotation in solution