Review of organic functional groups
 
 

Slides from General Organic and Biochemistry-University of Akron

The important sugars for our purposes are

the aldohexoses
 

glucose	galactose	mannose

and aldopentoses
 

arabinose	xylose

 

in addition to the aldoses, other functional groups may occur

Configuration of monosaccharides

Differences are due to chiral centers

Number of stereoisomers that may exist depends on the number of chiral centers

• Number of stereoisomers=2^{number of chiral centers}
• Number of pairs of enantiomers=2^{number of chiral centers -1}
• D and L designations only refer to the configuration about the penultimate carbon
• + and - designate rotational direction

epimers-differ only in the configuration about C-2

Anomers differ only in configuration about C-1

C-1 becomes chiral by cyclization of the sugars

In three dimensions the pyranose ring can adopt several conformations with varying energies

Reactions of carbohydrates

isomerization

anomerization or mutarotation



• optical rotation changes with time
• occurs because the isomers all have different rotations and occur in varying concentrations
• final optical rotation depends on what isomers are present
• optical rotation of each
• concentration of each

epimerization reaction
Beginning with a given sugar, the products of this reaction will be
• original sugar
• the epimer of the original sugar
• the corresponding keto sugar
• the corresponding 1,2 enediol
• for example

Upon exposure to additional base, the products from epimerization can continue to react

formation of an isosaccharinic acid

formation of metasaccharinic acids

Reduction reactions

Bromine-water gives the corresponding aldonic acid



Nitric acid (a stronger oxidizing agent) oxidizes both the aldehyde and 1° alcohol



Qualitiative tests for reducing sugars are oxidation reactions

• Tollens test
• silver ions precipitate in the presence of free aldehydes
• Fehling's test
• cupric ions are the oxidant
• These reagents are reduced by compounds with free aldehyde groups (i.e. aldoses)
• Sugars with free aldehydes are classified as reducing sugars
• free aldehyde may occur if the sugar is acyclic
• also if the sugar is cyclic (due mutarotation a small but finite amount of the sugar will be present in its open chain form)

• usually used as an analytical tool for structure determination
• periodic acid (HIO4) will oxidize
• hydroxyls
• carbonyls
• amines
• on adjacent carbons
• 1° alcohols give formaldehyde
• 2° alcohols give higher aldehydes
• 3° alcohols give ketones
• alpha-hydroxy aldehydes give formic acid and an aldehyde
• adjacent carbonyls give two carboxylic acids

Substitution reactions

• etherification
• glycoside formation (specialized ether formed through the anomeric position)
• this is generally an acid catalyzed alcoholysis reaction
• depending on the reaction conditions the products will be a mixture of
• pyranose and furanose rings
• alpha and beta anomers
• general mechanism

Acid hydrolysis of glycosides (catalytic addition of water) (glycosides are stable in base)

• This is basically the reverse of glycoside formation, with the products being the original sugar and the alcohol
• The reaction may occur through a cyclic or acyclic intermediate
• The sugar products will again be a mixture of ring sizes and anomers

Ethers may also form through the non-anomeric hydroxyls

• etherification is used
• to increase volatility for gc analysis
• structural analysis of polysaccharides
• common ethers are
• methyl (structural analysis)
• trimethylsilyl (TMS) (most common currently for gc)
• triphenylmethyl (trityl) (steric bulk used in syntheses)

• used extensively in synthetic carbohydrate chemistry



Anhydro-sugars (internal acetals)

glycosans
• formed through the anomeric position
• 1-6 linkage is common to make levoglucosan



epoxides
also internal ethers that do not form through the anomeric position

relies on specific orientation of hydroxyl groups



Esters

• recall that esters are the products of reactions between acids and alcohols
• the acids may be organic acids or mineral acids
• saccharides are usually esterified through acid anhydrides (but reactions may also involve free acids and acid halides)
• esterification is used in gc analysis of sugars
• esterification of polysaccharides results in a number of important derivatives
• General mechanism for ester formation

Other reactions

• Kiliani-Fischer synthesis (chain lengthening)
• epimeric pairs are produced
• reaction with HCN

Chain shortening reaction

Ruff degradation