Chapter 24: Carbohydrates Flashcards
Natural configuration of carbohydrates
Natural sugars are D-sugars
Determined by examining the chiral center furthest from the most oxidized carbon
Naming carbohydrates
- Use the prefix aldo or keto to indicate whether the compound is an aldehydy or a ketone
- Next add the term designating the cumber of carbon atoms: tri-, tetra-, pent-, hex-
- Use the suffix -ose to indicate that it is a carbohydrate
- Assign (D) or (L) to indicate the configuration of the sugar
Cyclization of monosaccharides
Under acidic conditions, monosaccharides cyclize- aldehydes react with alcohols to form hemiacetals
Equilibrium prefers the cyclic form
New chiral center is formed at the anomeric carbon; formerly the carbonyl carbon
- α anomer- the anomeric –OH group is trans to the –CH2OH group
- β anomer- the anomeric –OH is cis to the –CH2OH group
The term mutorotation refers to the alternating nature of carbohydrate rings between the α and β forms, although one form will usually be favored
Ring nomenclature based on number of members
A furanose ring has 5 members
A pyranose ring has 6 members
Drawing a Haworth projection form a Fischer projection
- Draw the skeleton ring of the Haworth projetion; convention places the oxygen at the back right position with carbons numbered in a clockwise fashion
- Draw the CH2OH group up connected to the last carbon in the ring
- Draw the hydroxy group at the anomeric position up or down based of the configuration of the ring
- Draw the remaining hydroxy groups up or down based off the Fischer projection; -OH groups on the left are oriented above the ring, while -OH groups of the right are oriented below the ring
Drawing the more stable chair conformation of a pyranose ring
- Draw the Haworth projection
- Draw the skeleton of the chair conformation with the oxygen at the upper, rear-right corner
- Draw all groups up or down as indicated
- Analyze each possible chair configuration: the most stable configuration will have the largerst group (CH2OH) occupying an equatorial position
Ester formation of monosaccharides
Reagents
Excess Ac2O & Pyridine
Acetic anhydride or acid chloride
Mechanism
Installs acetyl groups at the location of each hydroxyl group
Hydroxy groups of a monosaccharide can be converted to esters to improve their solubility in organic solvents
Williamson ether synthesis of monosaccharides
Reagents
Excess CH3I (iodomethane) & Ag2O
Mechanism
Installs methyl ether groups at the location of each hydroxyl group
Glycoside formation
Reagents
ROH & [H+]
Mechanism
Cyclic hemiacetal of a sugar can react with an alcohol under acidic conditions to form an acetal
- Formed through a resonance stabilized carbocation intermediate and thus can result in a mixture of α and β enantiomers
- Only the anomeric hydroxyl group is replaced
Glycosides are named by placing the alkyl group as a prefix and replacing the suffix -ose with -oside
Epimerization
Reagents
NaOH, H2O
Mechanism
Under strongly basic conditions, aldoses undergo epimerization at C2 via an enediol intermediate
Forms a mixture of the two products
Reduction of monosaccharides
Reagents
NaBH4, H2O
Mechanism
The carbonyl group of an aldose can be reduced to yield a product called an alditol
The carbonyl group of an ketose can be reduced to a secondary alcohol
Oxidation of monosaccharides
Mild oxidant
Reagents
Br2, H2O & pH = 6
Mechanism
The small amount of open-chain molecules that form are oxidized at the carbonyl carbon forming an aldonic acid
- Named by replacine the “-ose” suffix with -onic acid
A mild oxidizing agent is used so as to not oxidize all of the hydroxyl groups and only the carbonyl carbon
- Mild oxidizing agents can only oxidize aldoses
Oxidation of monosaccharides
Stron oxidant
Reagents
HNO3, H2O & heat
Mechanism
A stronger oxidant will oxidize the aldehyde and 1° alcohol of an aldose, forming an aldaric acid
Selective oxidation
A chemical test that is used to distinguish between aldoses and ketoses
- Bromine water test
- Tollen’s reagent- Ag+ in aqueous ammonia
Identifying a reducing sugar
Chemical tests
Glycosides (acetals) are NOT reducing sugars and not oxidized
Oxidation tests detect the presence of reducing sugars
- Fehling’s reagent- Cu2+ in aqueous sodium tartate
- Benedict’s reagent- Cu2+ in aqueous sodium citrate
Kiliani-Fischer sythesis
Reagents
- HCN
- H2, Pd/BaSO4
Mechanim
A cyanohydrin is first fromed and then subsequently hydrogenated to result in the overall addition of one carbon atom to the chain
Produces a mixture of product about the previously carbonyl carbon
Wohl degredation
Reagents
- NH2OH (hydroxylamine)
-
Ac2O
Acetic anhydride or acid chloride - NaOMe
Mechanism
Shortens the carbon chain by one carbon via the conversion to a cyanhydrin followed by the loss of HCN
Glycosidic bonds
Can be an O-glycosidic bond or an N-glycosidic bond
Can be an α-glycosidic bond or a β-glycosidic bond
- α-glycosidic- oxygen attached to the anomeric carbon is oriented DOWN
- β-glycosidic- oxygen attached to the anomeric carbon is oriented UP
Cellulose
Polysaccharide composed of glucose monomers liked by β-1,4- glycosidic bonds
Chitin
Polysacchardie composed of glucosamine monomers that have an N-acyl group installed in place of the -OH group on carbon-2
The N-acyl group allows for even stronger H-bonding between neighboring chains than cellulose
N-glycoside formation
Reagents
[H+], RNH2
The nitrogenous base pairs in DNA are connected to the deoxyribose units via β-N-glycosidic linkages, forming a nucleoside
Lactose
Disaccharide sugar composed of a galactose and glucose subunit
Joined by a β-1,4-glycosidic linkage
Is a reducing sugar
Sucrose
Disaccharide sugar composed of a glucose and fructose subunit
Joined by an α-1,β-2-glycosidic linkage
Is a non-reducing sugar because both anomeric carbons are involved in the glycosidic bonding
Maltose
Disaccharide formed from two units of glucose molecules
Joined by an α-1,4-glycosidic bond
Is a reducing sugar
