Carbohydrates are in the very heart of biomolecular chemistry. Analysis of carbohydrates and product — Physical Chemistry — Thermodynamics Chemistry Question
Osazone of glucose
Carbohydrates are in the very heart of biomolecular chemistry. Analysis of carbohydrates and products of their transformations is often hardly possible due to their appearance as oils or syrups with no characteristic melting point. The sophisticated stereochemistry of carbohydrates does not make their investigation easier. In the year 1880 the German chemist Emil Fischer found that heating of some monosaccharides with an excess of phenylhydrazine results in formation of crystalline products, which he named “osazones”. Different phenylosazones existed as distinctive crystals, and formed at different rates from various parent sugars. The crystallinity of these products helped in their analysis, whereas the loss of chirality at the 2nd carbon atom was of great importance in establishing stereochemical details of many monosaccharides. In this task you will prepare phenylhydrazine derivative of carbohydrate D-glucose (I).
[VISUAL]
Chemicals and Reagents
* D-Glucose
* Phenylhydrazine
* Water
* Acetic acid solution, 50 %
* Ethanol, 96 %
Equipment and Glassware
* Magnetic stirrer with heating
* Magnetic bar
* Water bath
* Round-bottom flask, 50 cm3
* Reflux condenser
* Laboratory stand with metal rings and clamps
* Filter flask
* Porous Shott’s glass filter
* Water- or vacuum pump
* Analytical balance (± 0.001 g)
* Pipette pump
* Capillary for melting point determination (2-3 ea.)
* Glass tube for capillary filling
* Melting point apparatus
* Glass rod
Procedure
D-glucose osazone
To a round bottom flask equipped with a reflux condenser and a water bath add 200 mg of glucose, 4 cm3 of water, 400 mg of freshly distilled phenylhydrazine (caution – poisonous!) and 0.4 cm3 of 50% acetic acid. Using the magnetic stirrer with a heating mantle, heat the reaction mixture until the water in the bath starts boiling. In 5 min, the yellow precipitate of osazone will start forming. Continue heating for 1 h, then carefully remove the bath, remove the condenser and let the reaction mixture slowly cool down to the room temperature.
Knead the content of the flask and transfer it on the glass Shotts’ filter. Turn on the water-pump, connect it to the filtration flask and filter the precipitate off. After the mother liquor stops dropping down, disconnect the flask and take the glass filter off. Wash the reaction flask with mother liquor, place the glass filter back, pour the content of the reaction flask onto the filter, and connect to vacuum. After the mother liquor stops dropping down, disconnect the flask. Add 3 cm3 of ethanol to the precipitate, knead it with a glass bar, and connect to vacuum again. Repeat the rinsing procedure with ethanol once more. To provide for effective drying, keep the precipitate pressing with the glass rod from time to time. Keep drying the product under vacuum for at least 10 min. Weigh the product and calculate the yield. Pick out a few crystals of the product for further determination of its melting point.
Determination of melting point
Determine the melting point of the product according to the directions in Problem 31.
Put the stoichiometry coefficients for the reaction between D-glucose and phenylhydrazine. What are the other products of this reaction?
Model Answer
[VISUAL] 3 PhNHNH2 + D-glucose → D-glucose osazone + NH3 + PhNH2 + 2 H2O
Which starting substance would you use to calculate the yield of your product?
Model Answer
D-Glucose, since phenylhydrazine is taken in an excess.
What is the product of the glucose reaction with equimolar amount of phenylhydrazine under mild conditions?
Model Answer
The appropriate phenylhydrazone of aldehyde.
Draw the osazones of D-glucose, D-mannose and D-fructose. What can you say about the similarity in stereochemistry of the starting sugars?
Model Answer
[VISUAL]
It is one and the same product for all the starting substances. These means the stereochemistry of C3, C4 and C5 of the starting sugars is the same. The initial difference in nature and/or stereochemistry at 1st and 2nd carbon atoms of the monosaccharides is equalizes by hydrazone formation.
Do the pairs of osazones of the hereunder sugars represent the same or different molecules?
a) D-glucose and L-glucose
b) D-allose and D-talose
c) D-galactose and D-talose
d) D-ribose and D-allose
Model Answer
a), b), d) are different; c) are the same.