Mutarotation Procedure 2006

CHM 266 Spring 2008
Mutarotation of Glucose
CH2OH
O
HO
HO
HO
CH2OH
OH
HO
HO
H
HO
OH
-D-glucose
m.p. 146C, = +112
CH2OH
O
HO
HO
OH
HO
open-chain form
-D-glucose
m.p. 150C, = +19
When -D-(+)-glucose is dissolved in water, the specific rotation of the sample

decreases from a value of +112 to a value of +53. The specific rotation of 112 corresponds to
-D-(+)-glucose. The specific rotation of +53 corresponds to an equilibrium mixture of -D-(+)glucose and -D-(+)-glucose. A change in the optical rotation of freshly prepared solutions of
sugars is called mutarotation. The forms of glucose involved in mutarotation are the open chain
form and two cyclic forms, which are in equilibrium. The 2 cyclic forms are diastereomers, which
have different physical properties (different m.p. and specific rotations). They are referred to as
anomers since they differ only in the configuration at the hemiacetal carbon. If either cyclic
compound is dissolved in water, the specific rotation changes to +53. The specific rotation
decreases from +112 to +53 when -D-(+)-glucose is dissolved in water and increases from
+19 to +53 when -D-(+)-glucose is dissolved in water. The mutarotation of glucose is acidcatalyzed and proceeds rapidly at a pH of 7.
Procedure
1. Turn on the polarimeter. Approximately 10 to 20 minutes are required before the polarimeter
will operate at full efficiency.
2. Prepare an H2PO4-/HPO4-2 buffer by dissolving 6.80g of KH 2PO4 in 200 ml. of water in a 400
ml. beaker. Using a calibrated pH meter (see *), add 1M NaOH until the pH is 7.0
(calculated amount is about 20 ml of 1 M NaOH). When the pH is 7.0, transfer the buffer
into a 250 ml. volumetric flask and dilute to the mark. This supplies enough buffer for four
kinetic experiments.
3. Clean the sample cells with water. Place a cell with solution (no glucose) into the polarimeter
and press reset to have the zero reading on polarimeter. Set the polarimeter to read the
rotations every 30 seconds for a total of 10 minutes. For this purpose press 2 on control device
and then 20 for the number of readings and 30 seconds for intervals.
4. Weigh out 5.00 g of D-(+)-glucose, which has been pulverized to insure the absence of large
crystals. To a clean dry beaker, add 40 ml. of the previously prepared phosphate buffer. Stir
it vigorously with a magnetic stirrer. Rapidly add the glucose to this solution. Start the
stopwatch as soon as the glucose comes in contact with the solution. As soon as it dissolves,
place it in a clean, dry 50 ml. volumetric flask and dilute to the mark with phosphate buffer.
Fill the 20 cm polarimeter tube as rapidly as possible. Fill the tube so that no air bubble will
be trapped in the cell. The sample cell length is 2 dm. It is preferable if you perform these
operations near the polarimeter.
5. As soon as you put the tube in polarimeter, press the start button. The first reading should be
made at 3 minutes (or before if possible) and successive readings every 30 s for a total of 10
minutes. At the end printer will print out the times and observed rotations. Write your class,
name, instructor, and experiment on the form. Take out the polarimeter tube and keep it for
obtaining infinity reading after 30 minutes. Record the exact time the readings were made.
6. You need 2 runs (experiments that agree closely for the value of k (1st order rate constant).
Obtain another polarimeter tube and repeat 3-5 again and record the time with another
stopwatch. Record the temperature at during the experiment.
Calculations
1. Calculate what the observed rotation should be at zero time (o) assuming a specific rotation
of +112 (corresponding to the -anomer). The equation for the relationship of specific rotation
to observed rotation of solution is
[]D25 = obs / lc
obs = observed rotation
l = length of tube in decimeters
c = concentration of g per ml.
2. Calculate what the observed rotation should be at infinite time () assuming a specific
rotation of +53 (corresponding to the equilibrium mixture).
3. Plot obs vs. time (s). (Use a plotting program)
4. Plot ln (t - ) vs. time (s). This is a linear relationship. Fit the best line to the data. (Use a
plotting program)
a. Determine t at to (This is o = observed polarimeter at zero time)
b. From the slope calculate k. The early points should be in a line.
Many calculators, spreadsheet programs, and plotting programs are capable fitting a best line
and obtaining slopes and y-intercepts through a linear regression analysis.
5. Assume a first order reaction and calculate the rate constant k for each reading by the
equation:
k = (1/t) ln ((o-)/(t - ))
o = observed polarimeter reading at zero time
= reading after equilibrium is reached
t = reading at time t
Notebook
The notebook data tables should include all data necessary to do the calculations. All results
from these calculations and all plots should be included.
*Adjusting the pH of the HPO4-2/H2PO4- buffer to 7
1. burets are available
2. use the meter to check the pH
3. the HPO4-2/H2PO4- buffer should be thoroughly mixed after each 1M NaOH addition
before measuring the pH
4. follow the instructions provided with the pH meter
5. do a 2-point calibration
6. pH 4 buffer = slope buffer
7. remove the cap from the electrode
8. rinse the electrode with distilled water

9. dry the electrodes by blotting with tissue, do not wipe the electrodes

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CHM 266 Spring 2008

When -D-(+)-glucose is dissolved in water, the specific rotation of the sample

8. rinse the electrode with distilled water

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