Louise Quelch
CHEM2054 Experiment B10: Report
30/04/13
Introduction:
The aim of this experiment is to synthesis glucose pentaacetate through the acetylation of α-D-glucose:
(where R is any of the carbons on α-D-glucose)
Once synthesised and purified the 1H NMR was used to determine whether the product was β-Glucose
pentaacetate or α-Glucose pentaacetate.
Experimental:
α-D-glucose (5.0007 g, 0.027757 mol) was added to anhydrous sodium acetate (2.51 g, 0.0306 mol) and
refluxed for an hour with acetic anhydride (25 mL, 0.264 mol). The solution was added to distilled ice
water for 15 minutes. The solution was vacuum filtered and the precipitate collected. This product was
then recrystallised by dissolving in minimal boiling methanol and allowed to crystallise again. The
product was collected and rinsed with cold methanol. Yield 7.37 g, 68.0 %. m.p. 108.5 – 123.2 ºC
Results and Discussion:
The mechanism of the glucose being acetated is as follows (where the R group is any of the carbons on
the glucose ring):
s
One literature value for the yield for a similar procedure was 85 % [3], the calculated yield was decrease
due to loss of both products and reactants throughout the reaction. One of the major causes would have
been that during reflux, not all of the reactants had dissolved. Product was also lost every time it was
transferred between glassware, this is because the solid in solution was very ‘sticky’ whilst moist and
made 100% transfer impossible. A way to increase the yield would be to reflux the solution for a longer
period of time and at a higher temperature. This would lower the amount of initial reactants lost.
The difference between the α and β anomers is that acetate groups attached the C1 and C2 are cis in
α, but trans in β. These cyclic anomers are in constant equilibrium with the open chained formation.
When the cyclic structure reforms the bond will form in one of two directions, each one these
relates to an anomer.
Page 1 of 4
Louise Quelch
CHEM2054 Experiment B10: Report
30/04/13
The literature values for the melting points of β-Glucose pentaacetate or α-Glucose pentaacetate are
131 – 132 °C[1] and 111 – 112 °C[2] respectively. The melting point recorded does not seem very
reliable as it has such a large range and the product started to melt before the temperature had
reached either of the literature values. This could be due to the product containing contaminants or
not being completely dry, ie still containing methanol, this would decrease the overall melting point.
The melting point was unable to determine what the major product of the reaction was so the 1H NMR
was used to determine the product. When considering C1 and C2 the α isomer has cis acetate groups and
therefore one axial and the other equatorial. This means that the two hydrogens on these carbons will
also be in one in axial and the other equatorial (opposite to the acetate group). The β isomer however
has trans acetate groups this means that both hydrogens will be in the axial positions. This axial – axial
interaction causes the coupling constant to be 7-10Hz, compared to a coupling constant of 2-6Hz where
there is no axial – axial interactions (ie the α isomer). The coupling constant determined by the 1H NMR
for the C1 hydrogen was 10.88Hz; while this is not in the defined range, it is significantly more likely to
belong to the β isomer.
The hydrogen in position 1 is the least shielded, since it is relatively close to the electron withdrawing
oxygen, it was represented as a couplet since it has one neighbouring hydrogen. The hydrogen in the 3
position is split into triplets which are caused by the hydrogen on either side. The position 5 hydrogens
are represented by a triplet of doublets closely after the position 3 hydrogens. Both position 4 and 2
hydrogens are represented as a doublet of doublets, this is because although they have two neighbouring
hydrogens, the energy difference is great and therefore the first doublet is split again by the influence of
the second hydrogen. The 5 position is represented by a double of quartets, again the firsts splitting is re
split by another hydrogen or different energy to the first. At approximately 2ppm are where the
hydrogens from positions 7 -11 are; they are the most shield. (The 1H NMR assignments have been
summarise in Appendix 1.)
Conclusion:
The major product produced was β-Glucose pentaacetate, this was determined through 1H NMR, by
using relationships with axial and equatorial hydrogens and analysing the 1H NMR of know sample of α
and β-Glucose pentaacetate. While the final product was not a pure as desired (shown by the large range
and inaccuracy of the melting point) the product was able to produce a reasonable 1H NMR by which
the major product could be determined
Page 2 of 4
Louise Quelch
CHEM2054 Experiment B10: Report
30/04/13
Appendix 1: Assignment of 1H NMR peaks
Chemical shift –
average (ppm)
5.7146
5.2527
5.1285
4.289
No. of Hydrogens
4.11125
1
3.83775
1
2.1141
2.0856
2.0325
2.0126
3
3
6
3
1
1
2
1
Multiplicity
Coupling
constant
Doublet
10.88
Triplet
24.88
Doublet of triplets 27.84
Doublet of
22.72
doublets
Doublet of
19.64
doublets
Doublet of
22.2
quartets
Singlet
0
Singlet
0
Singlet
0
Singlet
0
β-D-Glucose pentaacetate
Page 3 of 4
Assignment
1
3
6
4
2
5
7
11
8 and 10
9
Louise Quelch
CHEM2054 Experiment B10: Report
30/04/13
References
[1] U. Niedballa, H. Vorbruggen, J. Org. Chem., 1974, 39, 3659
[2] F. Dasgupta, P. P. Singh, H. C. Smvsrava, Ekevier Scientific Publishing Company, 1980, 80, 346
[3] A. Pulsipher and M. N. Yousaf Chem. Commun., 2011, 47, 523
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