Organic chemistry, i need to write lab report for Preparation and Isomerization
ID: 522193 • Letter: O
Question
Organic chemistry, i need to write lab report for Preparation and Isomerization of D-Glucose Pentaacetate, and i need help with the anlysis of the HNMR and the calculation of J value
Analysis 1. Compare the NMR spectra of the D-glucose pentaacetate samples prepared in Parts I and II (these spectra are posted online). Without trying to assign every signal in the NMR, convince yourself (and your TA in your lab report) that in both reactions, you isolated an isomer of glucose pentaacetate. Identify the anomeric proton signal in both NMRs and label them. 2. Determine which anomer, a- or B-D-glucose pentatcetate, was formed in Part I, and which anomer was formed in Part II. Does your melting point data support your conclusions from the NMR spectra? 3. Is part I of the experiment under kinetic or thermodynamic control?Explanation / Answer
1. The NMR shows 11 major peaks. There is a splitting of these peaks into ‘sub-peaks’ due to the presence of neighbouring hydrogens. Each peak can correspond to multiple hydrogens which was then determined by integration of the major peak. The peaks are: 5.71 (1H, d, J = 10.88), 5.25 (1H, t, J = 24.88), 5.12 (2H, doublet of triplets, J = 27.84), 4.29 (1H, dd, J = 22.72), 4.11 (1H, dd, J = 19.64), 3.83 (1H, doublet of doublets of doublets, J = 22.2), 2.11 (3H, s, J = 0), 2.08 (3H, s, J = 0), 2.03 (6H, s, J = 0), 2.01 (3H, s, J = 0).
The hydrogen attached to C-1 has a greater number of electron withdrawing oxygen atoms in close proximity and is less shielded by electrons. This causes the peak to have a higher chemical shift. In the case of the hydrogen on C-1 in the anomer is approximately 5.7 ppm whereas in the anomer, it is around 6.3 ppm. When the product was tested, peaks at both 5.7 and 6.3 ppm were observed. In part I experiment, anomer is observed at 5.7 ppm. In part II experiment, anomer is observed at 6.3 ppm.
2. In part I experiment, anomer is formed and observed at 5.7 ppm. In part II experiment, anomer is formed and observed at 6.3 ppm.
Yes, melting point data support the conclusions from NMR data. The literature values for the melting points of -Glucose pentaacetate and -Glucose pentaacetate are 131 – 132 °C[3] and 111 – 112 °C respectively.
Another means to distinguish the axial and equatorial hydrogens comes from the coupling constants (J values). The coupling constant value depends on both the number of bonds separating the nuclei (the distance between the peaks in Hertz) and their conformation. It is calculated by the difference in chemical shift of the lowest and highest sub-peaks multiplied by the frequency. Since the C-1 hydrogen changes conformation during the interconversion of - to -D-(+)-glucose, the dihedral angle changes and therefore the coupling constants change. In anomer, the hydrogen in position 1 is axial. This causes diaxial interactions between itself and the hydrogen in position 2 which causes the coupling constant to be in the range of of 7 – 10 Hz. Conversely the hydrogen in position 1 in the isomer is equatorial causing equatorial-axial interactions, resulting in coupling constants of 2 - 6 Hz.
3. Part I experiment is under kinetic control. This is because:
- anomer occurs more in solution because it is more stable than the anomer. In the chair conformation, there is a symmetry axis. In the - anomer, the OH group and other substituents in the ring lie equatorial to the axis and this reduces steric hindrance, making this conformation more stable. Thus, - anomer forms in the kinetic control.
-D-glucose penta-acetateis the stable,therefore, it forms in thethermodynamic control.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.