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15240672380's Question
Posted 3 months ago

Q5. In the 1H NMR of 1,3,4,6-tetra-O-acetyl-2-amino-2-deoxy- β\beta-Dglucopyranoside, the H1\mathrm{H}-1 proton appears as a doublet (d) at δ=5.46ppm\delta=5.46 \mathrm{ppm} ( J=9 HzJ=9 \mathrm{~Hz} ) while the H2\mathrm{H}-2 proton appears as a doublet of doublet (dd) at δ=\delta= 3.03ppm3.03 \mathrm{ppm} with (j=9,10 Hz)(j=9,10 \mathrm{~Hz}). From these data determine which of the following chair conformations (1C4\left({ }^{1} C_{4}\right. or 4C1)\left.{ }^{4} C^{1}\right) is adopted by the title compound and explain your answer.
\| \rightarrow \delta=5.46 \\
\|\| \rightarrow \delta=3.03

{ }^{1} \mathrm{C}_{4}

The compound adopts the
1C4{ }^{1} C_{4}
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Answer from Sia
Posted 3 months ago
Analyze the NMR data for H-1: The H-1 proton appears as a doublet at δ = 5.46 ppm with a coupling constant (J) of 9 Hz. This suggests that H-1 is coupled to only one neighboring proton with a relatively large coupling constant, indicative of a dihedral angle of about 60° between the coupled protons, which is typical for an axial-axial coupling in a chair conformation
Analyze the NMR data for H-2: The H-2 proton appears as a doublet of doublets at δ = 3.03 ppm with coupling constants J = 9 Hz and J = 10 Hz. This indicates that H-2 is coupled to two different protons with similar coupling constants, suggesting that H-2 is in an equatorial position, as axial-equatorial couplings are typically smaller than axial-axial couplings
Determine the chair conformation: Considering the coupling constants and the chemical shifts, the data suggest that H-1 is axial (due to the larger coupling constant) and H-2 is equatorial (due to the presence of two coupling constants). The ^1C_4 conformation would place H-1 in an equatorial position and H-2 in an axial position, which is inconsistent with the NMR data. Therefore, the ^4C_1 conformation is adopted by the compound, as it places H-1 in an axial position and H-2 in an equatorial position, consistent with the observed NMR data
The compound adopts the ^4C_1 conformation.
Key Concept
Coupling constants in NMR spectroscopy can help determine the spatial arrangement of protons in a molecule.
The larger coupling constant for H-1 suggests an axial position, while the doublet of doublets for H-2 suggests an equatorial position. The ^4C_1 conformation is consistent with these observations.

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