The 1H NMR spectrum of a sample that contains a mixture of 3-methyl-2-butanone and

Question

2,3-dimethyl-3-pentanol (determine the structures) was acquired, and the following data
was obtained. Answer the following questions based on this data. Assume that protons
attached to oxygen atoms do not appear in the spectrum, and consider only three bond
coupling for the multiplicities. Show any calculations.
a. Draw the structures of the two compounds described above using ChemDraw (see
posted installation instructions). Label each chemically distinct proton.
b. Assign the distinct protons in 3-methyl-2-butanone and 2,3-dimethyl-3-pentanol
to the chemical shifts of the signals described below using the labelled structures.
c. What is the approximate mole ratio of 2,3-dimethyl-3-pentanol to 3-methyl-2-
butanone?
d. What are the masses of 3-methyl-2-butanone and 2,3-dimethyl-3-pentanol that are
present in the sample if the sample weighs 5.13 g and contains only those two
compounds?
Chemical Shift Integration Multiplicity (n+1 rule)
0.90 35.4 doublet (2)
1.00 17.5 triplet (3)
1.12 72.7 doublet (2)
1.22 17.9 singlet (1)
1.23 12.0 quartet (4)
1.62 6.1 septet (7)
2.15 36.4 singlet (1)
2.59 12.1 septet (7

Answer 1

To determine the structures of the two compounds, we can analyze the chemical shift values and the multiplicities provided in the NMR spectrum data.

a. Drawing the structures of the two compounds using ChemDraw:

3-methyl-2-butanone:
- Proton 1: CH3
- Proton 2: CH3
- Proton 3: CH3
- Proton 4: CH2
- Proton 5: CH2

2,3-dimethyl-3-pentanol:
- Proton 6: CH3
- Proton 7: CH3
- Proton 8: CH3
- Proton 9: CH2
- Proton 10: CH
- Proton 11: CH
- Proton 12: CH2
- Proton 13: OH

(Note: The structures cannot be directly shown in this text-based format, but you can draw them using ChemDraw based on the description provided.)

b. Assigning the distinct protons in 3-methyl-2-butanone and 2,3-dimethyl-3-pentanol to the chemical shifts:

3-methyl-2-butanone:
- Proton 1: 1.23 (singlet)
- Proton 2: 1.12 (doublet)
- Proton 3: 2.15 (singlet)
- Proton 4: 1.22 (quartet)
- Proton 5: 1.00 (triplet)

2,3-dimethyl-3-pentanol:
- Proton 6: 1.62 (septet)
- Proton 7: 2.59 (septet)
- Proton 8: 0.90 (doublet)
- Proton 9: 1.12 (doublet)
- Proton 10: 36.4 (singlet)
- Proton 11: 0.90 (doublet)
- Proton 12: 72.7 (doublet)
- Proton 13: 2.15 (singlet)

c. To approximate the mole ratio of 2,3-dimethyl-3-pentanol to 3-methyl-2-butanone, we can use the integration values (in parentheses) for the protons of each compound.

Mole ratio = (Integration value of protons of 2,3-dimethyl-3-pentanol) / (Integration value of protons of 3-methyl-2-butanone)

Mole ratio = (36.4 + 0.90 + 72.7 + 2.15) / (1.23 + 1.12 + 2.15 + 1.22 + 1.00)

Mole ratio = 111.15 / 5.72 ≈ 19.4

Therefore, the approximate mole ratio of 2,3-dimethyl-3-pentanol to 3-methyl-2-butanone is 19.4.

d. To determine the masses of 3-methyl-2-butanone and 2,3-dimethyl-3-pentanol present in the sample, we need to calculate the mass percentages of each compound.

Mass percentage of 3-methyl-2-butanone = (Mass of 3-methyl-2-butanone) / (Total mass of the sample) x 100%

Mass percentage of 2,3-dimethyl-3-pentanol = (Mass of 2,3-dimethyl-3-pentanol) / (Total mass of the sample) x 100%

We can use the chemical formulas of each compound and the mole ratio obtained in part c.

Let x = mass of 3-methyl-2-butanone
Then, (5.13 - x) = mass of 2,3-dimethyl-3-pentanol

From part c, the mole ratio of 2,3-dimethyl-3-pentanol to 3-methyl-2-butanone is approximately 19.4.

Mass percentage of 3-methyl-2-butanone = (x / 5.13) x 100% ≈ ((19.4 * (5.13 - x)) / 5.13) x 100%

Mass percentage of 2,3-dimethyl-3-pentanol = ((5.13 - x) / 5.13) x 100%

We know that the sum of the mass percentages of both compounds is 100%.

((19.4 * (5.13 - x)) / 5.13) + ((5.13 - x) / 5.13) = 100

Solving the equation will give the values of x and (5.13 - x), representing the masses of 3-methyl-2-butanone and 2,3-dimethyl-3-pentanol present in the sample, respectively.