Chapter 13.SE, Problem 48AP

Interpretation Introduction

a)

Interpretation:

The structure of an given molecular formula C₈H₁₆ to be predicted using ¹³CNMR spectra.

Concept introduction:

The ¹³CNMR spectrum gives information on the different electronic environments of carbon. As like ¹HNMR, the number of signals generated in ¹³CNMR are predicted by performing symmetry operations (rotation or reflection symmetry). Only chemical shift values are reported in the spectrum but not the multiplicity and integration values because the coupling between two neighboring ¹³C - ¹³C nuclei are weakly involved due to the low abundance of ¹³C isotopes of carbon atom.

Broadband-decoupled ¹³CNMR spectrum: The spectra provide information regarding the total number of carbon environments.

DEPT (Distortionless enhancement by polarization transfer):

i) DEPT -90: The spectrum exhibits signal only from CH group and no signals from CH₃, CH₂, CH and quaternary carbon (carbon with no protons).

ii) DEPT -135: The spectrum exhibits CH₃ groups and CH groups as positive signals (pointing up); CH₂ groups appear as negative signals (pointing down) and quaternary carbon does not appear.

The signals appear in each type of spectrum:

To identify:

The structure of given molecular formula C₈H₁₆.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the presence of a double bond or a ring.

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows five signals whereas the given molecular formula also has eight carbon atoms. Thus all the eight carbons have chemically different electronic environments showing signals.

i) The five signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene or Methine groups.

DEPT -90: Spectrum has no signals led to confirmation of no CH group in the structure.

DEPT -135 (gives signals of CH₂, CH₃ and C groups):

ii) The two positive signals indicate the presence of two methyl groups as no signals appear in the DEPT -90 spectrum.

iii) The three negative signals indicate the presence of four methylene groups; only methylene groups appear negative in the spectrum.

iv) So far the group of fragments obtained is

Two -CH₃, -C-, four -CH₂

From the fragments (CH₃ + CH₃ + (CH₂)₅ + C = 16 protons) sixteen protons are obtained whereas the total number of protons from the molecular formula is also sixteen.

The possible structures are:

The first structure possesses rotational symmetry and exhibits fewer signals in the spectrum and gets cancelled out.

Conclusion

The structure of a given molecular formula C₈H₁₆ is predicted using ¹³CNMR spectra.

Interpretation Introduction

b)

To identify:

The structure of given molecular formula C₃H₈O.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the absence of a double bond or a ring.

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows three signals whereas the given molecular formula also has three carbon atoms. Thus all the three carbons have chemically different electronic environments showing signals.

i) The two signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene groups.

DEPT -90: Spectrum has no signals led to confirmation of no CH group in the structure.

DEPT -135 (gives signals of CH₂, CH₃ groups):

ii) The two positive signals indicate the presence of two methyl groups as no signals appear in the DEPT -90 spectrum.

iii) The one negative signals indicate the presence of methylene groups; only methylene groups appear negative in the spectrum.

iv) So far the group of fragments obtained is

Two -CH₃, -CH₂

From the fragments (CH₃ + CH₃ + CH₂ = 8 protons) eight protons are obtained whereas the total number of protons from the molecular formula is also eight.

Conclusion

The structure of a given molecular formula C₃H₈O is predicted using ¹³CNMR spectra.

Interpretation Introduction

c)

To identify:

The structure of given molecular formula C₁₀H₂₀.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the presence of a double bond or a ring.

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows six signals whereas the given molecular formula also has ten carbon atoms. Thus all the ten carbons have chemically different electronic environments showing signals.

i) The two signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene groups.

DEPT -90: Spectrum has signals led to confirmation of CH group in the structure.

DEPT -135 (gives signals of CH₂, C, CH₃ groups):

ii) The two positive signals indicate the presence of three methyl groups as no signals appear in the DEPT -90 spectrum.

iii) The three negative signals indicate the presence of five methylene groups; only methylene groups appear negative in the spectrum.

iv) So far the group of fragments obtained is

Two -CH₃, -CH₂

From the fragments (CH₃)₃ + (CH₂)₅ + CH + C = 20 protons) twenty protons are obtained whereas the total number of protons from the molecular formula is also twenty.

Conclusion

The structure of a given molecular formula C₁₀H₂₀ is predicted using ¹³CNMR spectra.

Interpretation Introduction

d)

To identify:

The structure of given molecular formula C₆H₁₀.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the presence of a double bond or a ring. (two unsaturated degree).

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows six signals whereas the given molecular formula also has six carbon atoms. Thus all the six carbons have chemically different electronic environments showing signals.

i) The two signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene groups.

DEPT -90: Spectrum has signals led to confirmation of CH group in the structure.

DEPT -135 (gives signals of CH₂, C, CH₃ groups):

ii) The four positive signals indicate the presence of two methyl groups as no signals appear in the DEPT -90 spectrum.

iii) The negative signals indicate the presence of methylene groups; only methylene groups appear negative in the spectrum.

iv) So far the group of fragments obtained is

Two -CH₃, -CH₂

From the fragments (CH₃)₂ + CH₂ + (CH)₂ + C = 10 protons) ten protons are obtained whereas the total number of protons from the molecular formula is also ten.

Conclusion

The structure of a given molecular formula C₆H₁₀ is predicted using ¹³CNMR spectra.

Interpretation Introduction

e)

To identify:

The structure of given molecular formula C₈H₁₆.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the presence of a double bond or a ring.

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows four signals whereas the given molecular formula also has eight carbon atoms. Thus all the eight carbons have chemically different electronic environments showing signals.

i) The four signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene or Methine groups.

DEPT -90: Spectrum has no signals led to confirmation of CH group in the structure.

DEPT -135 (gives signals of CH₂, CH₃ and CH groups):

ii) The two positive signals indicate the presence of two methyl groups as no signals appear in the DEPT -90 spectrum.

iii) The two negative signals indicate the presence of four methylene groups; only methylene groups appear negative in the spectrum.

iv) So far the group of fragments obtained is

Two -CH₃, -CH-, four -CH₂

From the fragments (CH₃ + CH₃ + (CH₂)₄ + (CH)₂ = 16 protons) sixteen protons are obtained whereas the total number of protons from the molecular formula is also sixteen.

Conclusion

The structure of a given molecular formula C₈H₁₆ is predicted using ¹³CNMR spectra.

Interpretation Introduction

f)

To identify:

The structure of given molecular formula C₆H₁₀O.

Answer to Problem 48AP

Explanation of Solution

Calculate HDI:

The HDI calculation led to confirm about the presence of a double bond or a ring.

Interpret the given ¹³CNMR spectrum:

Broadband - decoupled spectrum:

The spectrum shows four signals whereas the given molecular formula also has six carbon atoms. Thus all the six carbons have chemically different electronic environments showing signals.

i) The four signals in the region of 0-50ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene or Methine groups.

DEPT -135 (gives signals of CH₂, C groups):

ii) The two negative signals indicate the presence of four methylene groups; only methylene groups appear negative in the spectrum.

iii) So far the group of fragments obtained is

C=O, -C-, four -ch₂

From the fragments (CH₂)₅ + C = 10 protons) ten protons are obtained whereas the total number of protons from the molecular formula is also ten.

Conclusion

The structure of a given molecular formula C₆H₁₀O is predicted using ¹³CNMR spectra.