Chapter 21, Problem 21.81P

Use the ${}^1\text{H}\text{NMR}$ and $\text{IR}$ data to determine the structure of each compound.

Compound A Molecular formula: ${\text{C}}_5{\text{H}}_{\text{10}}\text{O}$

$\text{IR}$ absorptions at $1728,2791,\text{and}\text{2700}{\text{cm}}^{-1}$

${}^1\text{H}\text{NMR}$ data: $\begin{array}{l}\text{1}\text{.08}\left(\text{singlet,}\text{9}\text{H}\right)\text{and}\\ \text{9}\text{.48}\left(\text{singlet,}\text{1}\text{H}\right)\text{ppm}\end{array}$

Compound B Molecular formula: ${\text{C}}_5{\text{H}}_{\text{10}}\text{O}$

$\text{IR}$ absorptions at $1718{\text{cm}}^{-1}$

${}^1\text{H}\text{NMR}$ data: $\begin{array}{l}\text{1}\text{.10}\left(\text{doublet,}\text{6}\text{H}\right),\text{2}\text{.14}\\ \left(\text{singlet,}\text{3}\text{H}\right),\text{and}\\ \text{2}\text{.58}\left(\text{septet,}\text{1}\text{H}\right)\text{ppm}\end{array}$

Compound C Molecular formula: ${\text{C}}_{10}{\text{H}}_{\text{12}}\text{O}$

$\text{IR}$ absorptions at $1686{\text{cm}}^{-1}$

${}^1\text{H}\text{NMR}$ data: $\begin{array}{l}\text{1}\text{.21}\left(\text{triplet,}3\text{H}\right),\text{2}\text{.39}\\ \left(\text{singlet,}\text{3}\text{H}\right),\text{2}\text{.95}\\ \left(\text{quartet,}\text{2}\text{H}\right),\text{7}\text{.24}\\ \left(\text{doublet,}\text{2}\text{H}\right),\text{and}\text{7}\text{.85}\\ \left(\text{doublet,}\text{2}\text{H}\right)\text{ppm}\end{array}$

Compound D Molecular formula: ${\text{C}}_{10}{\text{H}}_{\text{12}}\text{O}$

$\text{IR}$ absorptions at $1719{\text{cm}}^{-1}$

${}^1\text{H}\text{NMR}$ data: $\begin{array}{l}\text{1}\text{.02}\left(\text{triplet,}3\text{H}\right),\text{2}\text{.45}\\ \left(\text{quartet,}\text{2}\text{H}\right),\text{3}\text{.67}\\ \left(\text{singlet,}\text{2}\text{H}\right),\text{and}\\ \text{7}\text{.06-7}\text{.48}\\ \left(\text{multiplet,}\text{5}\text{H}\right)\text{ppm}\end{array}$

Interpretation Introduction

Interpretation: The structure of each compound from the given $\text{IR}$, and ${}^1\text{H}\text{NMR}$ data is to be stated.

Concept introduction: Spectroscopy method is used to identify the structure of the molecule. It is based on the interactions between matter and electromagnetic radiations. Proton $\text{NMR}$ spectroscopy identifies the number of hydrogen atoms present in a molecule and the nature of the functional group. The value of chemical peaks depends upon the chemical environment around the hydrogen atom.

Answer to Problem 21.81P

The structure of A compound from the given data of $\text{IR}$, and ${}^1\text{H}\text{NMR}$ is shown below:

The structure of B compound from the given data of $\text{IR}$, and ${}^1\text{H}\text{NMR}$ is shown below:

The structure of C compound from the given data of $\text{IR}$, and ${}^1\text{H}\text{NMR}$ is shown below:

The structure of D compound from the given data of $\text{IR}$, and ${}^1\text{H}\text{NMR}$ is shown below:

Explanation of Solution

Structure determination of Compound A:

The molecular formula of the Compound A is ${\text{C}}_5{\text{H}}_{\text{10}}\text{O}$.

Information from $\text{IR}$ spectrum:

The $\text{IR}$ absorption at $1728{\text{cm}}^{-1}$ suggests the presence of $\text{C}=\text{O}$ bond, carbonyl group.

The $\text{IR}$ absorption at $2791,\text{and}\text{2700}{\text{cm}}^{-1}$ suggests the presence of $\text{CHO}$ group.

Information from ${}^1\text{H}\text{NMR}$ spectrum:

The observed chemical shift value at $\text{1}\text{.08}\left(\text{singlet,}\text{9}\text{H}\right)\text{ppm}$ suggests the presence of three methyl groups, bonded to carbon that has no hydrogen atoms.

The observed chemical shift value at $\text{9}\text{.48}\left(\text{singlet,}\text{1}\text{H}\right)\text{ppm}$ suggests the presence of $\text{CHO}$ group.

Thus, the structure of Compound A from the given $\text{IR}$, and ${}^1\text{H}\text{NMR}$ data is,

Figure 1

Structure determination of Compound B:

The molecular formula of the Compound B is ${\text{C}}_5{\text{H}}_{\text{10}}\text{O}$.

Information from $\text{IR}$ spectrum:

The $\text{IR}$ absorption at $1718{\text{cm}}^{-1}$ suggests the presence of $\text{C}=\text{O}$ bond, carbonyl group.

Information from ${}^1\text{H}\text{NMR}$ spectrum:

The observed chemical shift value at $\text{1}\text{.10}\left(\text{doublet,}\text{6}\text{H}\right)\text{ppm}$ suggests the presence of two methyl groups, bonded to carbon that has $1\text{H}$.

The observed chemical shift value at $\text{2}\text{.14}\left(\text{singlet,}\text{3}\text{H}\right)\text{ppm}$ suggests the presence of one methyl group, bonded to carbon that has no hydrogen atoms.

The observed chemical shift value at $\text{2}\text{.58}\left(\text{septet,}\text{1}\text{H}\right)\text{ppm}$ suggests the presence of $\text{CH}$ group bonded to two methyl groups, and carbon that has no hydrogen atoms.

Thus, the structure of Compound B from the given $\text{IR}$, and ${}^1\text{H}\text{NMR}$ data is,

Figure 2

Structure determination of Compound C:

The molecular formula of the Compound C is ${\text{C}}_{10}{\text{H}}_{\text{12}}\text{O}$.

Information from $\text{IR}$ spectrum:

The $\text{IR}$ absorption at $1686{\text{cm}}^{-1}$ suggests the presence of $\text{C}=\text{O}$ bond, carbonyl group.

Information from ${}^1\text{H}\text{NMR}$ spectrum:

The observed chemical shift value at $\text{1}\text{.21}\left(\text{triplet,}3\text{H}\right)\text{ppm}$ suggests the presence of methyl group, bonded to carbon that has $2\text{H's}$.

The observed chemical shift value at $\text{2}\text{.39}\left(\text{singlet,}\text{3}\text{H}\right)\text{ppm}$ suggests the presence of one methyl group, bonded to carbon that has no hydrogen atoms.

The observed chemical shift value at $\text{2}\text{.95}\left(\text{quartet,}\text{2}\text{H}\right)\text{ppm}$ suggests the presence of $-{\text{CH}}_2-$ group bonded to carbon that has $3\text{H's}$. The slightly downfield absorption indicates that some electron withdrawing group, $\left(\text{C}=\text{O}\right)$ is also bonded to it.

The observed chemical shift values at $\text{7}\text{.24}\left(\text{doublet,}\text{2}\text{H}\right)\text{ppm}$, and $\text{7}\text{.85}\left(\text{doublet,}\text{2}\text{H}\right)\text{ppm}$ suggest the presence of benzene ring on which substituents are present at para position to each other.

Thus, the structure of Compound C from the given $\text{IR}$, and ${}^1\text{H}\text{NMR}$ data is,

Figure 3

Structure determination of Compound D:

The molecular formula of the Compound D is ${\text{C}}_{10}{\text{H}}_{\text{12}}\text{O}$.

Information from $\text{IR}$ spectrum:

The $\text{IR}$ absorption at $1719{\text{cm}}^{-1}$ suggests the presence of $\text{C}=\text{O}$ bond, carbonyl group.

Information from ${}^1\text{H}\text{NMR}$ spectrum:

The observed chemical shift value at $\text{1}\text{.02}\left(\text{triplet,}3\text{H}\right)\text{ppm}$ suggests the presence of methyl group, bonded to carbon that has $2\text{H's}$.

The observed chemical shift value at $\text{2}\text{.45}\left(\text{quartet,}\text{2}\text{H}\right)\text{ppm}$ suggests the presence of $-{\text{CH}}_2-$ group bonded to carbon that has $3\text{H's}$. The slightly downfield absorption indicates that some electron withdrawing group, $\left(\text{C}=\text{O}\right)$ is also bonded to it.

The observed chemical shift value at $\text{3}\text{.67}\left(\text{singlet,}\text{2}\text{H}\right)\text{ppm}$ suggests the presence of $-{\text{CH}}_2-$ group bonded to carbon that has no hydrogen atoms. The slightly downfield absorption indicates that some electron withdrawing group, $\left(\text{C}=\text{O}\right)$ is also bonded to it.

The observed chemical shift value at $\text{7}\text{.06-7}\text{.48}\left(\text{multiplet,}\text{5}\text{H}\right)\text{ppm}$ suggests the presence of monosubstituted benzene ring.

Thus, the structure of Compound D from the given $\text{IR}$, and ${}^1\text{H}\text{NMR}$ data is,

Figure 4

Conclusion

The structures of A, B, C, and D compounds from the given data of $\text{IR}$, and ${}^1\text{H}\text{NMR}$ are shown in Figure 1, Figure 2, Figure 3, and Figure 4 respectively.