Chapter 10.6, Problem 28E

(a)

Interpretation Introduction

Interpretation: Absorption associated with carboxylic acid and alkene functional groups and trans $\text{C}=\text{C}$ in the IR spectrum of $\left(E\right)\text{-cinnamic acid}$ should be identified.

Concept introduction:The IR spectrum of compound helps to detect the presence of the functional group in molecule. The functional groups that are present in compound absorb certain IR radiation at a characteristic wavelength or wavenumber. For example, the carbonyl group present in compound shows a characteristic peak around $1700{\text{ cm}}^{-1}$ in the IR spectrum.

Explanation of Solution

The typical infrared absorption frequencies values are given as follows:

  

IR spectrum of $\left(E\right)\text{-cinnamic acid}$ is given as follows:

  

The sharp absorption signal corresponding to carboxylic acid appears near $1700{\text{ cm}}^{-1}$ . The signal corresponding to alkene appears at $1660{\text{ cm}}^{-1}$ .

The fingerprint region that lies within $1250–600{\text{ cm}}^{-1}$ shows the evidence for trans- $\text{C}=\text{C}$ bond. The signal near $965{\text{ cm}}^{-1}$ is characteristic for $\text{C}=\text{C}-\text{H}$ bending.

(b)

Interpretation Introduction

Interpretation: The various resonances corresponding to ${}^1\text{H}$ NMR of hydrogen nuclei found in $\left(E\right)\text{-cinnamic acid}$ should be assigned.

Concept introduction:The scale used for the NMR spectrum is delta scale. $\text{δ}$ denotes parts per million units. The lower values of $\text{δ}$ denote upfield while higher values of $\text{δ}$ denote downfield region. Each peak in the NMR spectrum corresponds to distinct hydrogen.

The area within each peak corresponds to the number of equivalent protons found at that chemical shift values.

Spin-spin splitting is observed as a result of the interaction amongst non-equivalent NMR active nuclei. This is independent of the strength of the external magnetic field. The formula to calculate the number of peaks in the ${}^1\text{H}$ NMR spectra is as follows:

  $\text{Number of peaks}=N+1$

Where,

• $N$ is the number of equivalent protons on the adjacent carbon atoms.

Thus if a fragment is $-{\text{CH}}_{\text{2}}-{\text{CH}}_{\text{3}}-$ then it would show a three proton triplet due to ${\text{CH}}_{\text{3}}$ and a two proton quartet in accordance with $N+1$ the rule.

Explanation of Solution

The typical chemical shifts values are given as follws:

  

The ${}^1\text{H}$ NMR spectra corresponding to $\left(E\right)\text{-cinnamic acid}$ is given as follows:

  

The signals are magnified and hence various resonances in ${}^1\text{H}$

NMR are assigned as follows:

  

(c)

Interpretation Introduction

Interpretation: The manner the resonances from the vinylic hydrogens can be used to distinguish E and Z on the basis of ${}^1\text{H}$ the NMR spectrum of $\left(E\right)\text{-cinnamic acid}$ should be determined.

Concept introduction:The scale used for the NMR spectrum is delta scale. $\text{δ}$ denotes parts per million units. The lower values of $\text{δ}$ denote upfield while higher values of $\text{δ}$ denote downfield region. Each peak in the NMR spectrum corresponds to distinct hydrogen.

The area within each peak corresponds to the number of equivalent protons found at that chemical shift values.

Spin-spin splitting is observed as a result of the interaction amongst non-equivalent NMR active nuclei. This is independent of the strength of the external magnetic field. The formula to calculate the number of peaks in the ${}^1\text{H}$ NMR spectra is as follows:

  $\text{Number of peaks}=N+1$

Where,

• $N$ is the number of equivalent protons on the adjacent carbon atoms.

Thus if a fragment is $-{\text{CH}}_{\text{2}}-{\text{CH}}_{\text{3}}-$ then it would show a three proton triplet due to ${\text{CH}}_{\text{3}}$ and a two proton quartet in accordance with $N+1$ the rule.

Explanation of Solution

The typical chemical shifts values are given as follows:

  

The signals are magnified and hence various resonances in ${}^1\text{H}$

NMR are assigned as follows:

  

The coupling constant values indicate whether the alkene is E or Z. If E alkene is present the coupling constant should come tobe much higher than in the case of Z alkene that has a lower value of coupling constant.

This coupling constant is essentially the difference between two adjacent peaks in the NMR spectrum. Since the space between protons on olefin is more, therefore, it can be concluded that more coupling constant is indicative of E stereochemistryin $\left(E\right)\text{-cinnamic acid}$ .

(d)

Interpretation Introduction

Interpretation: The resonances of ${}^{13}\text{C}$ NMR should be assigned.

Concept introduction: The scale used for the ${}^{13}\text{C}$ NMR spectrum is the delta scale with the only difference is larger chemical shift. The range lies from $\text{δ 0}-\text{250}$ denotes parts per million unit. The lower values of $\text{δ}$ denote upfield while higher values of $\text{δ}$ denote downfield region. Each peak in the NMR spectrum corresponds to a distinct carbon.

Explanation of Solution

The typical chemical shifts values are given as follows:

  

The symmetry in the structure suggests there are four types of aromatic carbon that appear $\text{δ}$ at 128.5, 128.6, 127.9 and 135.2.

The ethylene benzylic carbon appears at 116.6 while others olefinic and 144.2 respectively. The carbonyl carbon of carboxylic acid appears at 171.5.

Therefore these seven distinct carbon ${}^{13}\text{C}$

NMRresonances are assigned as follows: