Analytical Techniques

Compound A is analysed by carbon-13 NMR and proton ($^1\text{H}$) NMR spectroscopy.

State the reference substance and a solvent that can be used in NMR spectroscopy.

reference ..................................................................................................................................

solvent ......................................................................................................................................

Predict the number of peaks in the carbon-13 NMR spectrum of A.

The proton ($^1\text{H}$) NMR spectrum of A shows peaks in four different chemical environments.

Complete Table 8.1 for the proton ($^1\text{H}$) NMR spectrum of A.

Table 8.1

Define the following terms used in chromatography.

• $R_{\text{f}}$ value
• retention time

Each type of chromatography makes use of a stationary phase and a mobile phase.

Complete Table 6.1 with a description of each of these.

A mixture of two substances A and B is analysed by thin-layer chromatography.

The $R_{\text{f}}$ value of substance A is larger than that of substance B.

Suggest why substance A has a larger $R_{\text{f}}$ value.

Complete Table 6.2 to predict the number of peaks observed in the proton ($^1\text{H}$) NMR spectra for Y and Z.

Name all the different splitting patterns observed in the proton ($^1\text{H}$) NMR spectra for Y and Z.

Define the following terms used in chromatography.

$R_{\text{f}}$ value .............................................................................................................................

retention time ....................................................................................................................

Each type of chromatography makes use of a stationary phase and a mobile phase.

Complete Table 6.1 with a description of each of these.

Table 6.1

A mixture of two substances A and B is analysed by thin-layer chromatography.

The $R_{\text{f}}$ value of substance A is larger than that of substance B.

Suggest why substance A has a larger $R_{\text{f}}$ value.

Complete Table 6.2 to predict the number of peaks observed in the proton ($^1\text{H}$) NMR spectra for Y and Z.

Table 6.2

Name all the different splitting patterns observed in the proton ($^1\text{H}$) NMR spectra for Y and Z.

Y ........................................................................................................................................

Z ........................................................................................................................................

Name, or describe in detail, a suitable substance that could be used for each phase.

stationary:

mobile:

A mixture of three organic compounds is separated by gas-liquid chromatography. The chromatogram obtained is shown in Fig. 9.1. The amount of each substance is proportional to the area under its peak.

Explain the meaning of retention time.

Calculate the percentage of B in the mixture. Show your working.

Complete Table 9.1 to give the number of peaks in the carbon-13 NMR spectrum of each of the five isomers of $\text{C}_5\text{H}_{10}\text{O}_2$ that has an ester group.

Table 9.1

State the number of peaks that would be seen in the proton ($^1\text{H}$) NMR spectrum of methyl butanoate, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CO}_2\text{CH}_3$. Name all the splitting patterns seen in this spectrum.

number of peaks:

splitting patterns:

Deduce the structures of the two esters D and E and draw their displayed formulae in the boxes below.

The spectrum of D includes a quartet at $\delta\ 4.1$.

Identify the protons responsible for this quartet on your structure in (i) by labelling these protons with the letter F.

Explain why this peak is split into a quartet.

The spectrum of E has a doublet at $\delta\ 1.1$.

Identify the protons responsible for this doublet on your structure in (i) by labelling these protons with the letter G.

Explain why this peak has a chemical shift of 1.1.

Name, or describe in detail, a suitable substance that could be used for each phase.

stationary ............................................................................................................................

mobile .................................................................................................................................

A mixture of three organic compounds is separated by gas-liquid chromatography. The chromatogram obtained is shown in Fig. 9.1. The amount of each substance is proportional to the area under its peak.

Explain the meaning of retention time.

Calculate the percentage of B in the mixture. Show your working.

Complete Table 9.1 to give the number of peaks in the carbon-13 NMR spectrum of each of the five isomers of $\text{C}_5\text{H}_{10}\text{O}_2$ that has an ester group.

Table 9.1

State the number of peaks that would be seen in the proton ($^1\text{H}$) NMR spectrum of methyl butanoate, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CO}_2\text{CH}_3$. Name all the splitting patterns seen in this spectrum.

number of peaks ........................................................................................................................

splitting patterns ........................................................................................................................

Deduce the structures of the two esters D and E and draw their displayed formulae in the boxes below.

The spectrum of D includes a quartet at $\delta\ 4.1$.

Identify the protons responsible for this quartet on your structure in (i) by labelling these protons with the letter F.

Explain why this peak is split into a quartet.

The spectrum of E has a doublet at $\delta\ 1.1$.

Identify the protons responsible for this doublet on your structure in (i) by labelling these protons with the letter G.

Explain why this peak has a chemical shift of $1.1$.

Explain why $\text{CDCl}_3$ is used as a solvent instead of $\text{CHCl}_3$.

Explain why TMS is added to give the small peak at chemical shift $\delta = 0$.

Compound E is hydrolysed by hot $\text{NaOH(aq)}$, giving two organic products only. One of these products is ethanol.

Name the functional group in compound E that is hydrolysed by hot $\text{NaOH(aq)}$.

Describe and explain the splitting patterns of the peaks at $\delta = 1.4$ and $\delta = 4.3$.

Each molecule of compound E contains five protons which give rise to the peaks between $\delta = 7.0$ and $\delta = 8.5$.

Identify the functional group in compound E which contains these protons.

Give the structural formula of compound E.

The mass spectrum of compound E includes fragment ions with $m/e$ values of 29 and 77.

Give the formulae of these fragment ions.

Suggest what is meant by the term retention time.

Give an example of a carrier gas used in gas chromatography.

Z spends the longest time in the chromatography column.

Suggest why this might be the case.

Explain a possible limitation of gas/liquid chromatography in separating two esters such as ethyl methanoate, $\text{HCO}_2\text{CH}_2\text{CH}_3$, and methyl ethanoate, $\text{CH}_3\text{CO}_2\text{CH}_3$.

A student works out the areas underneath the three peaks in the chromatogram.

Assuming the areas underneath the peaks are proportional to the masses of the respective components, what percentage of the original mixture was made up of the organic compound, X?

What compound is responsible for the absorption at $\delta = 0$?

Compound Y is an ester with the molecular formula $\text{C}_4\text{H}_8\text{O}_2$.

Complete the table for the NMR spectrum of Y.
The actual chemical shifts for three absorptions in Y and the splitting pattern for the resonance at $\delta = 3.7 \text{ ppm}$ have been given for you.
Use of the Data Booklet may be helpful.

Use your conclusions to suggest a structure for the ester Y.

The mass spectrum of a compound G contains M and M+1 peaks in the ratio of their heights of 74 : 2.5.
Use these data to calculate the number of carbon atoms present in G. Show your working.

Use the Data Booklet and your knowledge of NMR spectroscopy to identify the type of proton responsible for each of the three absorptions.

The addition of $\text{D}_2\text{O}$ causes one of these absorptions to disappear.
Explain why this happens and state which absorption is affected.

Draw the structural formula of G.

Draw the structural formula of an isomer of G with only two absorptions in its NMR spectrum.

Use the Data Booklet to suggest where these absorptions would occur.

The mass spectrum of a compound G contains M and M+1 peaks in the ratio of their heights of 74:2.5.
Use these data to calculate the number of carbon atoms present in G. Show your working.

Use the Data Booklet and your knowledge of NMR spectroscopy to identify the type of proton responsible for each of the three absorptions.

The addition of $\text{D}_2\text{O}$ causes one of these absorptions to disappear.
Explain why this happens and state which absorption is affected.

Draw the structural formula of G.

Draw the structural formula of an isomer of G with only two absorptions in its NMR spectrum.

Use the Data Booklet to suggest where these absorptions would occur.