11.1 Spectroscopic Identification

An alcohol can be prepared by hydrolysing the halogenoalkane C₂H₅CHBClCH₃ with aqueous sodium hydroxide. The infrared spectrum for C₂H₅CHClCH₃  is shown below with the C–Cl bond absorption labelled.

Using section 26 of the data booklet deduce how IR spectroscopy will change as a result of the above reaction.

The mass spectrum of (CH₃)₂CHCH₂OH is shown below.

Deduce which ion is responsible for the peak with the greatest relative intensity.

Alcohol X has the following percentage composition by mass. Carbon = 68.2%, hydrogen = 13.6%, oxygen = 18.2%. The molecular ion peak in the mass spectrum for alcohol X occurs at m/z = 88.

The mass spectrum of alcohol X has a major peak at m/z = 45.

Clenbuterol, shown below, is considered a performance enhancing drug and is believed to increase short term work rate and cardiovascular output.

Deduce the functional groups marked x and y and state to which class they belong to.

Determine the m/z value of the molecular ion, M⁺, of clenbuterol. Justify your answer.

Two students, P and Q, were provided with the mass spectrum of an alkane, shown in below. Student P analysed peaks a and b and concluded that the alkane was one of two structures. Student Q analysed peaks a, b and c and was able to identify one possible alkane structure.

Four samples containing isomeric alcohols with molecular formula C₄H₁₀O, were studied using ¹H NMR spectroscopy

Draw structural formulas of the alcohols and deduce the number of peaks in the NMR spectrum of each alcohol

Two of the alcohols produce the same number of peaks in an ¹H NMR spectrum. Suggest how they may be distinguished, by further spectroscopy analysis.

Compound A can be converted into compound B via an intermediate species.

Suggest how you would be able to determine the difference between Compounds A and B by analysis of their ¹H NMR spectra.

Toluene is a common organic chemical with many industrial and commercial applications. Toluene is also known as methylbenzene. 

Draw the displayed structure of toluene.  

State the number of ¹H NMR signals that would be seen on the NMR spectrum of toluene and state the ratio of the area under the peaks in which they would appear.  

Another derivative of benzene has the molecular formula C₈H₁₀. 

Draw the structures of the four possible isomers of this derivative.

Infrared spectroscopy is a common analytical tool used in chemistry, to identify or distinguish between different organic compounds. These could be compounds such as toluene which contain a benzene ring, or any other organic compound. IR is often used alongside other analytical tools, such as mass spectrometry, to help identify the unknown. 

An organic molecule, J, has the following composition by mass.  

            62.1% carbon

            10.3% hydrogen 

            27.6% oxygen

In the mass spectrum of J, the molecular ion peak has a value of m/z = 116. 

You are provided with the following selection of organic compounds, to investigate using test-tube reactions and spectroscopy. 

Figure 1 

State the index of hydrogen deficiency, IHD, of compound B and what this tells you about the molecule.  

State the simple test-tube reactions that could be done to distinguish between Compound A and Compound D from part (a), including the observations that could be made. 

Compound D in part (a) can be formed from an alcohol if the correct reagents and conditions are used for the reaction. 

A student claims to have fully oxidised a different primary alcohol than that in part
The student analysed their product and obtained the following IR spectrum.  

Figure 2

Use the spectrum in Figure 2 and section 26 of the data booklet to suggest whether full oxidation has taken place.

The analytical instruments used for identification of organic compounds are constantly being improved.            

 Mass spectroscopy is one such analytical tool which provides key information used to identify an unknown compound. 

An unknown compound has the empirical formula C₂H₄O, and its mass spectrum has a molecular ion peak at m/z 84.

Deduce the molecular formula of the compound.  

Figure 1 below shows the IR spectrum of the unknown compound in part (a). 

Identify the bonds which are causing peak X and peak Y on the spectrum, using section 26 of the data booklet.

Figure 1

The unknown compound is a carboxylic acid. Deduce the two possible carboxylic acid structural isomers.  

For each of the isomers drawn in part (c), state the number of signals which would be seen in an ¹H NMR spectrum and the ratio of the areas under the peaks.

A group of students are asked to distinguish between four samples of different organic compounds.            

The four samples are as follows: 

• A primary alcohol 
• A tertiary alcohol 
• An aldehyde 
• A carboxylic acid

Describe how the group of students could distinguish between the two different alcohols.

One of the students wanted to distinguish between three compounds using mass
spectrometry alone. The three compounds are shown below in Figure 1 and the mass spectrum for one of the compounds is shown in Figure 2.  

Figure 1

                              Figure 2

Explain, using Figure 2,  why determining the exact mass using mass spectrometry alone would not help in distinguishing between the samples of X, Y and Z shown in Figure 1.

The three compounds from Figure 1 in part (b) were analysed using IR spectroscopy. 
 The spectrum of one of the compounds is shown below: 

Figure 3

Identify which of the three compounds X, Y or Z this spectrum belongs to. Justify your choice. 

State, with a reason, how infrared spectroscopy would be used to distinguish between the compounds X and Y from Figure 1 in part (b).