3.6 Organic Analysis

Propanal, CH₃CH₂CHO, and propanone, CH₃COCH₃, are carbonyl compounds. When these compounds are compared using analytical methods, which of the following statements is not correct?

A         The compounds produce different patterns in the fingerprint region of the IR spectrum.

B         The carbonyl groups absorb at frequencies in the same region of the IR spectrum.

C         The compounds produce different fragmentation patterns in a mass spectrum.

D         The compounds have molecular ion peaks at different mass to charge ratios in a mass spectrum.

This question is about compounds with the molecular formula C₄H₈O.

Three different compounds, A, B and C, have the molecular formula C₄H₈O.

All three compounds have infrared absorptions at about 3500 cm^-1.
Only the infrared spectra of A and B each contain a peak at about 1650 cm^-1.
A is the only branched carbon chain.
B is the E-isomer of a pair of stereoisomers.

Deduce a possible displayed formula for each of the compounds A, B and C.

You must use all of the data and the information in the Data Sheet to justify each of your structures.

A student analysed the spectrum in Figure 2 and correctly deduced that the image was that of butanal.

Figure 2

Suggest why this spectrum cannot be that of butanoic acid, but-1-ene or butan-1-ol. Justify your answer.

The mass spectrum of 2-methylpropan-2-ol is shown in Figure 3. The relative molecular mass of 2-methylpropan-2-ol is 74.

Figure 3

The compound 2-bromopropane was heated at high temperatures under reflux with concentrated potassium hydroxide dissolved in a mixture of water and ethanol. There were two organic products, P and Q, formed in this reaction. P and Q belong to different homologous series.

After separating P and Q, their spectra were obtained.

The mass spectrum of P shows peaks at m/z = 45 and at m/z = 60.

Use this information to identify P. Justify your answer.

The infrared spectra of P and Q (from part a) are shown in Figure 1.

Figure 1

Explain how each spectrum shows the homologous series to which each product belongs.

A student predicted that the rate of hydrolysis must decrease as you go down Group 7. Fluorine is the most electronegative element in group 7 and must therefore react the fastest with the nucleophile.

Explain whether you agree or disagree with their conclusion. Justify your answer.

Another student wanted to measure the rate of hydrolysis of three haloalkanes: 1-chlorobutane, 1-bromobutane and 1-iodobutane.

Explain what chemical tests the student could carry out to prove their conclusions were correct from part (c). Include in your answer any relevant observations and overall equation.

Another student wanted to measure the rate of hydrolysis of three haloalkanes: 1-chlorobutane, 1-bromobutane and 1-iodobutane.

In a second experiment, the student used three structural isomers of C₄H₉Br, shown in Table 1 and noticed these isomers also hydrolysed at different rates. State the trend in the different reaction rates and explain your ideas.

Table 1

Compound F is a trans stereoisomer which is a useful intermediate in organic synthesis.

Analysis of compound F shows it to contain 68.6 % carbon, 8.6 % hydrogen and 22.8 % oxygen. The infrared spectrum of compound F is shown in Figure 1.

Figure 1

In the mass spectrum, shown in Figure 2, the peak with the greatest relative intensity is caused by the loss of a functional group from the molecular ion of compound F.

Figure 2

Determine the structure of compound F. Explain your reasoning and show your working.

Crotonaldehyde, CH₃CH=CHCHO, occurs in soybean oils.

Table 1 shows the reagents that crotonaldehyde will be reacted with in four separate reactions.

Table 1

Complete Table 1 by writing the structural formulae of the organic compounds and stating any colour changes you would expect to observe for each reaction.

Use the information from part (b) to draw the skeletal formula of crotonaldehyde.

Crontonaldehyde can be formed from alcohols.

State what reagents and conditions are needed for this reaction.

An alcohol can be prepared by hydrolysing the halogenoalkane C₂H₅CHBrCH₃ with aqueous sodium hydroxide.

The infrared spectrum for C₂H₅CHBrCH₃ is shown in Figure 1 with the C–Br bond absorption labelled.

Figure 1

Outline how IR spectroscopy will change as a result of the above reaction.

The mass spectrum of (CH₃)₂CHCH₂OH is shown below in Figure 1.

Figure 1

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

This question is about the identification of an alcohol, X.

Alcohol X has the following percentage composition by mass:

• Carbon, C = 68.2%
• Hydrogen, H = 13.6%
• Oxygen, O = 18.2%

The molecular ion peak in the mass spectrum for alcohol X occurs at m/z = 88.

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

Figure 1

State the simple test-tube reaction which could be done to distinguish between Compound B and Compound C, including the observations that could be made.

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 (c). The student analysed their product and obtained the following IR spectrum.

Figure 2

Use the spectrum in Figure 2 to suggest whether full oxidation has taken place.

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 four types of organic compounds stated above, using the minimum number of tests necessary on each sample.

One of the students wanted to distinguish between three compounds using mass spectrometry. The three compounds are shown below in Figure 1.

Figure 1

Explain why determining the exact mass using mass spectrometry 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 2

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

Explain why infrared spectroscopy alone could not be used to distinguish between the compounds X and Y from Figure 1 in part (b).

Infrared spectroscopy is a common analytical tool used in chemistry, to identify or distinguish between different organic compounds.

State how an IR  spectrum could be used to prove that a ketone was butan-2-one. 

Breathalysers are used to test whether someone has been drinking alcohol while driving. Early models of breathalysers used chemicals which changed colour during an oxidation reaction.

Inside the breathalyser, ethanol was oxidised to ethanoic acid using the same reagents and the same oxidation reaction which would be completed in a lab.

State the reagents which are used to cause this oxidation and state the colour change which would be observed.

The mass spectrum of the ketone, hexan-2-one has a large fragment ion at m/z = 43.

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.

A student was investigating the structure of three hydrocarbons, L, M and N.

L and M are isomers.

L has two stereoisomers, but neither of the other compounds do.

Define the term stereoisomer and explain the key feature which must be present in L for it to show stereoisomerism.

The homologous series that L and M belong to have the general formula C_nH_2n and the homologous series of N has the general formula C_nH_2n+2.

State a simple test-tube reaction that could be done to distinguish between L and N, including the observations you would see.

Simple test-tube reactions like those in part (b) can also be used to distinguish between the following organic compounds.

2-bromopropane and hexane.

State the test-tube reaction which could be used and any observations that would be seen.

Simple test-tube reactions like those in part (b) and part (c) can also be used to distinguish between the following pair of organic compounds.

Pentan-3-ol and 3-methyl-pentan-3-ol

State the test-tube reaction which could be used and any observations that would be seen.