Functional Groups: Classification of Organic Compounds (DP IB Chemistry: HL): Exam Questions

Organic compounds are classified into families called a homologous series.

State three features of members belonging to the same homologous series.

The table below shows the boiling points of the first five members of the alkane family.

 On the axes below, draw a graph of boiling point against the number of carbon atoms in the alkanes. Estimate the boiling point of the next member of the homologous series, hexane, C₆H₁₄, and show on your graph how you arrived at your estimated boiling point. 

Estimated boiling point of hexane : ________ ^oC

i) State the general formula for an alkyne.

[1]

ii) Give the molecular formula and name of the fifth member of the alkyne family.

[1]

The boiling point of ethyne, C₂H₂, is -84 ^oC.

State with, with a reason, whether the boiling point of ethyne would be expected to be higher or lower than the boiling point of ethane, C₂H₆.

Often, scientists will use analytical techniques such as infrared spectroscopy to determine if a desired reaction has taken place.

Outline how infrared spectroscopy generates useful information about an organic molecule.

A chemist uses infrared spectroscopy to distinguish between two organic molecules, a primary alcohol, and an aldehyde. 

Using section 20 of the data booklet, explain how the chemist could use the IR spectra produced to distinguish between these two molecules.

The chemist fully oxidised the alcohol from part (b) and used IR spectroscopy to prove that the reaction had taken place. 

State how the chemist could use infrared spectroscopy to determine that the reaction had taken place. Use section 20 of the data booklet to support your answer.

Toluene, also known as methylbenzene, is an important industrial solvent and precursor.

Draw the skeletal formula of toluene.

The ¹H NMR spectrum of toluene is unexpectedly complex.

i) Predict the number of signals that would be seen in the spectrum.

[1]

ii) State the integration ratio for these signals.

[1]

Benzene reacts with chloromethane in the presence of an AlCl₃ catalyst to form toluene.

i) State the role of chloromethane in this reaction.

[1]

ii) Explain the role of the AlCl₃ catalyst in this reaction, identifying the electrophile that is generated.

[2]

The molecular formula C₈H₁₀ corresponds to four aromatic isomers which all contain a single benzene ring.

One isomer is ethylbenzene. The other three are positional isomers of dimethylbenzene.

Draw the skeletal formula for ethylbenzene and 1,4-dimethylbenzene.

State the IUPAC name for the other two positional isomers of dimethylbenzene.

Explain how ¹H NMR spectroscopy could be used to distinguish between the three dimethylbenzene isomers, by predicting the number of signals in the aromatic region of each spectrum.

An unknown organic molecule, J, contains only carbon, hydrogen, and oxygen and has the following composition by mass:

• 62.1% carbon

• 10.3% hydrogen

• 27.6% oxygen

The mass spectrum of J shows a molecular ion peak at m/z = 116.

Determine the molecular formula of J.

The infrared spectrum of J shows a very broad absorption between 2500-3000 cm^-1 and a strong, sharp absorption at around 1710 cm^-1.

Deduce the functional group present in J.

Assuming J is an unbranched molecule, state its IUPAC name.

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 88.

Deduce the molecular formula of the compound.  

The IR spectrum of the unknown compound in part (a) is shown below.

Identify the bonds which are causing peak X and peak Y on the spectrum, using section 20 of the data booklet.

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, X, Y and Z, using mass spectrometry alone.

The mass spectrum for one of the compounds is shown below.

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

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

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

Explain how infrared spectroscopy would be used to distinguish between the compounds X and Y from part (b).

Unsaturated carboxylic acids are a class of organic compounds found widely in nature.

The structure of oleic acid is shown below.

State the molecular formula and the empirical formula of oleic acid.

State what is meant by the term stereoisomer.

Explain why oleic acid exhibits cis-trans isomerism.

Crotonic acid, C₄H₆O₂, is a simpler unsaturated carboxylic acid.

Draw the displayed formula of the two positional isomers of butenoic acid.

One of the isomers drawn in (d) also exhibits cis-trans isomerism. Draw the skeletal structures of these two stereoisomers.

Draw the displayed formula of a branched-chain isomer of butenoic acid.

A chemist is analysing the four compounds shown in the table.

Compound	Structure
1
2
3
4

i) Determine the molecular formula of Compound 1.

[1]

ii) Determine the empirical formula of Compound 4.

[1]

State the IUPAC name for Compound 2 and Compound 3.

A student suggests using acidified potassium dichromate(VI) to distinguish between Compound 1 and Compound 3.

i) Explain why this test would not distinguish between the two compounds.

[1]

ii) Describe a different simple chemical test that would distinguish between the two compounds, stating the expected observation for each.

[2]

Describe a chemical test, other than using bromine water, that would distinguish Compound 2 from Compound 4. State the expected results for both compounds.

Explain why Compound 3 exhibits cis-trans isomerism, but Compound 4 does not.

2-methylbut-2-ene can be converted into 2-methylbutan-2-ol, a liquid that smells of camphor. 

State the reagents needed to convert 2-methylbut-2-ene into 2-methylbutan-2-ol.

The reaction in part (a) produces a small amount of an isomeric co-product, X, which is optically active. 

i) State the meaning of optical activity. 

[1]

ii) Draw the structure of X.

[1]

What does optical activity indicate about the structure of X?

Explain how optical activity can be detected using a polarimeter

1,2-dichloroethene exists as two stereoisomers. Draw the structures of these isomers. 

Explain why 1,2-dichloroethene has stereoisomers.

Draw the structures of the stereoisomers of 1-bromo-1-chloroethane, C₂H₄BrCl, and show the relationship between them.

Explain the differences in chemical and physical properties between the isomers of C₂H₄BrCl.

Ethane-1,2-diol, C₂H₆O₂, can be distinguished from ethanedioic acid,C₂H₂O₄, by a number of analytic techniques including MS, IR and NMR.

The MS of these molecules is shown below. 

Which spectrum belongs to each molecule? Justify your answer.

  Spectrum A

Spectrum B

The IR spectra of ethane-1,2-diol, C₂H₆O₂, and ethanedioic acid dihydrate, C₂H₂O₄.2H₂O, are shown in spectrum C and D. Use Section 20 of the Data Booklet to answer this question.

Spectrum C

Spectrum D

Which spectrum belongs to each molecule? Justify your answer.

The ¹H NMR spectrum of ethane-1,2-diol is shown in spectrum E. Explain the significance of the spectrum.

Spectrum E

Predict the number of ¹H NMR signals and splitting pattern for ethanedioic acid. 

Interpreting a ¹H NMR spectrum involves understanding the significance of the reference standard, the number of signals, their chemical shifts, and their splitting patterns.

Tetramethylsilane (TMS) is used as the reference standard.

i) Draw the structural formula of TMS. 

[1]

ii) State two reasons why TMS is a suitable reference standard.

[2]

The number of signals in a spectrum depends on molecular symmetry.

Predict the number of signals in the ¹H NMR spectrum of 1,3-dichlorobenzene.

The structural formula of ethylbenzene is shown below.

i) Predict the number of peaks in the ¹H NMR spectrum of ethylbenzene.

[1]

ii) Predict the splitting pattern for the signal from the -CH₃ protons.

[1]

iii) Predict the splitting pattern for the signal from the -CH₂- protons.

[1]

iv) One of the hydrogen atoms in the structure of ethylbenzene shown above is labelled with an asterisk (*).

Use section 21 from the data booklet to suggest a range of δ values for the peak due to this proton in the ¹H NMR spectrum of ethylbenzene.

[1]

¹H NMR spectra are recorded against a reference standard.

Identify this standard compound and state its purpose in the spectrum.

Methyl cinnamate, C₁₀H₁₀O₂, is a white crystalline solid used in the perfume industry. The simplified ¹H NMR spectrum of methyl cinnamate is shown below.

The peak at 3.8 ppm has a relative peak area of 3 and is a singlet.

i) Identify the group of protons responsible for this peak by circling it on the structure below.

[1]

ii) Justify your choice with reference to the integration ratio and the splitting pattern.

[2]

The molecular formula C₆H₁₂O₂ corresponds to many structural isomers.

Draw the skeletal structure of an ester that shows only one signal in its ¹H NMR spectrum.

Draw the skeletal structures of the two esters with the formula C₆H₁₂O₂ that each show only two signals, both of which are singlets, with an integration ratio of 3:1.

Four isomers of C₆H₁₂O₂ are shown below.

Which isomer matches the ¹H NMR spectrum below? Justify your choice.

Isomers are molecules with the same molecular formula but different structural formulas. The properties and classification of isomers depend on their structure.

The molecular formula C₆H₁₄ corresponds to several alkane isomers.

i) Deduce the number of structural isomers of C₆H₁₄.

[1]

ii) State the IUPAC name of two branched-chain isomers of C₆H₁₄.

[2]

The formula C₆H₁₂ represents a different homologous series.

i) Draw the skeletal formula of one cyclic isomer of C₆H₁₂.

[1]

ii) State the general formula for a non-cyclic alkene.

[1]

Draw the structural formula and state the IUPAC name for a branched-chain isomer of C₆H₁₂ that has the parent name "-pent-1-ene".

An unknown compound, X, was analyzed using a range of techniques.

• Elemental analysis showed X has the percentage composition by mass of 68.1% C and 13.7% H.

• The mass spectrum of X showed a molecular ion peak at m/z = 88.

• When X is refluxed with acidified potassium dichromate(VI), it is oxidised to form compound Y.

• The IR spectrum of Y shows a strong, sharp absorption at ~1700 cm⁻¹ but no broad absorption between 2500-3600 cm⁻¹.

Determine the molecular formula of compound X.

Use the reaction and infrared evidence to deduce:

i) the functional group present in compound Y.

[1]

ii) the class of alcohol of compound X.

[1]

i) State the IUPAC names of the three possible structural isomers for X that fit the evidence from part (b).

[2]

ii) Explain how ¹H NMR spectroscopy can be used to distinguish between these isomers.

[4]

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

Identify the class of amine for the two amine functional groups, labelled x and y, present in the molecule

Determine the m/z value of the molecular ion peak, M⁺, in the mass spectrum of clenbuterol. Show your working

The mass spectrum shows major fragment ion peaks at m/z = 220 and m/z = 203.

i) Deduce the formula of the neutral radical lost to form the fragment ion at m/z = 220.

[1]

ii) Deduce the formula of the fragment ion responsible for the peak at m/z = 203.

[1]

Clenbuterol is a chiral molecule. Mark the chiral centre on the structure with an asterisk (*)

The molecular formula C₄H₁₀O corresponds to four structural isomers that are alcohols.

Draw the skeletal formula and state the IUPAC name for each of the four alcohol isomers.

Predict the number of signals that would be seen in the ¹H NMR spectrum of each of the four isomers.

Two of the alcohols produce the same number of signals.

Suggest one other spectroscopic method that could be used to distinguish between these two specific isomers.

Malonic acid, C₃H₄O₄, is naturally occurring and found in many fruits and vegetables. It contains only carbon, hydrogen and oxygen.

The MS of malonic acid is show below.

Determine the relative molecular mass of malonic acid from the spectrum and account for the peak at m/z 45, using section 22 of the Data booklet to support your answer.

The ¹H NMR spectrum of malonic acid is shown below. Use section 21 of the Data booklet to help you with this question.

Suggest the identity of the proton environments seen in the spectrum and comment on the type of signals shown.

The IR spectrum of malonic acid is shown below:

i) Identify two characteristic peaks and bonds that can be found in the spectrum of malonic acid.

[2]

ii) Explain how the spectrum can be used to distinguish malonic acid from ethanoic acid.

[1]

Draw a displayed structure for malonic acid.

An organic compound, Q, of molecular formula C_xH_yO_z, has the following MS. Use section 22 of the data booklet to help you answer this question.

i) Determine the relative molecular mass of Q and account for the peaks at m/z 15 and m/z 29.

[2]

ii) Comment on the size of the peak at m/z 43.

[1]

iii) Write an equation for the formation of the fragment at m/z 29.

[1]

The IR spectrum of Q is shown below. 

Suggest which functional group(s) could be present in Q.

The ¹H NMR spectrum of Q is shown below.

Explain the relative peak heights and splitting patterns.

Suggest the identity of Q, giving your reasons.

The molecular formula C₅H₁₀ corresponds to several structural isomers, including both alkenes and cycloalkanes.

One isomer, A, produces only a single peak in its ¹H NMR spectrum.

Deduce and draw the displayed formula of A.

The IR spectrum of A is shown below.

Using scetion 20 of the data booklet, explain how this spectrum helps to confirm the identity of isomer A.

State a chemical test that could be used to distinguish isomer A from its non-cyclic isomer, pent-1-ene. Describe the expected observation in each case.

For the isomer 2-methylbut-2-ene:

i) Predict the number of signals in its ¹H NMR spectrum.

[1]

ii) State the integration ratio of the signals.

[1]

iii) Describe the splitting pattern of the signal at the lowest chemical shift (highest field).

[1]

Oseltamivir is a drug used to treat and prevent influenza A and influenza B.

Predict the number of different proton environments in the molecule.

Using section 21 of the data booklet, predict the chemical shift and the splitting pattern seen for the hydrogens on the carbon atom circled in the diagram. Justify your answer.

Predict three absorptions you would expect to see in the IR spectrum of oseltamivir. Use section 20 of the data booklet. 

Ibuprofen is an important painkilling drug. Its structure is shown below, with key proton environments numbered 1-7.

The ¹H NMR spectrum of ibuprofen is also shown, with peaks labelled A-F.

The high resolution expansion of the peaks in B-F is:

Complete the table below to assign the peaks B, C, D, E, and F to the corresponding numbered proton environments. The assignment for peak A is given. 

A sample of ibuprofen shows strong absorptions at 1716 cm^-1 and 3345 cm^-1.

Suggest the bonds responsible for these absorptions using section 20 of the data booklet.

A sample of ibuprofen rotates plane polarised light.

i) State the feature in ibuprofen responsible for this.

[1]

ii) Mark this feature on the structure below with an asterisk (*)

[1]

Cholesterol, a vital component of cell membranes, is a complex chiral molecule.

State the number of chiral carbons in the cholesterol structure.

Explain why a sample of naturally occurring cholesterol is optically active, whereas a laboratory synthesis might produce an optically inactive sample.

Lactic acid has the molecular formula of C₃H₆O₃, and the structural formula of CH₃CHOHCOOH.

Illustrate the types of isomerism shown by C₃H₆O₃.

The general structure of polylactic acid is shown below:

Draw two possible structures formed from two repeating units. 

Your answer should keep the main polymer chain in the same plane but show the 3D representation of the chiral carbons.

Explain why the polymer formed from the uncontrolled condensation polymerisation of lactic acid, is not a racemate.