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3.2.11 Analytical Techniques
Infra red spectroscopy
151 minutes
146 marks
Page 1 of 32
Q1. (a) Alcohols can be classed as primary, secondary or tertiary. Draw possible structures for a primary, a secondary and a tertiary alcohol which have the molecular formula C
4H
8O.
Which of the structures you have drawn cannot be oxidised by potassium dichromate in acid solution?
(4)
(b) Explain what is meant by the fingerprint region of an infra-red spectrum. State how it is used to confirm the identity of organic molecules such as the primary, secondary and tertiary alcohols of molecular formula C
4H
8O.
(2)
(c) Each of the parts below concerns a different pair of isomers. Deduce one possible structural formula for each of the species A to F. Use, where appropriate, the table of infra-red absorption data given on the data sheet.
(i) A and B have the molecular formula C3H
8O. A has a broad absorption band at
3300 cm–1 in its infra-red spectrum, but B does not.
(ii) C and D have the molecular formula C5H
10. C has a weak absorption band at
1650 cm–1 in its infra-red spectrum, but D does not.
(iii) E and F have the molecular formula C3H
6O and both have strong absorption bands
at about 1700 cm–1 in their infra-red spectra. E reacts with Tollens’ reagent but F does not.
(6) (Total 12 marks)
Q2. Butan-2-ol can be oxidised by acidified potassium dichromate(VI) to form butanone as shown by the following equation.
CH3CH
2CH(OH)CH
3 + [O] → CH
3CH
2COCH
3 + H
2O
(a) State the class of alcohol to which butan-2-ol belongs.
...................................................................................................................... (1)
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(b) The infrared spectrum shown below is either that of butan-2-ol or that of butanone.
Identify the compound to which this infrared spectrum refers.
Explain your answer.
You may find it helpful to refer to the table of infrared absorption data on the back of the Periodic Table (Table 1).
Identity of the compound .............................................................................
Explanation ..................................................................................................
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...................................................................................................................... (3)
(c) Draw the displayed formula of the alcohol C4H
9OH which is resistant to oxidation by
acidified potassium dichromate(VI).
...................................................................................................................... (1)
(Total 5 marks)
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Q3. There are four isomeric alcohols with the molecular formula C4H
10O
(a) Two of these are butan-l-ol (CH3CH
2CH
2CH
2OH) and butan-2-ol.
The other two isomers are alcohol X and alcohol Y.
Draw the displayed formula for butan-2-ol.
Alcohol X does not react with acidified potassium dichromate(VI) solution. Give the structure of alcohol X.
Name the fourth isomer, alcohol Y.
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(b) The infrared spectrum of one of these isomeric alcohols is given below.
Identify one feature of the infrared spectrum which supports the fact that this is an alcohol. You may find it helpful to refer to Table 1 on the Data Sheet.
Explain how infrared spectroscopy can be used to identify this isomeric alcohol.
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(c) British scientists have used bacteria to ferment glucose and produce the biofuel butan-1-ol.
Write an equation for the fermentation of glucose (C6H
12O
6) to form butan-1-ol, carbon
dioxide and water only.
State one condition necessary to ensure the complete combustion of a fuel in air.
Write an equation for the complete combustion of butan-1-ol and state why it can be described as a biofuel.
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(d) Butan-1-ol reacts with acidified potassium dichromate(VI) solution to produce two organic compounds.
State the class of alcohols to which butan-1-ol belongs.
Draw the displayed formula for both of the organic products.
State the type of reaction that occurs and the change in colour of the potassium dichromate(VI) solution.
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(Total 15 marks)
Q4. Consider the following scheme of reactions.
(a) State the type of structural isomerism shown by propanal and propanone.
...................................................................................................................... (1)
(b) A chemical test can be used to distinguish between separate samples of propanal and propanone.
Identify a suitable reagent for the test. State what you would observe with propanal and with propanone.
Test reagent ................................................................................................
Observation with propanal ...........................................................................
Observation with propanone ........................................................................ (3)
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(c) State the structural feature of propanal and propanone which can be identified from their
infrared spectra by absorptions at approximately 1720 cm–1.
You may find it helpful to refer to Table 1 on the Data Sheet.
...................................................................................................................... (1)
(d) The reaction of chlorine with propane is similar to the reaction of chlorine with methane.
(i) Name the type of mechanism in the reaction of chlorine with methane.
............................................................................................................. (1)
(ii) Write an equation for each of the following steps in the mechanism for the reaction of chlorine with propane to form l-chloropropane (CH
3CH
2CH
2Cl).
Initiation step
.............................................................................................................
First propagation step
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Second propagation step
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A termination step to form a molecule with the empirical formula C3H
7
............................................................................................................. (4)
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(e) High resolution mass spectrometry of a sample of propane indicated that it was contaminated with traces of carbon dioxide.
Use the data in the table to show how precise Mr values can be used to prove that the
sample contains both of these gases.
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(Total 12 marks)
Atom Precise relative atomic mass
12C
12.00000
1H
1.00794
16O
15.99491
Q5. The alkene (Z)-3-methylpent-2-ene reacts with hydrogen bromide as shown below.
(a) (i) Name the major product P.
............................................................................................................. (1)
(ii) Name the mechanism for these reactions.
............................................................................................................. (1)
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(iii) Draw the displayed formula for the minor product Q and state the type of structural isomerism shown by P and Q.
Displayed formula for Q
Type of structural isomerism .............................................................. (2)
(iv) Draw the structure of the (E)-stereoisomer of 3-methylpent-2-ene.
(1)
(b) The infrared spectra of two compounds R and S are shown below. R and S have the molecular formula C
6H
12 and are structural isomers of 3-methylpent-2-ene. R is an
unsaturated hydrocarbon and S is a saturated hydrocarbon.
Spectrum 1
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Spectrum 2
(i) Identify the infrared Spectrum 1 or 2 that represents compound R. Use information from the infrared spectra to give one reason for your answer. You may find it helpful to refer to Table 1 on the Data Sheet.
R is represented by Spectrum ................
Reason ...............................................................................................
............................................................................................................. (2)
(ii) State the type of structural isomerism shown by R and S.
............................................................................................................. (1)
(iii) Name one possible compound which could be S.
............................................................................................................. (1)
(Total 9 marks)
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Q6. A student read the following passage on the Internet.
(a) Explain the meaning of each of the following terms in the information given above.
(i) nucleophile
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Haloalkanes contain a polar covalent bond. The carbon atom of the polar covalent bond can be attacked by nucleophiles. Nucleophilic attack enables haloalkanes to undergo substitution reactions. A nucleophilic substitution reaction occurs when a haloalkane undergoes hydrolysis; the rate of hydrolysis of the haloalkane is influenced by the carbon–halogen bond enthalpy.
(ii) substitution, as applied to nucleophilic substitution in a haloalkane
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(iii) hydrolysis
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(iv) bond enthalpy, as applied to a carbon–halogen bond.
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(b) Outline a mechanism for the nucleophilic substitution reaction in which 2-bromopropane (CH
3CHBrCH
3) reacts with potassium hydroxide to form
propan-2-ol.
(2)
(c) Haloalkanes also undergo elimination reactions to produce alkenes.
(i) Outline a mechanism for the elimination reaction in which 2-bromopropane reacts with potassium hydroxide to form propene.
(3)
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(ii) A student obtained the following infrared spectrum for the product from this elimination reaction.
Use information from the infrared spectrum to state and explain how the student deduced that the product was an alkene. You may find it helpful to refer to Table 1 on the Data Sheet.
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(Total 11 marks)
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Q7. A student used the infrared spectra of water vapour and of carbon dioxide to try to find a link between infrared radiation and global warming.
(i) Use information from the infrared spectra to deduce one reason why the student concluded that water vapour is a more effective greenhouse gas than carbon dioxide.
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(ii) Use your knowledge of the bonds in CO2 to state why the infrared spectrum of carbon
dioxide is not as might be predicted from the data provided in Table 1 on the Data Sheet.
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(Total 3 marks)
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Q8. The table below shows the structures of three isomers with the molecular formula C5H
10O
(a) Complete the table by naming Isomer 3. (1)
Isomer 1
(E)-pent-3-en-2-ol
Isomer 2
pentanal
Isomer 3
(b) State the type of structural isomerism shown by these three isomers.
..................................................................................................................... (1)
(c) The compound (Z)-pent-3-en-2-ol is a stereoisomer of (E)-pent-3-en-2-ol.
(i) Draw the structure of (Z)-pent-3-en-2-ol.
(1)
(ii) Identify the feature of the double bond in (E)-pent-3-en-2-ol and that in (Z)-pent-3-en-2-ol that causes these two compounds to be stereoisomers.
............................................................................................................. (1)
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(d) A chemical test can be used to distinguish between separate samples of Isomer 2 and Isomer 3. Identify a suitable reagent for the test. State what you would observe with Isomer 2 and with Isomer 3.
Test reagent ...............................................................................................
Observation with Isomer 2...........................................................................
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Observation with Isomer 3............................................................................
..................................................................................................................... (3)
(e) The following is the infrared spectrum of one of the isomers 1, 2 or 3.
(i) Deduce which of the isomers (1, 2 or 3) would give this infrared spectrum. You may find it helpful to refer to Table 1 on the Data Sheet.
............................................................................................................. (1)
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(ii) Identify two features of the infrared spectrum that support your deduction. In each case, identify the functional group responsible.
Feature 1 and functional group ...........................................................
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Feature 2 and functional group ...........................................................
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(Total 10 marks)
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Q9. Consider the five cyclic compounds, A, B, C, D and E.
(a) The infrared spectra of compounds A, B, C and D are shown below.
Write the correct letter, A, B, C or D, in the box next to each spectrum. You may find it helpful to refer to Table 1 on the Data Sheet.
(1)
(i)
(1)
(ii)
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(1)
(iii)
(1)
(iv)
(b) A simple chemical test can be used to distinguish between cyclohexane (A) and cyclohexene (D). Give a reagent for this test and state what you would observe with each compound.
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(c) Cyclohexanol (B) can be converted into cyclohexanone (C).
Give a reagent or combination of reagents that can be used for this reaction and state the type of reaction.
State the class of alcohols to which cyclohexanol belongs.
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(d) Cyclohexane (A) can be converted into bromocyclohexane (E) by a reaction that is similar to the reaction of methane either with chlorine or with bromine.
Name and outline a mechanism for the reaction of methane (CH4) with bromine to form
bromomethane (CH3Br). Give one condition for this reaction to occur.
Write an equation for each step in your mechanism.
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(Total 15 marks)
Q10. The reaction of butane-1,4-diol with butanedioic acid produces the polymer PBS used in biodegradable packaging and disposable cutlery. Butanedioic acid is produced by two different processes.
Process 1
• Aqueous sodium hydroxide reacts with 1,4-dibromobutane to make butane-1,4-diol.
• Butane-1,4-diol is oxidised to butanedioic acid.
Process 2
• Glucose reacts with carbon dioxide in the presence of microorganisms to produce butanedioic acid directly.
• The carbon dioxide used in this process is obtained from a local factory that produces bioethanol.
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(a) Deduce one safety reason and one environmental reason why Process 2 is preferred to Process 1.
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(b) (i) Name and outline a mechanism for the following reaction that occurs in Process 1.
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(3)
BrCH2CH
2CH
2CH
2Br + NaOH BrCH
2CH
2CH
2CH
2OH + NaBr
(ii) The infrared spectra shown are those of three compounds.
Compound A 1,4-dibromobutane Compound B butane-1,4-diol Compound C butanedioic acid
Identify the compound responsible for each spectrum by writing the correct letter, A, B or C, in the box next to each spectrum. You may find it helpful to refer to Table 1 on the Data Sheet.
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(3)
(c) In the production of bioethanol, glucose (C6H
12O
6) is converted into a dilute aqueous
solution of ethanol and carbon dioxide.
Give the name of this process and state three essential conditions necessary to produce a good yield of ethanol.
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(d) State the class of alcohols to which the diol butane-1,4-diol belongs.
Identify a suitable reagent or combination of reagents for the conversion of butane-1,4-diol into butanedioic acid (HOOCCH
2CH
2COOH).
Write an equation for this oxidation reaction using [O] to represent the oxidising agent.
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(Total 15 marks)
Q11. Chloromethanes, such as dichloromethane and trichloromethane, are produced in industry as they have many uses. Trichloromethane has been used in the manufacture of the refrigerant chlorodifluoromethane.
(a) Chlorine can react with dichloromethane (CH2Cl
2) to form trichloromethane (CHCl
3).
(i) Write an equation for each of the following steps in the mechanism for this reaction.
Initiation step
...............................................................................................................
First propagation step
...............................................................................................................
Second propagation step
............................................................................................................... (3)
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(ii) Give one essential condition for this reaction and name the type of mechanism.
Essential condition ...............................................................................
Type of mechanism .............................................................................. (2)
(b) An organic product, X, with Mr = 154.0 is obtained when chlorine reacts with
trichloromethane.
(i) Write an equation for the overall reaction of chlorine with trichloromethane to form X, by the same mechanism as that outlined in part (a)(i).
............................................................................................................... (1)
(ii) The following infrared spectrum was obtained for a sample of X produced in this reaction.
Use this infrared spectrum to explain why it is possible to deduce that this sample of X contains no trichloromethane. You may find it helpful to refer to Table 1 on the Data Sheet.
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(c) Explain, with the aid of equations and the intermediates that form in the ozone layer, why the European Union has banned the use of chlorodifluoromethane (CHClF
2) as a
refrigerant.
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(d) The compound 2,3,3,3-tetrafluoropropene is the refrigerant used in all new car air conditioners.
(i) Draw the displayed formula for 2,3,3,3-tetrafluoropropene.
(1)
(ii) Give one reason why 2,3,3,3-tetrafluoropropene is a more environmentally acceptable refrigerant than chlorodifluoromethane.
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(Total 14 marks)
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Q12. Glucose is an organic molecule. Glucose can exist in different forms in aqueous solution.
(a) In aqueous solution, some glucose molecules have the following structure.
(i) Deduce the empirical formula of glucose.
............................................................................................................... (1)
(ii) Consider the infrared spectrum of solid glucose.
Wavenumber / cm–1
State why it is possible to suggest that in the solid state very few molecules have the structure shown. You may find it helpful to refer to Table 1 on the Data Sheet.
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(b) In the absence of oxygen, an aqueous solution of glucose can be fermented to produce ethanol for use in alcoholic drinks.
Write an equation for this fermentation reaction. Give two other essential conditions for the production of ethanol in this fermentation.
Equation
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Condition 1 ......................................................................................................
Condition 2 ...................................................................................................... (3)
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(c) Any ethanol present in the breath of a drinker can be detected by using a breathalyser. The ethanol is converted into ethanoic acid. The breathalyser has negative and positive electrodes. A current is measured and displayed in terms of alcohol content.
The overall redox equation is as follows
CH3CH
2OH(I) + O
2(g) CH
3COOH(I) + H
2O(I)
(i) Draw the displayed formula for ethanoic acid.
(1)
(ii) Deduce a half-equation for the reduction of atmospheric oxygen to water in acidic solution at one electrode of the breathalyser.
............................................................................................................... (1)
(iii) Deduce a half-equation for the oxidation of ethanol in water to ethanoic acid at the other electrode of the breathalyser.
............................................................................................................... (1)
(iv) The earliest breathalysers used laboratory chemicals to oxidise the ethanol to ethanoic acid. Detection was by a colour change.
Identify a reagent or combination of reagents that you would use in the laboratory to oxidise ethanol to ethanoic acid. State the colour change that you would expect to see.
Reagent or combination of reagents .......................................................
Colour change ......................................................................................... (2)
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(d) The fermentation of glucose from crops is the main method for the production of ethanol. The product is called bioethanol. The European Union has declared that bioethanol is carbon-neutral.
(i) State the meaning of the term carbon-neutral.
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(Extra space) .......................................................................................
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(ii) Other than carbon-neutrality, state the main advantage of the use of glucose from crops as the raw material for the production of ethanol.
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(iii) Give one disadvantage of the use of crops for the production of ethanol.
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(Total 13 marks)
Q13. Haloalkanes are used in the synthesis of other organic compounds.
(a) Hot concentrated ethanolic potassium hydroxide reacts with 2-bromo-3-methylbutane to form two alkenes that are structural isomers of each other. The major product is 2-methylbut-2-ene.
(i) Name and outline a mechanism for the conversion of 2-bromo-3-methylbutane into 2-methylbut-2-ene according to the equation.
(CH3)
2CHCHBrCH
3 + KOH (CH
3)
2C=CHCH
3 + KBr + H
2O
Name of mechanism ...........................................................................
Mechanism
(4)
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(ii) Draw the displayed formula for the other isomer that is formed.
(1)
(iii) State the type of structural isomerism shown by these two alkenes.
............................................................................................................... (1)
(b) A small amount of another organic compound, X, can be detected in the reaction mixture formed when hot concentrated ethanolic potassium hydroxide reacts with 2-bromo-3-methylbutane. Compound X has the molecular formula C
5H
12O and is a secondary alcohol.
(i) Draw the displayed formula for X. (1)
(ii) Suggest one change to the reaction conditions that would increase the yield of X.
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(iii) State the type of mechanism for the conversion of 2-bromo-3-methylbutane into X.
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(iv) Identify one feature of this infrared spectrum of a pure sample of X that may be used to confirm that X is an alcohol. You may find it helpful to refer to Table 1 on the Data Sheet.
Wavenumber / cm−1
Feature .................................................................................................
............................................................................................................... (1)
(Total 10 marks)
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