OCR A2 CHEMISTRY

TOPIC 4.3.2 SPECTROSCOPY

BOOKLET OF PAST EXAMINATION QUESTIONS

TOTAL 308 MARKS

Name:………………………………………………………….

MARK:                      /308	                              %	GRADE:

1.	The proton n.m.r. spectrum of compound X is shown below.

	Compound X, C7H12O3, contains both a ketone and an ester functional group. The measured integration trace for the peaks in the n.m.r. spectrum of X gives the ratio shown in the table below.
Chemical shift, d/ppm	4.13	2.76	2.57	2.20	1.26	
Integration ratio	0.8	0.8	0.8	1.2	1.2	
	Refer to the spectrum, the information given above and the data below the Periodic Table provided to answer the following questions.
(a)	How many different types of proton are present in compound X?
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(1) 
(b)	What is the whole-number ratio of each type of proton in compound X?
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(1)
(c)	Draw the part of the structure of X which can be deduced from the presence of the peak at d2.20.
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(1)
(d)	The peaks at d4.13 and d1.26 arise from the presence of an alkyl group. Identify the group and explain the splitting pattern.
Alkyl group .............................................................................................................
Explanation .............................................................................................................
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(3)
(e)	Draw the part of the structure of X which can be deduced from the splitting of the peaks at d2.76 and d2.57.
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(1) 
(f)	Deduce the structure of compound X.
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(2)
(Total 9 marks)

2.	Compound Q has the molecular formula C4H7ClO.
(a)	The infra-red spectrum of Q contains a major absorption at 1724 cm–1. Identify the bond responsible for this absorption.
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(1)
(b)	The mass spectrum of Q contains two molecular ion peaks at m/z = 106 and m/z = 108.
It also has a major peak at m/z = 43.
(i)	Suggest why there are two molecular ion peaks.
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(ii)	A fragment ion produced from Q has m/z = 43 and contains atoms of three different elements. Identify this fragment ion and write an equation showing its formation from the molecular ion of Q.
Fragment ion ....................................................................................................
Equation ...........................................................................................................
(3)
(c)	The proton n.m.r. spectrum of Q was recorded.
(i)	Suggest a suitable solvent for use in recording this spectrum of Q.
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(ii)	Give the formula of the standard reference compound used in recording proton n.m.r. spectra.
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(2)
(d)	The proton n.m.r. spectrum of Q shows three peaks. Complete the table below to show the number of adjacent, non-equivalent protons responsible for the splitting pattern.
	Peak 1	Peak 2	Peak 3	
Integration value	3	3	1	
Splitting pattern	doublet	singlet	Quartet	
Number of adjacent,
non-equivalent protons	1			
(1)
(e)	Using the information in parts (a), (b) and (d), deduce the structure of compound Q.

(1)
 (Total 8 marks)
3.	(a)	Draw the structure of ethyl propanoate.

(1)
(b)	The mass spectrum of ethyl propanoate contains a major peak at m/z = 57. Write an equation showing the fragmentation of the molecular ion to form the species responsible for the peak at m/z = 57. Show the structure of this species in your answer.
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(2) 
(c)	Draw the structure of another ester which is an isomer of ethyl propanoate and which gives a major peak at m/z = 71 in its mass spectrum.

(1)
(Total 4 marks)

4.	(a)	Fragmentation of the molecular ion of methylbutanone, (CH3)2CHCOCH3, gives rise to dominant peaks at m/z = 71 and m/z 43.
(i)	Construct a balanced equation to show how fragmentation of the molecular ion gives rise to the peak at m/z = 71.
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(ii)	In the fragmentation of (CH3)2CHCOCH3, two fragments with m/z  43 are formed. Give the structural formula of each of these fragments.
	Structure 1                               Structure 2

(5)
(b)	Two molecular ion peaks appear in the mass spectrum of 2-chloropropane at m/z 78
and m/z = 80. Explain why two molecular ion peaks are found and why the relative intensities of the peaks are approximately 3 to 1, respectively.
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(3)
(Total 8 marks)
5.	Three sections of the proton n.m.r. spectrum of CH3CHClCOOH are shown below.

(a)	Name the compound CH3CHClCOOH
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(1)
(b)	Explain the splitting patterns in the peaks at d 1.72 and d 4.44
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(2)
(c)	Predict the splitting pattern that would be seen in the proton n.m.r. spectrum of the isomeric compound ClCH2CH2COOH
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(1)
 (Total 4 marks)

6.	Spectral data for use in this question are provided below the Periodic Table (first item on the database).
	Compound Q has the molecular formula C4H8O2
(a)	The infra-red spectrum of Q is shown below.

	Identify the type of bond causing the absorption labelled R and that causing the absorption labelled S.
R ............................................................................................................................
S .............................................................................................................................
(2)
(b)	Q does not react with Tollens’ reagent. Identify a functional group which would react with these reagents and therefore cannot be present in Q.
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(1)
(c)	Proton n.m.r. spectra are recorded using a solution of a substance to which tetramethylsilane (TMS) has been added.
(i)	Give two reasons why TMS is a suitable standard.
Reason 1 .......................................................................................................
Reason 2 .......................................................................................................
(ii)	Give an example of a solvent which is suitable for use in recording an n.m.r. spectrum. Give a reason for your choice.
Solvent ..........................................................................................................
Reason ..........................................................................................................
(4)
 
(d)	The proton n.m.r. spectrum of Q shows 4 peaks.
	The table below gives d values for each of these peaks together with their splitting patterns and integration values.
d/ppm	2.20	2.69	3.40	3.84	
Splitting pattern	singlet	triplet	singlet	triplet	
Integration value	3	2	1	2	
	What can be deduced about the structure of Q from the presence of the following in its n.m.r. spectrum?
(i)	The singlet peak at d = 2.20
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(ii)	The singlet peak at d = 3.40
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(iii)	Two triplet peaks
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(3)
(e)	Using your answers to parts (a), (b) and (d), deduce the structure of compound Q.

(1)
(Total 11 marks)

7.	The proton n.m.r. spectrum of an ester, A, is shown below.

Chemical shift, d/ppm
	The measured integration trace gives the ratio 0.50 to 0.50 to 0.75 to 0.75 for the peaks at d 4.13, 2.32, 1.33 and 1.09, respectively.
(a)	The mass spectrum of compound A has a molecular ion peak at m/z = 102.
Deduce the molecular formula of compound A.
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(3)
 
(b)	What is the ratio of the numbers of each type of proton?
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(1)
(c)	What can be deduced about the arrangement of protons from the splitting patterns in the n.m.r. spectrum?
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(1) 
(d)	Deduce the structure of compound A and label with the letters a, b, c and d the four groups of equivalent protons.

(1)
(e)	Assign the various signals in the table below to the groups of protons in compound A previously labelled a, b, c and d.
Chemical shift, d/ppm	1.09	1.33	2.32	4.13	
Label of group					
(4)
(Total 10 marks)

8.	The proton n.m.r. spectrum of an alcohol, A, C5H12O, is shown below

	The measured integration trace gives the ratio 0.90 to 0.45 to 2.70 to 1.35 for the peaks at
d 1.52, 1.39, 1.21 and 0.93, respectively.
(a)	What compound is responsible for the signal at d 0?
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(1)
(b)	How many different types of proton are present in compound A?
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(1)
 
(c)	What is the ratio of the numbers of each type of proton?
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(1)
(d)	The peaks at d 1.52 and d 0.93 arise from the presence of a single alkyl group. Identify this group and explain the splitting pattern.
Group..........................................................................................................................
Explanation.................................................................................................................
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(3)
(e)	What can be deduced from the single peak at d 1.21 and its integration value?
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(1)
(f)	Give the structure of compound A.

(1)
(Total 8 marks)

9.		The ester formed when ethanol reacts with ethanoic anhydride was analysed by high-resolution proton n.m.r. spectroscopy.
(i)	Give the structural formula of the ester.

 
(ii)	How many different types of proton are present in this ester?
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(iii)	Describe the splitting pattern of the ethyl group in the n.m.r. spectrum of the ester.
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(3)
 (Total 3 marks)

10.	(a)	Define the term structural isomerism.
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(2)
(b)	The graph below shows part of the mass spectrum of an organic compound, A, which has the molecular formula C4H10.

(i)	Draw graphical formulae for the structural isomers of C4H10.
	Isomer 1	Isomer 2

(2) 
(ii)	Suggest the formula of the fragment that corresponds to each of the following m/e values shown in the mass spectrum of compound A.
43 ......................................................................................................................
29 ......................................................................................................................
15 ......................................................................................................................
(3)
(iii)	Deduce which of the isomers drawn in (i) corresponds to compound A. Give a reason for your answer.
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(2)

(c)	(i)	Suggest graphical formulae for two structural isomers with the molecular formula C3H6O2. Each isomer contains a single functional group. The functional groups are different in the two isomers.
	Isomer 3	Isomer 4

(2)
(ii)	Give the name of the type of compound that each isomer represents.
Isomer 3 ............................................................................................................
Isomer 4 ............................................................................................................
(2)
(Total 13 marks)

11.	Each of the parts (a) to (f) below concerns a different pair of isomers. Deduce one possible structural formula for each of the species A to L.
(a)	A and B have the molecular formula C2H4O2. An aqueous solution of A reacts with calcium carbonate to liberate carbon dioxide, but B does not.
A		B

(2)
(b)	C and D have the molecular formula C3H8O. C has a broad absorption band at
3300 cm–1 in its infra-red spectrum, but D does not.
C		D

(2)
(c)	E and F have the molecular formula C3H6C12. E has only one peak in its low resolution proton n.m.r. spectrum but F has two.
E		F

(2)
(d)	G and H have the molecular formula C3H6O and both have strong absorption bands at about 1700 cm–1 in their infra-red spectra. G turns acidified potassium dichromate(VI) solution green but H does not.
G		H

(2) 
(e)	I and J have the molecular formula C4H9Br. I shows optical activity, but J does not.
I		J

(2)
 (Total 10 marks)

12.		Carbonyl compounds can be reduced to alcohols.
(i)	Write an equation for the reduction of propanone to the corresponding alcohol and identify a suitable reducing agent. In the equation you may use the symbol [H] for the reducing agent.
Equation ............................................................................................................
Reducing agent .................................................................................................
(ii)	Two isomeric carbonyl compounds, A and B, with molecular formula C4H8O both have 3 peaks in their low-resolution proton n.m.r. spectra.
	Reduction of A forms alcohol C which has 5 peaks in its low-resolution proton n.m.r. spectrum whereas reduction of B forms alcohol D which has 4 peaks in its low-resolution proton n.m.r. spectrum.
Draw one possible structure for each of these 4 compounds.
Carbonyl compound A	Alcohol C

 
Carbonyl compound B	Alcohol D

(6)
(Total 6 marks)

13.	Compound A, C5H10O, reacts with NaBH4 to give B, C5H12O. Treatment of B with concentrated sulphuric acid yields compound C, C5H10. Acid-catalysed hydration of C gives a mixture of isomers, B and D.
	Fragmentation of the molecular ion of A, [C5H10O]+·, leads to a mass spectrum with a major peak at m/z 57. The infra-red spectrum of compound A has a strong band at 1715 cm–1 and the infra-red spectrum of compound B has a broad absorption at 3350 cm–1 (Table). The proton n.m.r. spectrum of A has two signals at d 1.06 (triplet) and 2.42 (quartet), respectively (Spectrum).
	Use the analytical and chemical information provided to deduce structures for compounds A, B, C and D, respectively. Include in your answer an equation for the fragmentation of the molecular ion of A and account for the appearance of the proton n.m.r. spectrum of A. Explain why isomers B and D are formed from compound C.
Spectrum

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(Total 20 marks)

14.	(a)	The infra-red spectrum of compound A, C3H6O2, is shown below.

	Identify the functional groups which cause the absorptions labelled X and Y.

	
	Using this information draw the structures of the three possible structural isomers for A.

	

	Label as A the structure which represents a pair of optical isomers.
(6)

(b)	Draw the structures of the three branched-chain alkenes with molecular formula C5H10

	
	Draw the structures of the three dibromoalkanes, C5H10Br2, formed when these three alkenes react with bromine.

	One of these dibromoalkanes has only three peaks in its proton n.m.r. spectrum. Deduce the integration ratio and the splitting patterns of these three peaks.
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(10)
(Total 16 marks)

15.	(a)	The reaction of but-1-ene with chlorine produces 1,2-dichlorobutane, C4H8Cl2
(i)	Given that chlorine exists as a mixture of two isotopes, 35Cl and 37Cl, predict the number of molecular ion peaks and their m/z values in the mass spectrum of C4H8Cl2
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(ii)	The mass spectrum of 1,2-dichlorobutane contains peaks at m/z = 77 and 79. Draw the structure of the fragment ion which produces the peak at m/z = 77 and write an equation showing its formation from the molecular ion.

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(6)
(b)	The reaction of but-2-ene with hydrogen chloride forms a racemic mixture of the stereoisomers of 2-chlorobutane. 
	Name the type of stereoisomerism shown by 2-chlorobutane and give the meaning of the term racemic mixture. State how separate samples of the stereoisomers could be distinguished.
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 (4)

(c)	The reaction of but-2-ene with chlorine produces 2,3-dichlorobutane, C4H8Cl2
(i)	State the number of peaks, their integration ratio and any splitting of peaks in the proton n.m.r. spectrum of 2,3-dichlorobutane.
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(ii) 	Compound S, an isomer of C4H8Cl2, produces a proton n.m.r. spectrum which consists only of a singlet, a triplet and a quartet with an integration ratio of 3:3:2 respectively.
	Compound T, also an isomer of C4H8Cl2, produces a proton n.m.r. spectrum which consists only of two singlets with an integration ratio of 3:1
Draw the structures of S and of T.

(6)
(Total 16 marks)

16.	Butenedioic acid, HOOCCH=CHCOOH, occurs as two stereoisomers. One of the isomers readily forms the acid anhydride C4H2O3 when warmed.
(a)	Draw the structures of the two isomers of butenedioic acid and name the type of isomerism shown.

Use the structures of the two isomeric acids to suggest why only one of them readily forms an acid anhydride when warmed. Draw the structure of the acid anhydride formed.

(6)

(b)	Identify one electrophile which will react with butenedioic acid and outline a mechanism for this reaction.

(4) 
(c)	Write an equation for a reaction which occurs when butenedioic acid is treated with an excess of aqueous sodium hydroxide.

(2)
(d)	Describe and explain the appearance of the proton n.m.r. spectrum of butenedioic acid.

(3)
(Total 15 marks)

17.	(a)	Name ester X, CH3CH2COOCH3.

(1)
(b)	The proton n.m.r. spectrum of X is shown below together with that of an isomeric ester, Y. Deduce which of Spectrum 1 and Spectrum 2 is that obtained from X. Use Table 1 on the Data Sheetand the integration data on the spectra to help you to explain your deduction. Suggest a structure for Y.

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(4)
(Total 5 marks)

18.	The three compounds CH3CH2CH2CH2OH, (CH3)3COH and CH3CH2CH2CHO can be 
	distinguished by use of the following three reagents
	1.	potassium dichromate(VI) acidified with dilute sulphuric acid
2.	Tollens’ reagent
3.	ethanoic acid, together with a small amount of concentrated sulphuric acid.
(a)	Identify which of these three organic compounds would reduce acidified potassium dichromate(VI). Give the structures of the organic products formed. Write a half-equation for the reduction of dichromate(VI) ions in acidic solution.

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(6)
(b)	Identify which one of these three organic compounds would reduce Tollens’ reagent. Give the structure of the organic product formed. Write a half-equation for the reduction of Tollens’ reagent.

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(3) 
(c)	Identify which of these three organic compounds would react with ethanoic acid in the presence of concentrated sulphuric acid. In each case, give the structure of the organic product formed.

(4) 
(d)	State the number of peaks in the proton n.m.r. spectra of CH3CH2CH2CH2OH and of (CH3)3COH. (Analysis of peak splitting is not required.)
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(2)
(Total 15 marks)

19.	(a)	Draw the structure of each of the three ketones which have the molecular formula C5H10O. For each compound give the ratio of the areas under each peak in its low-resolution proton n.m.r. spectrum.

(6)
(b)	Draw the structure of each of the four aldehydes which also have the molecular formula C5H10O. Label with the letter X the compound which has only two peaks in its low-resolution proton n.m.r. spectrum. Label with the letter Y the compound which has five peaks with the ratios of the areas under each peak 3:3:2:1:1 in its low-resolution proton n.m.r. spectrum. Label with the letter Z the compound which shows optical isomerism.

  
(7)
(c)	Explain why aldehydes react with Tollen’s (or Fehling’s) reagent but ketones do not.

(3)
(Total 16 marks)

20.	(a)	Describe, by giving reagents and stating observations, how you could distinguish between the compounds in the following pairs using a simple test-tube reaction for each pair.
(i)	 
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(ii)	 
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(iii)	
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 (8)
(b)	State how compounds E and F in part (a)(iii) above could be distinguished by their infra-red spectra, without using the fingerprint region.  Explain how fingerprinting is used to identify a compound.
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(3) 
(c)	Suggest the structure of the fragment responsible for the major peak in the mass spectrum of compound E and state its m/z value.  Write an equation showing the formation of this fragment from the molecular ion.
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(4)
(Total 15 marks)
21.		       Compounds A to G are all isomers with the molecular formula C6H12O2
(a)	Isomer A, C6H12O2, is a neutral compound and is formed by the reaction between compounds X and Y in the presence of a small amount of concentrated sulphuric acid.
X and Y can both be formed from propanal by different redox reactions.
X has an absorption in its infra-red spectrum at 1750 cm–1.
Deduce the structural formulae of A, X and Y. Give suitable reagents, in each case, for the formation of X and Y from propanal and state the role of concentrated sulphuric acid in the formation of A.

(7) 
(b)	Isomers B, C, D and E all react with aqueous sodium carbonate to produce carbon dioxide.
Deduce the structural formulae of the three isomers that contain an asymmetric carbon atom.
The fourth isomer has only three singlet peaks in its proton n.m.r. spectrum. Deduce the structural formula of this isomer and label it E.

(4)
(c)	Isomer F, C6H12O2, has the structural formula shown below, on which some of the protons have been labelled.

	A proton n.m.r. spectrum is obtained for F. Predict a value of d for the protons labelled a and also for those labelled b. State and account for the splitting patterns of the peaks assigned to the protons a and b.

(6)
(d)	Isomer G, C6H12O2, contains six carbon atoms in a ring. It has an absorption in its infra-red spectrum at 3270 cm–1 and shows only three different proton environments in its proton n.m.r. spectrum. Deduce a structural formula for G.

(2)
(Total 19 marks)
22.	The conversion of compound A into compound B can be achieved in two steps as shown below.

Show how the number of peaks in their proton n.m.r. spectra would enable you to distinguish 
between compounds A and B.
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(2)
(Total 2 marks)

23.	Proton n.m.r. spectroscopy can be used to distinguish between the following isomers:
HOCH2CH2OCH3              HOCH2OCH2CH3              CH3OCH2OCH3
  A                                          B                                       C
	For each of these isomers, describe the appearance of the high-resolution proton n.m.r. spectrum and account for any splitting patterns. Assume that the OH protons are not coupled to any other protons.
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(Total 15 marks)
24.		The structural formulae of compounds A, B, C and D, which are the four structural isomers of molecular formula C4H9Cl, are given below.
CH3CH2CH2CH2Cl       CH3CH(Cl)CH2CH3       (CH3)2CHCH2Cl       (CH3)2C(Cl)CH3
	A	B	C	D
	Low resolution 1H NMR analysis of compounds A, B, C and D produces two spectra with 4 peaks, one spectrum with 3 peaks and one spectrum with a single peak.
(i)	Give the name of compound D.
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(1)
(ii)	Identify the compound (A, B, C or D) whose NMR spectrum shows a single peak.  Explain your reasoning.
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(2)
(iii)	Identify the compound (A, B, C or D) whose NMR spectrum shows three peaks and deduce the ratio of the areas under these peaks.  Explain your reasoning.
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(3)
 (v)	Draw the graphical formula for the product of the reaction between compound C and NaOH(aq).  Give an outline of the mechanism involved in this reaction.

(4)
(Total 10 marks)

25.	A molecular ion peak at mlz = 86 is present in the mass spectrum of each of three organic compounds, A, B and C. Each compound has only two peaks in its low-resolution proton n.m.r. spectrum.
A is a hydrocarbon.
B and C are isomers and both have a strong absorption near 1700 cm–1 in their infra-red spectra.
(a)	For each compound, give the molecular formula and draw a possible structure.  Deduce the ratio of the peak areas in the low-resolution proton n.m.r. spectrum of each compound.

(9)
(b)	Describe how you could distinguish between B and C using a chemical test. How could B or C be identified using its infra-red spectrum?

(5) 
(Total 14 marks)

26.	(a)	Explain what is meant by the fingerprint region of an infra-red spectrum and describe how it is used to confirm the identity of organic molecules.
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(4)
(b)	Each of three isomeric esters, Q, R and S, of molecular formula C5H10O2 has a branched structure.
	Q has only two peaks in its low resolution proton n.m.r. spectrum, but R and S each have three peaks.
	Hydrolysis of R in acid conditions forms compounds T and U. The low-resolution proton n.m.r. spectrum of T has three peaks and that of U has two peaks. Infra-red spectra of T and U are shown below.

Draw structures of the compounds Q, R, S, T and U and explain how the n.m.r. and i.r. data 
support your conclusions.

	 (11)
(Total 15 marks)

27.	Simple organic compounds, including structural isomers, can be distinguished from each other by chemical tests, or by spectroscopic methods, or by a combination of both. This question concerns the four compounds whose structural formulae are shown below.
CH3(CH2)2CHO	(CH3)2CHCHO	CH3COC2H5	CH3COOC2H5
A	B	C	D 
(a)	Draw graphical formulae for A, B, C and D, clearly indicating which is which. Give the names of A, B, C and D.

(8)
(b)	Compound D can be made by a reaction called esterification. State the names of two compounds needed for the reaction and give a condition used in the reaction.

(3)
(c)	Give the name and draw the graphical formula of an isomer of compound D that is not an ester.

(2)
(d)	Describe one chemical test that would distinguish between B and C, and state what you would observe in each case.

(3)

(e)	The 1H NMR spectra of A, B, C and D can be used to distinguish between some of the structures by considering the numbers of peaks and the ratios of the areas under them.
The following table shows some results of a 1H NMR investigation into A, B, C and D.
Each of the four spectra is referred to by a number.
Spectrum	Number of peaks	Ratio of areas under the peaks	
1	3	6 : 1 : 1	
2	4	3 : 2 : 2 : 1	
3	3	3 : 2 : 3	
4	3	3 : 2 : 3	
(i)	Which compound (A, B, C or D) corresponds to spectrum 1? Give a reason for your choice.

(2)
(ii)	Which compound (A, B, C or D) corresponds to spectrum 2? Give a reason for your choice.

(2)
(iii)	Explain why spectrum 3 and spectrum 4 have the same number of peaks and the same ratio of areas under them.

(1)
 (Total 21 marks)