Download Analytical Questions

Analytical Questions
(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 C4H8O.
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 C4H8O.
(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 C3H8O. 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 C5H10. 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 C3H6O 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)
Butan-2-ol can be oxidised by acidified potassium dichromate(VI) to form butanone as shown
by the following equation.
CH3CH2CH(OH)CH3 + [O] → CH3CH2COCH3 + H2O
(a)
State the class of alcohol to which butan-2-ol belongs.
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(1)
(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 C4H9OH which is resistant to oxidation by
acidified potassium dichromate(VI).
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(1)
(Total 5 marks)
There are four isomeric alcohols with the molecular formula C4H10O
(a)
Two of these are butan-l-ol (CH3CH2CH2CH2OH) 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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(3)
(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.
Explain how infrared spectroscopy can be used to identify this isomeric alcohol.
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(3)
(c)
British scientists have used bacteria to ferment glucose and produce the biofuel
butan-l-ol.
Write an equation for the fermentation of glucose (C6H12O6) to form butan-l-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-l-ol and state why it can be
described as a biofuel.
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(4)
(d)
Butan-l-ol reacts with acidified potassium dichromate(VI) solution to produce two
organic compounds.
State the class of alcohols to which butan-l-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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(5)
(Total 15 marks)
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, using, where appropriate, the table of infra-red
absorption data given below.
Table of infra-red absorption data
Bond
(a)
Wavenumber/cm–1
C–H
2850–3300
C–C
750–1100
C=C
1620–1680
C=O
1680–1750
C–O
1000–1300
O–H (alcohols)
3230–3550
O–H (acids)
2500–3000
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)
(f)
K and L have the molecular formula C5H10. K has a weak absorption band at
1650 cm–1 in its infra-red spectrum, but L does not.
K
L
(2)
a)
The fragmentation of a molecular ion (M–R)+, formed in the ionisation chamber of a
mass spectrometer, can be represented by the equation
(M–R)+  M+ + R
Identify the three types of species shown in the equation and explain what takes place in
this conversion.
(4)
(b)
The mass spectrum of chloroethane shows two molecular ion peaks at m/z values of 64
and 66. The peak at m/z = 64 is approximately three times as intense as that at m/z = 66.
Explain this observation and show, by means of an equation, how the molecular ion of
chloroethane fragments to give rise to a peak at an m/z value of 29.
(4)
(c)
Suggest why the mass spectrum of 1,2-dichloroethane shows peaks at m/z values of 98,
100 and 102.
(4)
(Total 12 marks)
(a)
(i)
Using mass spectrometry, what information in addition to Mr can be obtained from
the precise value of the mass of the molecular ion of an organic compound?
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(ii)
Suggest why, for most organic compounds, it is possible to detect a peak at one
mass unit higher than that of the molecular ion.
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(2)
(b)
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)
(c)
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 10 marks)