January 1998  Module 4 Exam

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1. The following passage gives a method for the preparation of nitrobenzene. Place 35 cm3 of concentrated nitric acid in a 500 cm3 flask, and add slowly 40 cm3 of concentrated sulphuric acid, keeping the mixture cool during the addition by immersing the flask in cold water.  Place a thermometer in the nitrating mixture, and add very slowly 29 cm3 of benzene.  The benzene should be added about 3 cm3 at a time, and the contents of the flask thoroughly mixed after each addition; the temperature of the mixture must not be allowed to rise above 40oC, and the flask must be cooled in cold water if necessary.
When all the benzene has been added, fit a reflux condenser to the flask, and heat the latter in a water bath at 60oC for 45 minutes.  During this period the flask should be removed from the water bath from time to time and vigorously shaken.
After this heating period, pour the contents of the flask into a large excess of cold water (about 300 cm3), and stir the mixture vigorously.  Decant off as much of the upper aqueous layer as possible, and transfer the residue to a separating funnel.  Run off the lower nitrobenzene layer into a beaker, and reject the aqueous layer.  Then wash the nitrobenzene successively first with an equal volume of cold water and then with dilute sodium carbonate solution.
Transfer the nitrobenzene to a small flask, add some granular calcium chloride, and leave until the liquid is quite clear.  Filter the nitrobenzene into a small, dry flask, and distil, collecting the fraction which boils between 207oC and 211oC.

(Adapted from Mann FG & Saunders BC, 'Practical Organic Chemistry', 4th ed: Longman, 1960.)
(a) Suggest reasons for each of the following:

(i) the use of a mixture of concentrated nitric and sulphuric acids.

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(ii) the slow addition of the benzene to the nitrating mixture.

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(iii) the need to keep the temperature below 40oC during the addition.

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(iv) the need to keep the reaction temperature at 60oC.

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(v) the necessity to shake the mixture from time to time.

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(vi) the addition of the reaction mixture to an excess of cold water.

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(vii) the need to wash with water and then with sodium carbonate solution.

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(viii) the distillation between 207oC and 211oC.

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(b) Nitrobenzene can be nitrated further to give 1,3-dinitrobenzene:

By analogy with the reduction of nitrobenzene to phenylamine, suggest reagents and conditions by which 1,3-dinitrobenzene can be converted to 1,3-diaminobenzene.

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(c) Nomex is a du Pont fibre, used for flame-retardant clothing, which can resist temperatures of 1000oC for 12 seconds, enough to have enabled the Benetton Formula 1 team to have survived a serious fire during the 1994 Grand Prix season.  Nomex is a polymer which could in principle be made from 1,3-diaminobenzene and benzene-1,3-dicarboxylic acid:

(i) Draw a representative length of the Nomex polymer chain.
 
 
 
 
 
 

(2)
(ii) What type of polymer is Nomex?

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(iii) Dicarboxylic acids are not usually used in making this type of polymer.  They are generally made from acid chlorides.  Suggest why this is so.

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(iv) Suggest how benzene-1,3-dicarboxylic acid could be converted to its diacid chloride, and draw its structure.

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(2)

Total 20 marks

2. This question concerns the compounds in the following reaction scheme:


 

(a) F and G are compounds which both decolorise bromine water.  F has two stereoisomers.

(i) What functional group is present in both F and G?

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(ii) Give the structural formulae of both stereoisomers of F.
 
 
 

(2)
(iii) Explain how these two isomers arise.

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(iv) Write the structural formula of G.
 
 
 
 
 

(1)
(b) B cannot be oxidised by acidified potassium dichromate(VI) solution.

(i) Write the structural formula of B.
 
 
 
 

(1)
(ii) Draw the general structural features of molecules which can be detected by the reaction with iodine and alkali.
 
 
 
 
(2)
(iii) Give the structure of the substance in solution D, and of the product E.
 
 
 
 
(2)
(c) The mass spectrum of A gives peaks at m/e 29 and 45, amongst others.  That at 45 is the largest.  This spectrum shows no molecular ion peak, which would be expected at m/e 74.  A is chiral.

(i) Give the structural formula for A.
 
 
 
 

(1)
(ii) Identify the ions responsible for the peaks at m/e 45 and 29, and hence suggest why the molecule shows no molecular ion peak.

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(d) A is miscible with benzene and the mixture formed shows a positive deviation from Raoult's Law and forms an azeotrope.

(i) A boils at 99oC, benzene at 80oC.  Sketch a possible boiling point/composition diagram for a mixture of A with benzene.
 
 
 
 
 
 

(3)
(ii) Explain why a mixture of A with benzene shows a large positive deviation from Raoult's Law.

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(iii) The infra-red spectrum of A is shown below.  The very broad peak at 3500cm-1 is due to the presence of an -OH group.  This peak becomes much narrower when diluted with benzene and moves to 3600cm-1.

Suggest why the -OH absorption peak changes as A is diluted with benzene.

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Total 23 marks

3. Propenal, CH2=CHCHO, is one of the materials that gives crispy bacon its sharp odour.  In the following question assume that the carbon-carbon double bond and the aldehyde group in propenal behave independently.

(a) Give the structural formulae of the compounds formed when propenal reacts with:

(i) hydrogen bromide;
 
 
 

(2)
(ii) hydrogen cyanide;
 
 
 
(1)
(iii) 2,4-dinitrophenylhydrazine.
 
 
 
(2)
(b) (i) Give the mechanism for the reaction between hydrogen cyanide and the aldehyde group.  You may represent the aldehyde group as
R
  \
   C=O
  /
H
 
 
 
 
 
 
 
(3)
(ii) The reaction in (i) occurs best in slightly acidic conditions.  It is slower if the pH is high or low.  Suggest reasons why this is so.

High pH..........................................................................................................................

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Low pH..........................................................................................................................

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(c) Explain why lithium tetrahydridoaluminate(III) (lithium aluminium hydride), LiAlH4, reacts only with the >C=O bond and not with the >C-C< bond, even though these bonds have the same electronic structure.

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(d) Suggest reactions, giving equations and conditions, which would convert propenal into a compound which would react with iodine in the presence of sodium hydroxide solution.
 
 
 
 
 
 
 
 
 

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Total 17 marks