Unit 2 – Mechanisms
Unit 2 - Jan 2009
Q1.
What type of species forms when a bond breaks homolytically?
   A     Nucleophile
   B     Electron
   C     Electrophile
   D     Free radical
(Total for question = 1 mark)

Unit 2 - Jun 2009
Q2.
When 2-chloro-2-methylpropane is heated in a mixture of water and ethanol at 65 °C, two types of reaction occur.  A mixture of two organic products, 2-methylpropan-2-ol and 2-methylpropene, is formed.

(a)  (i)  Name the two reaction types that are taking place.
(2)
Reaction 1
 ............................................................................................................................................. 
Reaction 2
 ............................................................................................................................................. 
*(ii)  Explain how the two products form, by describing the role of the water in each case.
(4)
Reaction 1
 ............................................................................................................................................. 
 ............................................................................................................................................. 
Reaction 2
 ............................................................................................................................................. 
............................................................................................................................................. 

 (b)  A proposed mechanism for Reaction 1 involved the formation of the carbocation,

(i)  What type of bond breaking must have occurred during the carbocation formation?
(1)
 ............................................................................................................................................. 
(ii)  Suggest why 1-chlorobutane reacts with water via a different mechanism.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (c)  Another halogenoalkane, 2-chlorobutane, behaves in a similar way to 2-chloro-2-methylpropane but in Reaction 2 can form three different alkenes. Suggest how three different alkenes can form and give their displayed formulae.
(4)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(d)  Suggest why 2-fluoro-2-methylpropane would react more slowly than 2-chloro-2-methylpropane in Reaction 1.
        What reagent could you use instead of water to increase the rate of this reaction involving 2-fluoro-2-methylpropane? Explain why the reagent would have this effect.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (Total for question = 16 marks)

Unit 2 - Jan 2010
Q3.
Chloroethane reacts with aqueous potassium hydroxide solution, producing ethanol as the organic product.

(a)  The hydroxide ion is acting as
(1)
   A     an electrophile.
   B     a nucleophile.
   C     an oxidizing agent.
   D     a reducing agent.

(b)  Which of the following shows the correct electron-pair movements in this reaction?
(1)

(Total for question = 2 marks)

Unit 2 - Jan 2010
Q4.
Which of the following is essential if a species is to act as a nucleophile?
   A     A lone pair of electrons.
   B     A negative charge.
   C     An unpaired electron.
   D     A strongly polar bond.
(Total for question = 1 mark)

Q5.
(a)  Propene, C3H6, reacts with hydrogen bromide, HBr, in an electrophilic addition reaction.
2-bromopropane is formed as the major product.

(i)  Complete the mechanism for the reaction, using 'curly arrows' where appropriate. Show clearly the structure of the intermediate carbocation formed.
(3)

(ii) Draw the structure of the alternative carbocation that can be formed in the reaction between propene and hydrogen bromide.
(1)

(b)  Four isomers, each with the molecular formula C4H10O, are shown below.
Isomer A:     CH3CH2CH2CH2OH
Isomer B:     CH3CH2CH(OH)CH3
Isomer C:     (CH3)3COH
Isomer D:     CH3CH(CH3)CH2OH
(i)  Which isomer is a secondary alcohol? Justify your answer.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (ii)  Which isomer is resistant to oxidation when heated with acidified potassium dichromate(VI)? Justify your answer in terms of the structure of the isomer.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iii)  Which isomer can be oxidized to a ketone? Draw the displayed formula of the ketone produced.
(1)
 ............................................................................................................................................. 

(iv)  Which isomers can be oxidized to an aldehyde?
(1)
 ............................................................................................................................................. 
(v)  Phosphorus(V) chloride (phosphorus pentachloride), PCl5, is used to test for the presence of an −OH group.

     What would you expect to see when any of the above four isomers, A, B, C or D, are reacted with phosphorus(V) chloride?
(1)
 ............................................................................................................................................. 
 (vi)  Complete the equation for the reaction shown below. State symbols are not required.
(2)

(Total for question = 13 marks)

Q6.
Four organic reactions are given below:

(a) Which reaction is a substitution reaction?
(1)
   A
   B
   C
   D

(b) Which reaction is an electrophilic addition reaction?
(1)
   A
   B
   C
   D

(c) Which reaction involves initial attack by a nucleophile?
(1)
   A
   B
   C
   D

(d) Which reaction requires an initiator?
(1)
   A
   B
   C
   D
(Total for question = 4 marks)

Unit 2 - Jun 2010
Q7.
Unsaturated vegetable oils are hardened to make margarine by reaction with hydrogen and a nickel catalyst. Which terms could both be used to describe this type of reaction?
   A    Substitution and oxidation
   B    Substitution and reduction
   C    Addition and oxidation
   D    Addition and reduction
(Total for Question = 1 mark)

Unit 2 - Jan 2011
Q8.
This question is about nitrogen monoxide, NO, which can be described both as a friend and a foe.

(a) (i)   What is meant by the term free radical?
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(a) (ii)  Suggest a dot and cross diagram for nitrogen monoxide, showing outer shell electrons only, remembering that it is a free radical.
(2)

(b) (i)   Part of the unbalanced equation for the preparation of nitrogen monoxide from nitric acid is shown below.

Identify the elements which are oxidized and reduced and give their oxidation numbers.
(3)
Element oxidized .........................

Oxidation number initial .........................    final .........................

Element reduced .........................

Oxidation number initial .........................    final .........................

     (ii)  Complete and balance the equation for the reaction between silver and nitric acid.
(2)

(c) The reaction between nitrogen and oxygen to form nitrogen monoxide reaches equilibrium.

     (i)   Explain why the yield of nitrogen monoxide is increased when the temperature is increased.
(1)
      ..............................................................................................................................................

  ∗(ii)  State and explain the effect, if any, on the yield of nitrogen monoxide when the pressure is increased.
(2)
      ..............................................................................................................................................

      ..............................................................................................................................................

   (iii)  State and explain how the rate of the reaction is affected by an increase in pressure.
(2)
      ..............................................................................................................................................

      ..............................................................................................................................................

∗ (d) (i)  Explain why a jet aeroplane in flight causes much more damage to the ozone layer than cars carrying the same number of passengers at sea level. You should assume that the nitrogen monoxide outputs for both methods of conveying the passengers are the same.
(2)
      ..............................................................................................................................................
      ..............................................................................................................................................

   (ii)  The reactions of chlorine free radicals with ozone may be represented by the following equations.

Write corresponding equations for the reactions of the free radical nitrogen monoxide with ozone. Combine your two equations to show the overall reaction.
Use these equations to explain why a small quantity of nitrogen monoxide can have a continuing effect on the ozone layer.
(5)
Equations

Explanation
      ..............................................................................................................................................

      ..............................................................................................................................................

      ..............................................................................................................................................
 (Total for Question = 20 marks)
Unit 2 - Jun 2011
Q9.
When iodomethane, CH3I, is heated in a sealed tube with an excess of alcoholic ammonia, which of the following cannot be formed?
   A    Methylamine, CH3NH2
   B    Ethylamine, CH3CH2NH2
   C    Dimethylamine, (CH3)2NH
   D    Ammonium iodide, NH4I
(Total for Question = 1 mark)
Q10.
Halogenoalkanes are important intermediates in organic chemistry. The scheme below summarises some important reactions of a halogenoalkane.

(a)  Identify the reagents and any specific conditions required for the reactions in the diagram. (You may assume that a suitable temperature is maintained in each reaction.)
(4)
A
 ............................................................................................................................................. 
B
 ............................................................................................................................................. 
C
 ............................................................................................................................................. 
D
 ............................................................................................................................................. 
(b)  (i)  Classify the type of reaction in each of A and D.
(2)
A
 ............................................................................................................................................. 
D
 ............................................................................................................................................. 
*(ii)  Reaction B can proceed via two possible reaction mechanisms, depending on the structure of the original compound. For each of the two isomers of C4H9Br shown below, draw the structure of the intermediate or transition state which is formed during the reaction. Explain in each case why the specified structure is more favourable.
(4)

Intermediate or transition state
Explanation
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

Intermediate or transition state
Explanation
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iii)  If C4H9I is used instead of C4H9Br in reaction D, the rate of formation of C4H9NH2 increases. Explain why the rate of reaction increases.
(1)
 ............................................................................................................................................. 
 (c)  Halogenoalkanes are widely used as refrigerants and belong to the class of refrigerants that cool by change of state (typically by boiling).
(i)  Suggest how halogenoalkanes cool by change of state.
(1)
 ............................................................................................................................................. 
 (ii)  Suggest two characteristics or properties desirable in a refrigerant.
(2)
1
 ............................................................................................................................................. 
2
 ............................................................................................................................................. 
(Total for question = 14 marks)

Unit 2 - Jan 2012
Q11.

(a) Write the equation for the formation of nitrogen monoxide from nitrogen and oxygen. State symbols are not required.
(1)

(b) The electronic structure of nitrogen monoxide is unusual in that it has an unpaired electron.

(i) What name is given to a chemical species such as nitrogen monoxide that has an unpaired electron?
(1)
 ............................................................................................................................................. 

(ii) Chemical species with unpaired electrons occur as intermediates in some chemical reactions. What type of bond breaking produces these species?
(1)
 ............................................................................................................................................. 

(c)(i) Suggest the most likely source of the hydrocarbons that react with nitrogen monoxide to form toxic compounds.
(1)
 ............................................................................................................................................. 

(ii) Suggest the type of reaction that is involved when a hydrocarbon is converted into an aldehyde.
(1)
 ............................................................................................................................................. 

(iii)  Draw the skeletal formula of the aldehyde with three carbon atoms.
(1)

(iv) By considering the equation

explain the effect of the reaction of hydrocarbons with nitrogen monoxide on the breakdown of ozone.
(1)
 ............................................................................................................................................. 
(d) Suggest why the proportion of nitrogen dioxide might be reduced at high altitudes.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (e) Explain why it is important to maintain the concentration of ozone in the upper atmosphere.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (f)(i) Write an equation for the reaction on a catalytic converter described in the passage. State symbols are not required.
(1)

(ii) Draw an energy profile for the exothermic reaction in (f)(i). Label the axes, the reactants and products, the enthalpy change and the activation energy.
(3)

*(iii) By referring to your energy profile, explain how a catalyst speeds up a chemical reaction.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (g) Jet aircraft are considered a greater threat to the ozone layer than road vehicles. Suggest an explanation for this.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (Total for question = 20 marks)

Q12.
The steps below show the reaction mechanism for the reaction of a halogenoalkane with sodium hydroxide in aqueous solution to form an alcohol.

(a) (i) Name X and Z.
(2)
X
 ............................................................................................................................................. 
Z
 ............................................................................................................................................. 
(ii) Draw the skeletal formula of X.
(1)

(iii) What type of alcohol is Z?.
(1)
 ............................................................................................................................................. 
(b) (i) Name the mechanism and type of reaction shown above.
(2)
 ............................................................................................................................................. 
(ii) Explain what the curly arrows shown in the mechanism represent.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
*(iii) Suggest the shape of the intermediate Y. Explain your answer.
(3)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (iv) If the reaction is carried out in alcoholic (ethanolic) rather than aqueous solution, a different type of reaction occurs and a different product is formed. Name the type of reaction that occurs in alcoholic (ethanolic) solution and identify the product by name or formula.
(2)
Type of reaction
 ............................................................................................................................................. 
Product

(c) The alcohol Z (shown below) resists oxidation. However, Z has three structural isomers which are readily oxidized. On complete oxidation, one isomer forms a ketone and the other two isomers form carboxylic acids.

(i) Draw the structural formula of the isomer of Z that forms a ketone.
(1)

(ii) Draw the structural formulae of the isomers of Z that form carboxylic acids.
(2)

(Total for question = 15 marks)

Unit 2 - Jun 2012
Q13.
      Nucleophiles are
   A       electron pair donors that attack regions of high electron density.
   B       electron pair donors that attack regions of low electron density.
   C       electron pair acceptors that attack regions of high electron density.
   D       electron pair acceptors that attack regions of low electron density.
(Total for question = 1 mark)

Unit 2 - Jun 2013
Q14.
C2H5Br + NaOH → C2H4 + NaBr + H2O

This reaction is an example of
   A    addition.
   B    elimination.
   C    hydrolysis.
   D    oxidation.
(Total for Question = 1 mark)

Q15.
(a)  The products of the reaction when 2-chlorobutane is heated with sodium hydroxide depend on the conditions.
(i)  What condition, other than a suitable temperature and sodium hydroxide concentration, would produce a mixture of but-1-ene and but-2-ene?
(1)
 ............................................................................................................................................. 

(ii)  What type of reaction occurs in (a)(i)?
(1)
 ............................................................................................................................................. 

(iii)  What condition, other than a suitable temperature and sodium hydroxide concentration, would produce butan-2-ol in the reaction of 2-chlorobutane with sodium hydroxide?
(1)
 ............................................................................................................................................. 

(iv)  Suggest the mechanism for the reaction of 2-chlorobutane with hydroxide ions to form butan-2-ol. Use curly arrows to show the movement of electron pairs.
(2)

(b)  Phosphorus(V) chloride, PCl5, can be used to test for the –OH group.
Describe what would be seen when phosphorus(V) chloride is added to butan-2-ol. Give the equation for the reaction. State symbols are not required.
(2)
Observation
 ............................................................................................................................................. 
Equation

(c)  A tertiary alcohol, A, is an isomer of butan-2-ol.
(i)  Butan-2-ol and A can be distinguished by warming separate samples with a mixture of potassium dichromate(VI) and sulfuric acid. State the observations which would be made with each alcohol.
(2)
Observation with butan-2-ol
 ............................................................................................................................................. 

Observation with A
 ............................................................................................................................................. 
(ii) Give the structural formula of the organic product which forms when butan-2-ol is oxidized.
(1)

(iii)  Explain how infrared spectroscopy could be used to detect whether butan-2-ol has been oxidized.
(1)
 ............................................................................................................................................. 
(Total for Question = 11 marks)
Q16.
The reaction of 1-chloropropane with water containing dissolved silver nitrate in the presence of ethanol is
   A    a redox reaction.
   B    a nucleophilic substitution.
   C    an electrophilic substitution.
   D    a free radical substitution.
(Total for Question = 1 mark)
Unit 2 - Jun 2014
Q17.
Ethane-1,2-diol, CH2OHCH2OH, is commonly used in antifreeze for cars to lower the freezing temperature of the water in the car radiator. It reacts in a similar way to ethanol but both of the alcohol groups can react.

(a)  Write an equation for the complete reaction between sodium and ethane-1,2-diol. State symbols are not required.
(2)

(b)  Ethane-1,2-diol is very quickly oxidized to ethanedioic acid, (COOH)2, even under the conditions shown below.

However, ethanol requires stronger oxidizing conditions to be converted into ethanoic acid.
Explain how you would change the above apparatus to achieve this oxidation of ethanol.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(c)  Draw the skeletal formula of ethanedioic acid.
(1)

(d)  Explain why phosphorus(V) chloride, PCl5, would not be a suitable reagent to be used to distinguish between ethane-1,2-diol and ethanedioic acid.
(1)
 ............................................................................................................................................. 

(e)  (i)  Depending on the reaction conditions, ethanol can be oxidized to either an aldehyde or to carboxylic acid. Infrared spectroscopy is a suitable technique for determining whether the oxidation product obtained is an aldehyde or a carboxylic acid.
Draw, on the spectrum below, any peak(s) that you would expect to see between 4000 and 1500 cm−1 if the product was an aldehyde and not a carboxylic acid.
(2)
DATA
The IR absorption ranges associated with some organic functional groups are given below:
O—H stretching in alcohols (variable, broad) at 	3750 − 3200 cm−1 	
O—H stretching in carboxylic acids (weak) at 	3300 − 2500 cm−1 	
CO stretching in aldehydes (strong) at 	1740 − 1720 cm−1 	
CO stretching in ketones (strong) at 	1700 − 1680 cm−1 	
CO stretching in carboxylic acids, alkyl (strong) at 	1725 − 1700 cm−1 	
C—H stretching in aldehydes (weak) at 	2900 − 2820 cm−1 	
                                       and (weak) at 	2775 − 2700 cm−1 	

(ii)  The infrared spectrum of the distillate from the reaction in (e)(i) included a peak at 3750–3200 cm−1.

What substance is likely to have caused this?
(1)
 ............................................................................................................................................. 

(iii)  Mass spectrometry can be used to identify the products of the oxidation of ethanol. Suggest the formula of a fragment that would show when ethanoic acid is produced and would not be present in either ethanol or ethanal.
(1)
 ............................................................................................................................................. 

(f)  Treatment of 2-bromoethanol, CH2BrCH2OH, with aqueous sodium hydroxide would be one way to produce ethane-1,2-diol.

(i)  Complete a possible mechanism for this reaction in the space below.
(3)

(ii)  Classify the mechanism and type of reaction in (f)(i):
(2)
Mechanism
 ............................................................................................................................................. 
Type
 ............................................................................................................................................. 

(g)  Aqueous silver nitrate can be used to test for the presence of bromide ions. Write an ionic equation for the reaction. Include state symbols in your answer.
Ionic Equation

*(h)  It can be difficult to distinguish between the colours of the silver halides. The use of solutions of ammonia can be very helpful.

A silver halide dissolved in concentrated ammonia to form a colourless solution.

Explain why this result does not prove conclusively that the silver halide was silver bromide and give a further test to confirm that the silver halide is silver bromide.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 (Total for Question = 19 marks)

Q18.
The meaning of homolytic fission is

   A    bond-breaking to form two free radicals.
   B    bond-making to form two free radicals.
   C    bond-breaking to form a cation and an anion.
   D    bond-making to form a cation and an anion.
(Total for Question = 1 mark)

Q19.
A reaction mechanism is shown below.

The hydroxide ion is acting as
   A    an electrophile.
   B    a catalyst.
   C    a free radical.
   D    a nucleophile.
(Total for question = 1 mark)

Q20.
This is a question about halogenoalkanes.
(a)  Halogenoalkanes can react with hydroxide ions in different ways depending on the conditions used. Using 1-chloro-1-fluoroethane, CH3CHClF, as an example of a halogenoalkane, the following reaction could occur in aqueous solution.
CH3CHClF + OH− → CH3CHOHF + Cl−
(i)  Suggest why it is unlikely that the fluorine atom in CH3CHClF would be substituted by the hydroxide ion.
(1)
 ............................................................................................................................................. 

*(ii)  A student attempted to draw the reaction mechanism for the reaction in (a)(i), but made a total of three errors.

Identify these errors and state how they should be corrected.
(3)
First error
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

Second error
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 

Third error
 ............................................................................................................................................. 
 ............................................................................................................................................. 
 ............................................................................................................................................. 
(iii)  In hot alcoholic solution, a different reaction may occur between halogenoalkanes and hydroxide ions.
Write the equation for the reaction between 1-chloro-1-fluoroethane, CH3CHClF, and hydroxide ions in alcoholic solution.
(2)

(b)  Chloroethane can be produced in various ways as shown below. Identify, by name or formula, the reagent needed for each of these reactions.
(3)

(c)  (i)  Chloroethane will react with alcoholic ammonia initially to produce ethylamine. Complete the equation for this reaction.
(1)
.............................. + NH3 → .............................. + ..............................

(ii)  Name the type and mechanism of the reaction occurring in (c)(i).
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(iii)  What feature of the ammonia molecule enables the reaction in (c)(i) to take place?
(1)
 ............................................................................................................................................. 

(iv)  If aqueous ammonia was used in (c)(i), instead of alcoholic ammonia, suggest the identity of the organic product that would be formed.
(1)
 ............................................................................................................................................. 

(d)  Dichlorodifluoromethane, CCl2F2, is also known as Freon 12 and its manufacture was banned in 1994 under the terms of the Montreal Protocol.

(i)  Complete the equation for the initiation stage and suggest equations for two of the propagation stages and a termination stage for the mechanism of the reaction that this molecule might undergo with ozone.
(4)

Initiation           CCl2F2 →

Propagation 1

Propagation 2

Termination

*(ii)  Explain why the effect of Freon 12 molecules on the ozone layer was such a serious issue that scientists recommended its use to be discontinued.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(iii)  Freon 12, CCl2F2, could also be described as a "greenhouse gas". Explain what the term 'greenhouse gas' means.
(2)
 ............................................................................................................................................. 
 ............................................................................................................................................. 

(iv)  Freon 12, and other similar molecules, are not normally viewed as contributors to the greenhouse effect. Suggest why this is so.
(1)
 ............................................................................................................................................. 
 ............................................................................................................................................. 
  
(Total for question = 23 marks