Q1. Pentanenitrile can be made by reaction of 1-bromobutane with potassium cyanide.

Transcription

1 Q1. Pentanenitrile can be made by reaction of 1-bromobutane with potassium cyanide. Which of these is the correct name for the mechanism of this reaction? A B C D Electrophilic addition Electrophilic substitution Nucleophilic addition Nucleophilic substitution (Total 1 mark) Q2. Consider the following reaction in which an alkene is formed from a haloalkane. (a) Name the haloalkane used in this reaction.... (b) Name and outline a mechanism for this reaction. Name of mechanism... Mechanism (4) (c) Another alkene, which is a structural isomer of but-2-ene, is also formed during this reaction. (i) State what is meant by the term structural isomers. Page 1 of 27

2 (ii) Draw the structure of this other alkene. (Total 7 marks) Q3. How many different alkenes are formed when 2-bromo-3-methylbutane reacts with ethanolic potassium hydroxide? A 2 B 3 C 4 D 5 (Total 1 mark) Q4. Nucleophiles react with bromoethane in substitution reactions. This type of reaction is illustrated in the following scheme. (a) State what is meant by the term nucleophile.... Page 2 of 27

3 (b) Outline a mechanism for the reaction of potassium cyanide with bromoethane (Reaction 1). (c) Explain why an excess of ammonia is needed in Reaction 2 to produce a high yield of ethylamine (d) When potassium hydroxide reacts with bromoethane, ethene can also be formed. Name and outline a mechanism for this reaction. Name of mechanism... Mechanism (4) (Total 8 marks) Page 3 of 27

4 Q5. A student carried out an experiment to study the rates of hydrolysis of some haloalkanes. (a) In the experiment, two different haloalkanes were placed in separate test tubes containing silver nitrate solution. The haloalkanes reacted with the water in the silver nitrate solution. The student timed how long it took for the first appearance of the silver halide precipitate in each tube at a constant temperature. This time was used to provide a measure of the initial rate of reaction. The student obtained the following results. 1-bromobutane 1-iodobutane Time to form a precipitate / s (i) State the meaning of the term hydrolysis. (ii) State the colour of the precipitate formed when iodide ions react with silver nitrate and write the simplest ionic equation for this reaction. Colour of precipitate... Simplest ionic equation (iii) Use your knowledge of the reactions of halide ions with silver nitrate to suggest why the student did not include 1-fluorobutane in this experiment. (b) The student used the following enthalpy data to try to account for the different initial rates of hydrolysis of the haloalkanes used in part (a). The student deduced that the rate of hydrolysis of a haloalkane is influenced by the strength of the carbon halogen bond in the haloalkane. C Br C I Bond enthalpy / kj mol State how the experimental evidence enabled the student to make this deduction Page 4 of 27

5 (c) The student had read that the reaction of water with haloalkanes was similar to the reaction of aqueous sodium hydroxide with haloalkanes and was an example of a nucleophilic substitution reaction. (i) State the meaning of the term nucleophile. (ii) When a hydroxide ion collides with a molecule of 1-bromobutane, the following reaction occurs. CH 3 CH 2 CH 2 CH 2 Br + OH CH 3 CH 2 CH 2 CH 2 OH + Br Outline the nucleophilic substitution mechanism for this reaction. (d) The reaction of hydroxide ions with 2-bromo-2-methylpropane may occur by a different mechanism from the one in part (c). This different mechanism involves the formation of a carbocation. (i) Complete the following equation by drawing the structure of the carbocation formed when the C Br bond in 2-bromo-2-methylpropane is broken. (ii) Suggest one reason why this reaction occurs by a mechanism involving a carbocation, but the reaction in part (c) (ii) does not. (Total 11 marks) Page 5 of 27

6 Q6. (a) Consider the following reaction. (i) Name and outline a mechanism for this reaction. Name of mechanism... Mechanism (3) (ii) Name the haloalkane in this reaction. (iii) Identify the characteristic of the haloalkane molecule that enables it to undergo this type of reaction. Page 6 of 27

7 (b) An alternative reaction can occur between this haloalkane and potassium hydroxide as shown by the following equation. Name and outline a mechanism for this reaction. Name of mechanism... Mechanism (4) (c) Give one condition needed to favour the reaction shown in part (b) rather than that shown in part (a).... (d) Alkenes can be polymerised to produce poly(alkenes). (i) State the type of polymerisation that alkenes undergo. (ii) Name the alkene that gives a polymer with the repeating unit shown below. Name of alkene... (Total 12 marks) Page 7 of 27

8 Q7. A student read the following passage on the Internet. 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. (a) Explain the meaning of each of the following terms in the information given above. (i) nucleophile (ii) substitution, as applied to nucleophilic substitution in a haloalkane (iii) hydrolysis (iv) bond enthalpy, as applied to a carbon halogen bond. Page 8 of 27

9 (b) Outline a mechanism for the nucleophilic substitution reaction in which 2-bromopropane (CH 3 CHBrCH 3 ) reacts with potassium hydroxide to form propan-2-ol. (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) Page 9 of 27

10 (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. (Total 11 marks) Page 10 of 27

11 Q8. Organic reaction mechanisms help chemists to understand how the reactions of organic compounds occur. The following conversions illustrate a number of different types of reaction mechanism. (a) When 2-bromopentane reacts with ethanolic KOH, two structurally isomeric alkenes are formed. (i) Name and outline a mechanism for the conversion of 2-bromopentane into pent-2-ene as shown below. (4) (ii) Draw the structure of the other structurally isomeric alkene produced when 2- bromopentane reacts with ethanolic KOH. Page 11 of 27

12 (b) Name and outline a mechanism for the following conversion. (5) (c) Name and outline a mechanism for the following conversion. (5) (Total 15 marks) Page 12 of 27

13 Q9. Consider the following reactions. (a) Name and outline a mechanism for Reaction 1. Name of mechanism... Mechanism (5) (b) Name and outline a mechanism for Reaction 2. Name of mechanism... Mechanism (5) Page 13 of 27

14 (c) State the type of reaction in Reaction 3. Give the name of substance X (d) The haloalkane produced in Reaction 1 can be converted back into propene in an elimination reaction using ethanolic potassium hydroxide. Outline a mechanism for this conversion. CH 3 CHBrCH 3 H 2 C=CHCH 3 (3) (Total 15 marks) Q10. Oxygen and ozone (O 3 ) both occur as gases in the upper atmosphere. Chlorine atoms catalyse the decomposition of ozone and contribute to the formation of a hole in the ozone layer. These chlorine atoms are formed from chlorofluorocarbons (CFCs) such as CF 3 Cl (a) (i) Give the IUPAC name of CF 3 Cl (ii) Complete the following equation that shows the formation of a chlorine atom from a molecule of CF 3 Cl (iii) State what the represents in Cl Page 14 of 27

15 (b) Write two equations that show how chlorine atoms catalyse the decomposition of ozone into oxygen. Equation 1... Equation 2... (c) An equilibrium is established between oxygen and ozone molecules as shown below. 3O 2 (g) 2O 3 (g) ΔH = +284 kj mol 1 (i) State Le Chatelier s principle. (ii) Use Le Chatelier s principle to explain how an increase in temperature causes an increase in the equilibrium yield of ozone. (d) Chemists supported the legislation to ban the use of CFCs. Modern refrigerators use pentane rather than CFCs as refrigerants. With reference to its formula, state why pentane is a more environmentally acceptable refrigerant (Total 9 marks) Page 15 of 27

16 Q11. 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. (CH 3 ) 2 CHCHBrCH 3 + KOH (CH 3 ) 2 C=CHCH 3 + KBr + H 2 O Name of mechanism... Mechanism (4) (ii) Draw the displayed formula for the other isomer that is formed. (iii) State the type of structural isomerism shown by these two alkenes... (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 5 H 12 O and is a secondary alcohol. (i) Draw the displayed formula for X. (ii) Suggest one change to the reaction conditions that would increase the yield of X..... Page 16 of 27

17 (iii) State the type of mechanism for the conversion of 2-bromo-3-methylbutane into X... (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..... (Total 10 marks) Q12. In each of the following questions, you should draw the structure of the compound in the space provided. (a) Draw the structure of the alkene that would form 1,2-dibromo-3-methylbutane when reacted with bromine. (b) Draw the structure of the alcohol with molecular formula C 4 H 10 O that is resistant to oxidation by acidified potassium dichromate(vi). Page 17 of 27

18 (c) Draw the structure of the alkene that has a peak, due to its molecular ion, at m/z = 42 in its mass spectrum. (d) Draw the structure of the organic product with M r = 73, made from the reaction between 2- bromobutane and ammonia. (Total 4 marks) Q13. The refrigerant R410A, used in air conditioners, is a mixture of two fluoroalkanes, pentafluoroethane and difluoromethane. (a) (i) The mechanism for the reaction of fluorine with either an alkane or a fluoroalkane is similar to that for the reaction of chlorine with methane. Name the type of mechanism for the reaction of chlorine with methane... (ii) Write equations for the following steps in the mechanism for the reaction of fluorine with fluoromethane (CH 3 F) to form difluoromethane (CH 2 F 2 ). Initiation step.. First propagation step.. Second propagation step.. A termination step leading to the formation of 1,2-difluoroethane... (4) Page 18 of 27

19 (iii) Write an overall equation for the reaction of fluorine with ethane to form pentafluoroethane (CF 3 CHF 2 ) by this mechanism... (b) The refrigerant R112A (CCl 3 CF 2 Cl) has been banned because of concerns about ozone depletion. Give the IUPAC name for CCl 3 CF 2 Cl... (c) Nitrogen monoxide (NO) catalyses the decomposition of ozone into oxygen. (i) Write the overall equation for this decomposition... (ii) Use the overall equation to deduce Step 3 in the following mechanism that shows how nitrogen monoxide catalyses this decomposition. Step 1 O 3 O + O 2 Step 2 NO + O 3 NO 2 + O 2 Step 3... (Total 9 marks) Page 19 of 27

20 Q14. Chlorine can be used to make chlorinated alkanes such as dichloromethane. (a) Write an equation for each of the following steps in the mechanism for the reaction of chloromethane (CH 3 Cl) with chlorine to form dichloromethane (CH 2 Cl 2 ). Initiation step... First propagation step... Second propagation step... The termination step that forms a compound with empirical formula CH 2 Cl.... (4) (b) When chlorinated alkanes enter the upper atmosphere, carbon-chlorine bonds are broken. This process produces a reactive intermediate that catalyses the decomposition of ozone. The overall equation for this decomposition is 2O 3 3O 2 (i) Name the type of reactive intermediate that acts as a catalyst in this reaction... (ii) Write two equations to show how this intermediate is involved as a catalyst in them decomposition of ozone. Equation 1... Equation 2... (Total 7 marks) Page 20 of 27

21 Q15. This question is about a method that can be used to prepare ethylamine. CH 3 CH 2 Br + 2NH 3 CH 3 CH 2 NH 2 + NH 4 Br Which statement about the reaction is not correct? A B C D Ethylamine is a primary amine. The mechanism is a nucleophilic substitution. Using an excess of bromoethane will prevent further reaction to form a mixture of amine products. Ammonium bromide is an ionic compound. (Total 1 mark) Q16. Why are fluoroalkanes unreactive? A Fluorine is highly electronegative. B The F ion is very stable. C They are polar molecules. D The C F bond is very strong. (Total 1 mark) Page 21 of 27

22 Q17. This question is about a method that can be used to prepare ethylamine. CH 3 CH 2 Br + 2NH 3 CH 3 CH 2 NH 2 + NH 4 Br Which of the curly arrows in the mechanism is not correct? A 1 B 2 C 3 D 4 (Total 1 mark) Q18. Which of the following is a correct mechanism for the formation of 2-methylbut-2-ene from 2-bromo-3-methylbutane? A B C D (Total 1 mark) Page 22 of 27

23 Q19. 2-bromo-2-methylpentane is heated with potassium hydroxide dissolved in ethanol. Two structural isomers are formed. (a) State the meaning of the term structural isomers (b) Name and draw the mechanism for the formation of one of the isomers. Name of mechanism... Mechanism (5) (Total 6 marks) Q20. Refrigerants are substances used to cool refrigerators and freezers. Until recently, many of the compounds used as refrigerants were chlorofluorocarbons (CFCs), but these are now known to form chlorine radicals. CFCs have been phased out in many countries by international agreement. (a) Write two equations to show how chlorine radicals react with ozone molecules in the upper atmosphere Page 23 of 27

24 (b) Chloropentafluoroethane is a CFC that has been used as a refrigerant. Draw its displayed formula. (c) 1,1,1-trifluoroethane (CF 3 CH 3 ) is one of the molecules that has been used as a refrigerant in place of CFCs. Explain why 1,1,1-trifluoroethane does not lead to the depletion of the ozone in the upper atmosphere (d) One of the steps in the synthesis of 1,1,1-trifluoroethane (CF 3 CH 3 ) is the reaction of 1,1-difluoroethane (CHF 2 CH 3 ) with fluorine in a free-radical substitution reaction. Write two equations to represent the propagation steps in this conversion of CHF 2 CH 3 into CF 3 CH 3 Propagation step 1... Propagation step 2... Page 24 of 27

25 (e) A refrigerator contains 1.41 kg of 1,1,1-trifluoroethane (CF 3 CH 3 ). Calculate the number of molecules of 1,1,1-trifluoroethane in the refrigerator. Give your answer to an appropriate number of significant figures. (The Avogadro constant L = mol 1 ) (f) There are growing concerns about the use of 1,1,1-trifluoroethane as a refrigerant as it is a greenhouse gas that absorbs some of Earth s infrared radiation. Give one reason why bonds in molecules such as carbon dioxide and 1,1,1-trifluoroethane absorb infrared radiation (Total 9 marks) Q21. CCl 4 is an effective fire extinguisher but it is no longer used because of its toxicity and its role in the depletion of the ozone layer. In the upper atmosphere, a bond in CCl 4 breaks and reactive species are formed. (a) Identify the condition that causes a bond in CCl 4 to break in the upper atmosphere. Deduce an equation for the formation of the reactive species. Condition... Equation... (b) One of the reactive species formed from CCl 4 acts as a catalyst in the decomposition of ozone. Write two equations to show how this species acts as a catalyst. Equation 1... Equation 2... Page 25 of 27

26 (c) A small amount of the freon CF 3 Cl with a mass of kg escaped from a refrigerator, into a room of volume 100 m 3. Assuming that the freon is evenly distributed throughout the air in the room, calculate the number of freon molecules in a volume of 500 cm 3. Give your answer to the appropriate number of significant figures. The Avogadro constant = mol 1. Number of molecules =... (3) (Total 7 marks) Page 26 of 27

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