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ORGANIC CHEMISTRY AS REVISION QUESTIONS Q1. Petroleum is separated into fractions by fractional distillation.The petrol fraction (C4 to C12) is burned in internal combustion engines and the naphthafraction (C7 to C14) is cracked. (a) Petroleum is separated into fractions when it is heated and the vapour mixture is passed into a fractionating column. (i) Explain what is meant by the term fraction as applied to fractional distillation. (ii) State a property of the molecules in petroleum which allows the mixture to be separated into fractions. (iii) Describe the temperature gradient in the column. (3) (b) The fractions from petroleum contain alkane hydrocarbons. (i) Write an equation for the incomplete combustion of the alkane C8H18 to produce carbon monoxide and water only. (ii) One isomer of C8H18 is 2,2,3-­‐trimethylpentane. Draw the structure of this isomer.(2) (c) State one economic reason for the cracking of petroleum fractions. (1) (d) (i) Give the type of reactive intermediate formed during catalytic cracking. (ii) Identify a catalyst used in catalytic cracking. (2) (e) (i) Give the type of reactive intermediate formed during thermal cracking. State how this reactive intermediate is formed. Reactive intermediate .......................................How intermediate is formed ................................. (ii) Identify the different type of hydrocarbon produced in a high percentage by the thermal cracking of alkanes. (3) (Total 11 marks) Q2. Hexane is a member of the homologous series of alkanes. (a) State two characteristics of a homologous series. (2) (b) (i) Hexane can be converted into 2,2-­‐dichlorohexane.Draw the displayed formula of 2,2-­‐dichlorohexane and deduce its empirical formula. (2) (ii) Explain why 2,2-­‐dichloro-­‐3-­‐methylpentane is a structural isomer of 2,2-­‐dichlorohexane. (2) (c) A reaction of hexane with chlorine is shown by the equation below. C6H14 + 2Cl2 → C6H12Cl2 + 2HCl Calculate the percentage atom economy for the formation of C6H12Cl2 in this reaction. (2) (d) The boiling points of some straight-­‐chain alkanes are shown below. Alkane C4H10 C5H12 C6H14 Boiling point / °C – 0.5 36.3 68.7 (1)(Total 11 marks) Page 1
(i) Explain the trend in these boiling points. (2) (ii) Name a process which can be used to separate C5H12 from C6H14 Q3.(a) The hydrocarbon but-­‐1-­‐ene (C4H8) is a member of the homologous series of alkenes. But-­‐1-­‐ene has structural isomers. (i) State the meaning of the term structural isomers. (2) (ii) Give the IUPAC name of the position isomer of but-­‐1-­‐ene. (1) (iii) Give the IUPAC name of the chain isomer of but-­‐1-­‐ene. (1) (iv) Draw the displayed formula of a functional group isomer of but-­‐1-­‐ene.(1) (b) But-­‐1-­‐ene burns in a limited supply of air to produce a solid and water only. (i) Write an equation for this reaction. (1) (ii) State one hazard associated with the solid product in part (b)(i). (1) (c) One mole of compound Y is cracked to produce two moles of ethene, one mole of but-­‐1-­‐ene and one mole of octane (C8H18) only. (i) Deduce the molecular formula of Y. (1) (ii) Other than cracking, give one common use of Y. (1) (d) In cars fitted with catalytic converters, unburned octane reacts with nitrogen monoxide to form carbon dioxide, water and nitrogen only. (i) Write an equation for this reaction. (1) (ii) Identify a catalyst used in a catalytic converter. (1)(Total 11 marks) Q4. (a) (i) Name the process used to separate petroleum into fractions. (ii) Give the molecular formula for an alkane with nine carbon atoms. (iii) Write an equation for the complete combustion of the alkane C11H24 (iv) Write an equation for the incomplete combustion of C11H24 to produce carbon and water only. (4) (b) Alkenes can be produced by cracking the naphtha fraction obtained from petroleum. (i) Write an equation for the thermal cracking of one molecule of C10 H22 to give one molecule of propene and one molecule of an alkane only. (ii) Draw the structure of the chain isomer of but-­‐1-­‐ene. (2) (c) The alkanes and the alkenes are examples of homologous series of compounds. One feature of an homologous series is the gradual change in physical properties as the relative molecular mass increases. State two other general features of an homologous series of compounds. (2)(Total 8 marks) Page 2
Q5.Hexane (C6H14) is a member of the homologous series of alkanes. (a) (i) Name the raw material from which hexane is obtained. (1) (ii) Name the process used to obtain hexane from this raw material. (1) (b) C6H14 has structural isomers. (i) Deduce the number of structural isomers with molecular formula C6H14 (ii) State one type of structural isomerism shown by the isomers of C6H14 (1) (c) One molecule of an alkane X can be cracked to form one molecule of hexane and two molecules of propene. (i) Deduce the molecular formula of X. (1) (ii) State the type of cracking that produces a high percentage of alkenes. State the conditions needed for this type of cracking. (2) (iii) Explain the main economic reason why alkanes are cracked. (1) (d) Hexane can react with chlorine under certain conditions as shown in the following equation. C6H14 + Cl2 C6H13Cl + HCl (i) Both the products are hazardous. The organic product would be labelled ‘flammable’. Suggest the most suitable hazard warning for the other product. (1) (ii) Calculate the percentage atom economy for the formation of C6H13Cl (Mr = 120.5) in this reaction. (1) (e) A different chlorinated compound is shown below. Name this compound and state its empirical formula. (2)(Total 12 marks) Q6.The following table shows the boiling points of some straight-­‐chain alkanes. Boiling point / °C CH4 C2H6 C3H8 C4H10 C5H12 −162 −88 −42 −1 36 (a) State a process used to separate an alkane from a mixture of these alkanes. (1) (b) Both C3H8 and C4H10 can be liquefied and used as fuels for camping stoves.Suggest, with a reason, which of these two fuels is liquefied more easily. (1) (c) Write an equation for the complete combustion of C4H10 (1) (d) Explain why the complete combustion of C4H10 may contribute to environmental problems. (1) (e) Balance the following equation that shows how butane is used to make the compound called maleic anhydride. ..........CH3CH2CH2CH3 + .......... O2 ..........C2H2(CO)2O + .......... H2O (1) (f) Ethanethiol (C2H5SH), a compound with an unpleasant smell, is added to gas to enable leaks from gas pipes to be more easily detected. (i) Write an equation for the combustion of ethanethiol to form carbon dioxide, water and sulfur dioxide. (1) Page 3
(ii) Identify a compound that is used to react with the sulfur dioxide in the products of combustion before they enter the atmosphere.Give one reason why this compound reacts with sulfur dioxide. (2) (iii) Ethanethiol and ethanol molecules have similar shapes. Explain why ethanol has the higher boiling point. (2) (g) The following compound X is an isomer of one of the alkanes in the table on above. (i) Give the IUPAC name of X. (1) (ii) X has a boiling point of 9.5 °C. Explain why the boiling point of X is lower than that of its straight-­‐chain isomer. (2) (iii) The following compound Y is produced when X reacts with chlorine. Deduce how many other position isomers of Y can be formed 1) (h) Cracking of one molecule of an alkane Z produces one molecule of ethane, one molecule of propene and two molecules of ethene. (i) Deduce the molecular formula of Z. (1) (ii) State the type of cracking that produces a high proportion of ethene and propene. Give the two conditions for this cracking process. (2)(Total 17 marks) Q7.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. (CH3)2CHCHBrCH3 + KOH (CH3)2C=CHCH3 + KBr + H2O (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. (1) (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 C5H12O and is a secondary alcohol. (i) Draw the displayed formula for X.(1) (ii) Suggest one change to the reaction conditions that would increase the yield of X. (1) (iii) State the type of mechanism for the conversion of 2-­‐bromo-­‐3-­‐methylbutane into X. (1) (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. Page 4
(1) Feature ................................................................................... (1)(Total 10 marks) Q8. The reaction of bromine with ethane is similar to that of chlorine with ethane. Three steps in the bromination of ethane are shown below. Step 1 Br2 •
Step 2 Br + CH3CH3 •
Step 3 CH3CH2 + Br2
2Br• •
CH3CH2 + HBr CH3CH2Br + Br
• (a) (i) Name this type of mechanism. (ii) Suggest an essential condition for this reaction. (iii) Steps 2 and 3 are of the same type. Name this type of step (iv) In this mechanism, another type of step occurs in which free-­‐radicals combine. Name this type of step. Write an equation to illustrate this step(5) (b) Further substitution in the reaction of bromine with ethane produces a mixture of liquid organic compounds. (i) Name a technique which could be used to separate the different compounds in this mixture. (ii) Write an equation for the reaction between bromine and ethane which produces hexabromoethane, C2Br6, by this substitution reaction. (2) (c) The compound 1,2-­‐dibromo-­‐1,1,2,2-­‐tetrafluoroethane is used in some fire extinguishers. Draw the structure of this compound.(1) (d) Halothane is used as an anaesthetic and has the following structure. (i) Give the systematic name of halothane. (ii) Calculate the Mr of halothane. (iii) Calculate the percentage by mass of fluorine in halothane. (3)(Total 11 marks) Q9. (a) Dichloromethane, CH2Cl2, is one of the products formed when chloromethane, CH3Cl, reacts with chlorine. (i) Name the type of mechanism involved in this reaction and write an equation for each of the steps named below. Name of type of mechanism ..............................................................
Initiation step
First propagation step
Second propagation step
(ii) Write an overall equation for the formation of dichloromethane from chloromethane. (5) (b) A compound contains 10.1% carbon and 89.9% chlorine by mass. Calculate the molecular formula of this compound, given that its relative molecular mass (Mr) is 237.0 (3) (c) Suggest the formulae of two bromine-­‐containing organic compounds formed when dibromomethane, CH2Br2, reacts with bromine. (2)(Total 10 marks) Page 5
Q10. 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.(1) (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) Q11. (a) The reaction of bromine with propane is similar to that of chlorine with methane. Three steps in the mechanism for the bromination of propane to form 1-­‐bromopropane are shown below. Step 1 Br2 2Br• Step 2 Br• + CH3CH2CH3 CH3CH2CH2• + HBr Step 3 CH3CH2CH2• + Br2 CH3CH2CH2Br + Br• (i)
Name the type of mechanism in this reaction. (1)
(ii) Give an essential condition for Step 1 to occur. (1)
(iii) Name the type of step illustrated by Steps 2 and 3.(1)
(iv) In this mechanism, a different type of step occurs in which free radicals combine. Name this type of step. Write an equation to show how hexane could be formed from two free radicals in the mechanism of this reaction. (2)
(v) Write an overall equation for the reaction between bromine and propane by the same mechanism to produce octabromopropane (C3Br8). (1) (b) Bromine reacts with alkenes, even though bromine is a non-­‐polar molecule. (i) Explain why bromine molecules react with the double bonds in alkenes. (2) (ii) Name the type of mechanism involved in this reaction. (1) (iii) Draw the structure of the compound with Mr = 387.6 formed when penta-­‐1,4-­‐diene (H2C CHCH2CH CH2) reacts with an excess of bromine (1) (c) Two products are formed when propene reacts with hydrogen bromide. Draw the structure of the intermediate that leads to the formation of the major product in the reaction of propene with hydrogen bromide. Give the name of this type of intermediate. (2)(Total 12 marks) Page 6 Q12. 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. (1) (b) Outline a mechanism for the reaction of potassium cyanide with bromoethane (Reaction 1)(2) (c) Explain why an excess of ammonia is needed in Reaction 2 to produce a high yield of ethylamine. (1)
(d) When potassium hydroxide reacts with bromoethane, ethene can also be formed.
Name and outline a mechanism for this reaction.(4)(Total 8 marks)
Q13. Sulfuric acid is an important chemical in many industrial and laboratory reactions. Consider the following three reactions involving sulfuric acid. Reaction 1 Mg(OH)2 + H2SO4 → MgSO4 + 2H2O Reaction 2 The reaction of solid sodium bromide with concentrated sulfuric acid Reaction 3 H2C=CH2 + H2O (a)
(b)
(i)
(ii)
(iii)
CH3CH2OH Give a use for magnesium hydroxide in medicine. (1)
Sulfuric acid behaves as an oxidising agent in Reaction 2.
In terms of electrons, state the meaning of the term oxidising agent. (1)
Give the formula of the oxidation product that is formed from sodium bromide in Reaction 2. (1)
Deduce the half-equation for the reduction of H2SO4 to SO2 in Reaction 2. (1)
(c) The formation of ethanol in Reaction 3 uses concentrated sulfuric acid and proceeds in two stages according to the following equations. Stage 1 H2C=CH2 + H2SO4 → CH3CH2OSO2OH Stage 2 CH3CH2OSO2OH + H2O → CH3CH2OH + H2SO4 (i)
State the overall role of sulfuric acid in Reaction 3. (1)
(ii) Outline a mechanism for Stage 1 of this reaction (4)
(iii) State the class of alcohols to which ethanol belongs. (1)
(iv) Draw the displayed formula of the carboxylic acid formed when ethanol is oxidised by an excess of acidified potassium
dichromate(VI) solution.(1)(Total 11 marks)
Q14.Three different ways of producing ethanol are shown below. (a) Reaction 1 produces a 15% aqueous solution of ethanol. It is claimed that the ethanol produced in this way is a carbon-­‐
neutral biofuel. Write an equation for Reaction 1 and name the process. Write an equation for the complete combustion of ethanol. Explain why the ethanol produced by this process may not be a carbon-­‐neutral biofuel(5) (b) Give a reagent and conditions for Reaction 2. CH3CH2Br CH3CH2OH Name and outline a mechanism for Reaction 2. Page 7
Suggest one reason, other than safety, why this method is not used in industry to make ethanol. (6) (c) Reaction 3 is used in industry. H2C =CH2 CH3CH2OH Identify a suitable catalyst for Reaction 3.Identify the type of reaction.Give two conditions, in addition to the presence of a catalyst, necessary for Reaction 3 to produce a high yield of ethanol. (4)(Total 15 marks) Q15. Glucose, produced during photosynthesis in green plants, is a renewable source from which ethanol can be made. Ethanol is a liquid fuel used as a substitute for petrol. The processes involved can be summarised as follows. Process 1 Photosynthesis in green plants 6CO2 + 6H2O → C6H12O6 + 6O2 Process 2 Fermentation of glucose to form ethanol Process 3 Complete combustion of ethanol CH3CH2OH + 3O2 → 2CO2 + 3H2O (a) State three essential conditions for the fermentation of aqueous glucose in Process 2. Write an equation for the reaction that takes place during this fermentation. (4) (b) It has been claimed that there is no net carbon (greenhouse gas) emission to the atmosphere when ethanol made by Process 2 is used as a fuel.State the term that is used to describe fuels of this type.Use the equations for Processes 1, 2 and 3 to show why it can be claimed that there is no net emission of carbon-­‐containing greenhouse gases. (3) (c) Use the information from the equation for Process 3 above and the mean bond enthalpies from the table below to calculate a value for the enthalpy change for this process. Mean bond –1 enthalpy / kJ mol
C–H C–C C–O O–H C=O O=O +412 +348 +360 +463 +743 +496 Give one reason why the value calculated from mean bond enthalpies is different from the value given in a data book. (4) (d) A student carried out a simple laboratory experiment to measure the enthalpy change for Process 3. The student showed that the temperature of 200 g of water increased by 8.0 °C when 0.46 g of pure ethanol was burned in air and the heat –1
produced was used to warm the water.Use these results to calculate the value, in kJ mol , obtained by the student for –1 –1
this enthalpy change. (The specific heat capacity of water is 4.18 J K g )Give one reason, other than heat loss, why the value obtained from the student’s results is less exothermic than a data book value. (4)(Total 15 marks) Page 8
Q16.The reaction of butane-­‐1,4-­‐diol with butanedioic acid produces the polymer PBS used in biodegradable packaging and disposable cutlery. Butanedioic acid is produced by two different processes. Process 1 • Aqueous sodium hydroxide reacts with 1,4-­‐dibromobutane to make butane-­‐1,4-­‐diol. • Butane-­‐1,4-­‐diol is oxidised to butanedioic acid. Process 2 • Glucose reacts with carbon dioxide in the presence of microorganisms to produce butanedioic acid directly. • The carbon dioxide used in this process is obtained from a local factory that produces bioethanol. (a) Deduce one safety reason and one environmental reason why Process 2 is preferred to Process 1. (2) (b) (i) Name and outline a mechanism for the following reaction that occurs in Process 1. BrCH2CH2CH2CH2Br + NaOH BrCH2CH2CH2CH2OH + NaBr (ii) The infrared spectra shown are those of three compounds. Compound A 1,4-­‐dibromobutaneCompound B butane-­‐1,4-­‐diol Compound C butanedioic acid Identify the compound responsible for each spectrum by writing the correct letter, A, B or C, in the box next to each spectrum. You may find it helpful to refer to Table 1 on the Data Sheet. (c) In the production of bioethanol, glucose (C6H12O6) is converted into a dilute aqueous solution of ethanol and carbon dioxide.Give the name of this process and state three essential conditions necessary to produce a good yield of ethanol. (4) (d) State the class of alcohols to which the diol butane-­‐1,4-­‐diol belongs.Identify a suitable reagent or combination of reagents for the conversion of butane-­‐1,4-­‐diol into butanedioic acid (HOOCCH2CH2COOH).Write an equation for this oxidation reaction using [O] to represent the oxidising agent. (3) (Total 15 marks) Page 9