14.2. Alkenes
A subsection of Chemistry, 9701, through 14. Hydrocarbons
Listing 10 of 191 questions
In industry, ethanol is made by reacting ethene with steam in the presence of H3PO4. reaction 1 C2H4+ H2OC2H5OHUse the bond energy values in Table 4.1 to calculate the enthalpy change, ΔHr , for reaction 1. Table 4.1 bond bond energy / kJ mol–1 C–C C=C C≡C C–H C–O C=O O–H ΔHr = kJ mol–1 Reaction 1 reaches equilibrium at constant temperature and pressure. Deduce what effect increasing the pressure will have on the amount of ethanol in the new equilibrium mixture. Use Le Chatelier’s principle to explain your answer. effect of increasing pressure explanation The mechanism for reaction 1 can be described in three steps. Steps 1 and 2 for reaction 1 are shown in . step 1 step 2 H2C H HO OH O CH2 H H2C H P O HO OH O– P O C+ H H2C H H H H C+ H H3C CH2 O+ H H O Describe the behaviour of H3PO4 in step 1 in . Explain your answer. Identify the species that behaves as an electrophile in step 2 in . Explain your answer. Complete to show the mechanism for step 3 of reaction 1. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate. H HO OH O P O HO OH O– P O H3C CH2 O+ H H3C CH2 O H H Describe how a catalyst affects a reaction. Explain your answer. Use and to justify why H3PO4 is described as a catalyst in reaction 1. Propene also reacts with steam. A mixture of organic products is produced. Explain why propan-2-ol is produced in the higher yield. Describe the covalent bonds present between the carbon atoms in an ethene molecule by completing Table 4.2. Table 4.2 sigma (σ ) pi (π) type of orbitals involved in bond how the orbitals overlap
9701_s23_qp_22
THEORY
2023
Paper 2, Variant 2
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Y is formed from X in a single-step reaction, as shown in . X Y O O HO OH OH Deduce the empirical formula of Y. The formation of Y from X requires the addition of a suitable reducing agent. Construct an equation using molecular formulae and for the reaction in . Use to represent one atom of hydrogen from the reducing agent. Identify a suitable non-gaseous reducing agent for the formation of Y from X. Complete Table 5.1 to show the number of sp2 and sp3 hybridised carbon atoms in a molecule of X. Table 5.1 type of hybridisation sp2 sp3 number of carbon atoms in X Complete Table 5.2 with the expected observations that occur when the reagents shown are added to separate solutions of X and Y. Do not refer to temperature changes in your answer. Table 5.2 reagent observation on addition to X observation on addition to Y aqueous sodium carbonate 2,4-dinitrophenylhydrazine (2,4-DNPH reagent) alkaline aqueous iodine
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THEORY
2023
Paper 2, Variant 3
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Compound W, CH2=CHCN, is used to make an addition polymer which is present in carbon fibres. Draw one repeat unit of the addition polymer of W. CH3CHO is used in a two-step synthetic route to form W, as shown in . In step 1, CH3CHO is heated with HCN in the presence of KCN. CH3CHO CH3CH(OH)CN CH2=CHCN step 1 step 2 W Name the mechanism for the reaction in step 1 in . Complete to show the mechanism for the reaction in step 1. Include all products, charges, dipoles, lone pairs of electrons and curly arrows, as appropriate. O H CH3 C N C Suggest a suitable reagent and conditions for step 2 in . shows the infrared spectrum of W, CH2=CHCN. wavenumber / cm–1 S T transmittance / % 50 Table 6.1 bond functional groups containing the bond characteristic infrared absorption range (in wavenumbers) / cm–1 C–O hydroxy, ester 1040–1300 C=C aromatic compound, alkene 1500–1680 C=O amide carbonyl, carboxyl ester 1640–1690 1670–1740 1710–1750 C≡N nitrile 2200–2250 C–H alkane 2850–2950 N–H amine, amide 3300–3500 O–H carboxyl hydroxy 2500–3000 3200–3600 Use Table 6.1 to identify the bonds responsible for the absorptions marked S and T on . S T Molecules of W, CH2=CHCN, do not show stereoisomerism. Describe stereoisomerism. Describe the two essential features of an alkene molecule that cause it to show geometrical stereoisomerism. Molecules of CH3CH(OH)CN exist as a pair of optical isomers. Draw three-dimensional diagrams in the boxes to show the optical isomers of CH3CH(OH)CN. isomer 1 isomer 2 Propanenitrile is heated with hydrogen gas and a platinum catalyst. The only product is propylamine. Construct an equation for this reaction. Propylamine can also be formed in a two-step synthesis from propan-1-ol, as shown in . CH3CH2CH2OH CH3CH2CH2Cl CH3CH2CH2NH2 step 1 SOCl 2 step 2 Name the type of reaction in step 1 in . Identify the reagent and conditions for step 2 in .
9701_s23_qp_23
THEORY
2023
Paper 2, Variant 3
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Methylbenzene can undergo different reactions, as shown in . reaction 1 H2 / Pt Br2 / UV light Br2 / FeBr3 + reaction 2 reaction 3 Draw structures in for the possible organic products of the three reactions shown. Complete Table 5.1. Table 5.1 type of reaction mechanism reaction 1 reaction 2 When methylbenzene reacts with an electrophile, a substitution reaction occurs. No addition reaction takes place under these conditions. Explain why no addition reaction takes place. The reaction of methylbenzene with thionyl bromide, SOBr2, in the presence of an iron(bromide catalyst, FeBr3, is shown in . FeBr3 + SOBr2 + HBr S O Br The mechanism of this reaction is similar to that of the bromination of benzene. The first step of the mechanism generates the SOBr+ electrophile, as shown. SOBr2 + FeBr3 SOBr+ + FeBr4 – The reaction of methylbenzene with SOBr+ ions is shown in . Complete the mechanism in . Include all relevant curly arrows and charges. Draw the structure of the organic intermediate. S+ O Br intermediate + S O Br The reaction shown in produces a small amount of a by-product, P, with the molecular formula C14H14OS. Suggest a structure for by-product P. Acyl bromides, RCOBr, can be synthesised by the reaction of a carboxylic acid and SOBr2. This is a similar reaction to the synthesis of acyl chlorides using SOCl 2. Give an equation for the reaction between ethanoic acid and SOBr2. Suggest the relative ease of hydrolysis of acyl bromides, RCOBr, acyl chlorides, RCOCl, and alkyl chlorides, RCl. Explain your answer. > > easiest to hydrolyse hardest to hydrolyse
9701_s23_qp_42
THEORY
2023
Paper 4, Variant 2
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Hydrocarbon molecules contain covalent bonds. Define covalent bond. A C=C bond in an alkene is made from a σ bond and a π bond. Identify the hybridisation of the carbon atoms in a C=C bond in an alkene. Draw labelled diagrams to show, in terms of orbital overlap, how the σ and π bonds are made in a C=C bond. σ bond π bond In electrophilic reactions involving alkenes the π bond of C=C is broken. Suggest one difference between σ and π bonds that explains why the π bond is broken in electrophilic addition reactions involving alkenes. Complete to show the mechanism for the electrophilic addition of hydrogen bromide to 2-methylpropene to produce the major organic product. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate. CH3 H CH3 H C C H Br
9701_s24_qp_21
THEORY
2024
Paper 2, Variant 1
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State what is meant by partition coefficient, Kpc. The partition coefficient, Kpc, for a compound, X, between carbon disulfide, CS2, and water is 10.5. 1.85 g of X is dissolved in water and made up to 100.0 cm3 in a volumetric flask. 40.0 cm3 of this aqueous solution is shaken with 25.0 cm3 of CS2. The mixture is left to reach equilibrium. Calculate the mass of X, in g, extracted into the CS2 layer. mass of X = g The compound C6H6 has many structural isomers. Four suggested structures of C6H6 are shown in . delocalised benzene Kekulé benzene Dewar benzene Ladenburg benzene Using , complete Table 6.1 to predict the number of carbon atoms that have sp, sp2 and sp3 hybridisation in Kekulé benzene, Dewar benzene and Ladenburg benzene. Table 6.1 C6H6 structure sp hybridised sp2 hybridised sp3 hybridised Kekulé benzene Dewar benzene Ladenburg benzene Describe the shape of delocalised benzene. Include the geometry of each carbon, the C-C-H bond angle and the type of bondbetween the carbon atoms and between the carbon and hydrogen atoms. Suggest why Dewar benzene and Ladenburg benzene are unstable isomers of C6H6. Complete Table 6.2 to predict the number of peaks in the proton (1H) NMR spectrum for Dewar benzene, Ladenburg benzene and delocalised benzene. Table 6.2 number of peaks Dewar benzene Ladenburg benzene delocalised benzene The reaction of phenylethanone with 1,4-dibromobutane, BrCH2CH2CH2CH2Br, in the presence of FeBr3 is shown in . O O BrCH2CH2CH2CH2Br phenylethanone FeBr3 + Br + HBr The mechanism of this reaction is similar to that of the alkylation of benzene. Construct an equation for the formation of the electrophile, BrCH2CH2CH2CH2 +. Complete the mechanism in for the reaction of phenylethanone with BrCH2CH2CH2CH2 + ions. Include all relevant curly arrows and charges. Draw the structure of the organic intermediate. O O BrCH2CH2CH2CH2 organic intermediate + Br + The reaction shown in forms small amounts of two by-products, Y (C20H22O2) and Z (C12H14O). Suggest structures for Y and Z in the boxes in . Y (C20H22O2) Z (C12H14O)
9701_s24_qp_41
THEORY
2024
Paper 4, Variant 1
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State what is meant by partition coefficient, Kpc. The partition coefficient, Kpc, for a compound, X, between carbon disulfide, CS2, and water is 10.5. 1.85 g of X is dissolved in water and made up to 100.0 cm3 in a volumetric flask. 40.0 cm3 of this aqueous solution is shaken with 25.0 cm3 of CS2. The mixture is left to reach equilibrium. Calculate the mass of X, in g, extracted into the CS2 layer. mass of X = g The compound C6H6 has many structural isomers. Four suggested structures of C6H6 are shown in . delocalised benzene Kekulé benzene Dewar benzene Ladenburg benzene Using , complete Table 6.1 to predict the number of carbon atoms that have sp, sp2 and sp3 hybridisation in Kekulé benzene, Dewar benzene and Ladenburg benzene. Table 6.1 C6H6 structure sp hybridised sp2 hybridised sp3 hybridised Kekulé benzene Dewar benzene Ladenburg benzene Describe the shape of delocalised benzene. Include the geometry of each carbon, the C-C-H bond angle and the type of bondbetween the carbon atoms and between the carbon and hydrogen atoms. Suggest why Dewar benzene and Ladenburg benzene are unstable isomers of C6H6. Complete Table 6.2 to predict the number of peaks in the proton (1H) NMR spectrum for Dewar benzene, Ladenburg benzene and delocalised benzene. Table 6.2 number of peaks Dewar benzene Ladenburg benzene delocalised benzene The reaction of phenylethanone with 1,4-dibromobutane, BrCH2CH2CH2CH2Br, in the presence of FeBr3 is shown in . O O BrCH2CH2CH2CH2Br phenylethanone FeBr3 + Br + HBr The mechanism of this reaction is similar to that of the alkylation of benzene. Construct an equation for the formation of the electrophile, BrCH2CH2CH2CH2 +. Complete the mechanism in for the reaction of phenylethanone with BrCH2CH2CH2CH2 + ions. Include all relevant curly arrows and charges. Draw the structure of the organic intermediate. O O BrCH2CH2CH2CH2 organic intermediate + Br + The reaction shown in forms small amounts of two by-products, Y (C20H22O2) and Z (C12H14O). Suggest structures for Y and Z in the boxes in . Y (C20H22O2) Z (C12H14O)
9701_s24_qp_43
THEORY
2024
Paper 4, Variant 3
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Ethylbenzene is an important starting material for making polystyrene (poly). State the conditions needed to carry out reaction I in the laboratory. State the reagent and conditions needed for reaction II. Draw the structure of the repeat unit of polystyrene. There are several polymers that consist of phenylethene co-polymerised with other monomers. The following formula shows part of the chain of one such co-polymer. Deduce the structural formula of the other monomer. CH CH2 CH2 CH CN CH2 CH CN CH2 Use CH2CH3 + Cl 2 reaction I CHCl CH3 reaction II CH CH2 polystyrene ethylbenzene B phenylethene [T Compound B undergoes the following series of reactions. Suggest reagents and conditions for reaction III. What would you see when reaction IV was carried out? Draw structures for C and D in the boxes above. Ethylbenzene can react with chlorine under a different set of conditions to give compound E, an isomer of compound B. Compound E undergoes the following reaction. Draw a structure for E in the box above. Describe the conditions used for reaction V. State the reagents used for reaction VI. + CHCl CH3 reaction III B C D CH(OH)CH3 reaction IV I2 + NaOH CH2CH3 + Cl2 Cl reaction V CO2H reaction VI E
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THEORY
2006
Paper 4, Variant 0
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Questions Discovered
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