15.1. Halogenoalkanes
A subsection of Chemistry, 9701, through 15. Halogen compounds
Listing 10 of 284 questions
Explain what is meant by the term bond energy. Describe and explain the trend in bond energies of the C–X bond in halogenoalkanes, where X = F, Cl, Br or I. Describe the relationship between the reactivity of halogenoalkanes, RX, and the bond energies of the C–X bond. Use the Data Booklet to suggest an explanation as to why CFCs such as CF2Cl 2 are much more harmful to the ozone layer than fl uorocarbons such as CF4 or hydrocarbons such as butane, C4H10. Predict the products of the following reactions and draw their structures in the boxes below. The molecular formula of each product is given, where X = Cl , Br or I. H2O C3H5O2X + Cl Cl O H2O C3H7OX + I Cl H2O C7H7OX + Br Br Ethane reacts with chlorine according to the following equation. C2H6 + Cl 2 → C2H5Cl + HCl State the conditions needed for this reaction. State the type of reaction occurring here. One of the steps during this reaction is the following process. Cl ● + CH3CH3 → HCl + CH3CH2 ● Use the Data Booklet to calculate the enthalpy change, ∆H, of this step. ∆H = kJ mol–1 Use the Data Booklet to calculate the enthalpy change, ∆H, of the similar reaction: I● + CH3CH3 → HI + CH3CH2 ● ∆H = kJ mol–1 Hence suggest why it is not possible to make iodoethane by reacting together iodine and ethane. Complete the following equations of some possible steps in the formation of chloroethane. Cl 2 → Cl ● + CH3CH3 → HCl + CH3CH2 ● CH3CH2 ● + → + + → CH3CH2Cl
9701_w13_qp_41
THEORY
2013
Paper 4, Variant 1
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Explain what is meant by the term bond energy. Describe and explain the trend in bond energies of the C–X bond in halogenoalkanes, where X = F, Cl, Br or I. Describe the relationship between the reactivity of halogenoalkanes, RX, and the bond energies of the C–X bond. Use the Data Booklet to suggest an explanation as to why CFCs such as CF2Cl 2 are much more harmful to the ozone layer than fl uorocarbons such as CF4 or hydrocarbons such as butane, C4H10. Predict the products of the following reactions and draw their structures in the boxes below. The molecular formula of each product is given, where X = Cl , Br or I. H2O C3H5O2X + Cl Cl O H2O C3H7OX + I Cl H2O C7H7OX + Br Br Ethane reacts with chlorine according to the following equation. C2H6 + Cl 2 → C2H5Cl + HCl State the conditions needed for this reaction. State the type of reaction occurring here. One of the steps during this reaction is the following process. Cl ● + CH3CH3 → HCl + CH3CH2 ● Use the Data Booklet to calculate the enthalpy change, ∆H, of this step. ∆H = kJ mol–1 Use the Data Booklet to calculate the enthalpy change, ∆H, of the similar reaction: I● + CH3CH3 → HI + CH3CH2 ● ∆H = kJ mol–1 Hence suggest why it is not possible to make iodoethane by reacting together iodine and ethane. Complete the following equations of some possible steps in the formation of chloroethane. Cl 2 → Cl ● + CH3CH3 → HCl + CH3CH2 ● CH3CH2 ● + → + + → CH3CH2Cl
9701_w13_qp_42
THEORY
2013
Paper 4, Variant 2
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Halogenoalkanes are useful intermediates in the synthesis of a wide variety of compounds. 2-bromobutane reacts in two different ways with sodium hydroxide depending on the conditions. When warmed with aqueous sodium hydroxide, 2-bromobutane produces an alcohol that exists as a pair of optical isomers. Give the name of the mechanism of the reaction between 2-bromobutane and aqueous sodium hydroxide. Explain why the alcohol produced exists as a pair of optical isomers. Draw the three-dimensional structure of the two optical isomers of the alcohol produced in . Heating 2-bromobutane with ethanolic sodium hydroxide produces a mixture of three alkenes, two of which are a pair of geometrical isomers. Give the name of the mechanism of the reaction between 2-bromobutane and ethanolic sodium hydroxide. Draw and name the structures of the pair of geometrical isomers formed by reaction of 2-bromobutane with ethanolic sodium hydroxide. name name Name the third alkene produced by reaction of 2-bromobutane with ethanolic sodium hydroxide and explain why it does not show geometrical isomerism. Some reactions involving 2-bromopropane are shown. CH3CHBrCH3 2-bromopropane CH3CH2CO2H (and CH3CH(NH2)CH3 which is removed) CH3CH2CH2OH CH3CH(NH2)CH3 X Y reaction 1 reaction 2 reaction 5 reaction 4 reaction 3 hydrolysis reaction 6 HBr State the reagent needed for reaction 1. State the reagent needed for reaction 2. Give the structural formula of X. Name the type of reaction involved in reaction 4 and suggest a suitable reagent. State the name of a solid catalyst for reaction 5. Complete the mechanism for the production of 2-bromopropane from Y in reaction 6 shown below. Include the structure of Y and any necessary lone pairs, curly arrows, charges and partial charges. Y H Br intermediate H3C CH3 C H Br Give the name of the mechanism in . 1-bromopropane is a minor product of reaction 6. Explain why 2-bromopropane is the major product of reaction 6.
9701_w15_qp_22
THEORY
2015
Paper 2, Variant 2
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Some reactions based on 1-bromobutane, CH3(CH2)3Br, are shown. CH3(CH2)3Br CH3CH2CH=CH2 CH3(CH2)3OH reaction 1 reaction 2 reaction 3 reaction 5 CH3(CH2)3C≡N CH3(CH2)2CHO reaction 4 reaction 6 CH3(CH2)3COOH CH3(CH2)2COOH For each of the reactions state the reagent, the particular conditions required, if any, and the type of reaction. For the type of reaction choose from the list. Each type may be used once, more than once or not at all. Each reaction may be described by more than one type. elimination hydrolysis substitution oxidation addition condensation reaction reagentand conditions typeof reaction Complete the diagram to show the SN2 mechanism of reaction 1. R represents the CH3(CH2)2 group. Include all necessary charges, dipoles, lone pairs and curly arrows. R C Br H H R C O H H H 2-bromo-2-methylpropane is a tertiary halogenoalkane that is a structural isomer of 1-bromobutane. Define the term structural isomer and name the three different types of structural isomerism. definition types of structural isomerism 2-bromo-2-methylpropane is treated with the same reagents as in reaction 1. Methylpropan-2-ol is formed. Identify the mechanism for this reaction. Explain why this reaction proceeds via a different mechanism from that of reaction1. mechanism explanation  The product of reaction 2, but-1-ene, does not show stereoisomerism. However, but-1-ene reacts with HCl to form a mixture of structural isomers X and Y. but-1-ene + HCl X (exists as a pair of stereoisomers and is produced in higher yield than Y) Y (does not show stereoisomerism) Explain the meaning of the term stereoisomers. Give two reasons why but-1-ene does not show stereoisomerism. Name X and Y. X Y Name the type of stereoisomerism shown by X. Use the conventional representation to draw the two stereoisomers of X.
9701_w17_qp_21
THEORY
2017
Paper 2, Variant 1
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Some reactions based on 1-bromobutane, CH3(CH2)3Br, are shown. CH3(CH2)3Br CH3CH2CH=CH2 CH3(CH2)3OH reaction 1 reaction 2 reaction 3 reaction 5 CH3(CH2)3C≡N CH3(CH2)2CHO reaction 4 reaction 6 CH3(CH2)3COOH CH3(CH2)2COOH For each of the reactions state the reagent, the particular conditions required, if any, and the type of reaction. For the type of reaction choose from the list. Each type may be used once, more than once or not at all. Each reaction may be described by more than one type. elimination hydrolysis substitution oxidation addition condensation reaction reagentand conditions typeof reaction Complete the diagram to show the SN2 mechanism of reaction 1. R represents the CH3(CH2)2 group. Include all necessary charges, dipoles, lone pairs and curly arrows. R C Br H H R C O H H H 2-bromo-2-methylpropane is a tertiary halogenoalkane that is a structural isomer of 1-bromobutane. Define the term structural isomer and name the three different types of structural isomerism. definition types of structural isomerism 2-bromo-2-methylpropane is treated with the same reagents as in reaction 1. Methylpropan-2-ol is formed. Identify the mechanism for this reaction. Explain why this reaction proceeds via a different mechanism from that of reaction1. mechanism explanation  The product of reaction 2, but-1-ene, does not show stereoisomerism. However, but-1-ene reacts with HCl to form a mixture of structural isomers X and Y. but-1-ene + HCl X (exists as a pair of stereoisomers and is produced in higher yield than Y) Y (does not show stereoisomerism) Explain the meaning of the term stereoisomers. Give two reasons why but-1-ene does not show stereoisomerism. Name X and Y. X Y Name the type of stereoisomerism shown by X. Use the conventional representation to draw the two stereoisomers of X.
9701_w17_qp_23
THEORY
2017
Paper 2, Variant 3
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