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Questions on reaction types, curly arrows and how a mechanism is written
Addition, substitution, elimination, oxidation, reduction, hydrolysis, condensation and polymerisation; homolytic and heterolytic bond fission; radicals, nucleophiles and electrophiles with examples; the curly-arrow and fishhook conventions; and how to draw a mechanism with its dipoles, lone pairs and charges, shown on nucleophilic substitution and electrophilic addition.
21 original questions · 67 marks · the reaction types, curly arrows and how a mechanism is written notes · Introduction to organic chemistry · Flashcards for this unit
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Explain the difference between homolytic and heterolytic fission, giving an example of each and describing how the curly arrows used to show them differ.
Mark scheme
Indicative content
- in homolytic fission each atom takes one electron from the shared pair
- this forms two radicals, for example Cl₂ in ultraviolet light giving two Cl• radicals
- it is shown by two half-headed (fishhook) arrows starting on the bond, one ending on each atom
- in heterolytic fission one atom, the more electronegative, takes both electrons of the shared pair
- this forms a positive and a negative ion, for example (CH₃)₃CBr giving (CH₃)₃C⁺ and Br⁻
- it is shown by one full-headed curly arrow from the bond to the atom that keeps the pair
Levels
- Level 3 (5–6 marks) both kinds of fission are defined in terms of where the electrons go, each has a correct example with the species formed, and the curly arrows are distinguished with the fishhook arrows for homolysis and the full-headed arrow for heterolysis
- Level 2 (3–4 marks) both kinds of fission are defined with an example each; the two kinds of curly arrow are not told apart, or one example gives the wrong species
- Level 1 (1–2 marks) one kind of fission is defined, commonly with homolytic fission said to give ions, and the examples and arrows are largely missing
State what is meant by a nucleophile and by an electrophile, and identify the species acting as each in two reactions: cyanide ions with bromoethane, and hydrogen bromide with ethene.
Mark scheme
- B1 a nucleophile is an electron pair donor; reject 'attracted to a positive charge' as the definition
- B1 an electrophile is an electron pair acceptor
- B1 the cyanide ion is the nucleophile, donating the lone pair on its carbon to the δ+ carbon of bromoethane
- B1 hydrogen bromide is the electrophile, its δ+ hydrogen accepting a pair of electrons from the C=C; accept H⁺ or Hδ+
Draw the mechanism of the reaction between hydroxide ions and bromoethane, showing the dipole, the lone pair and both curly arrows.
Mark scheme
- B1 the C–Br bond is shown polar, with δ+ on the carbon and δ− on the bromine
- B1 the first arrow starts on a lone pair on the oxygen of OH⁻ and ends on the δ+ carbon; allow the arrow from the minus sign on O, reject an arrow from the H
- B1 the second arrow starts on the C–Br bond and ends on the bromine atom
- B1 the products are ethanol, CH₃CH₂OH, and a bromide ion, Br⁻
Outline the mechanism for the reaction of cyanide ions with 1-bromopropane, CH₃CH₂CH₂Br, and name the organic product.
Mark scheme
- B1 the C–Br bond is shown with δ+ on the carbon and δ− on the bromine
- B1 an arrow starts on the lone pair on the carbon of CN⁻ and ends on the δ+ carbon
- B1 an arrow starts on the C–Br bond and ends on the bromine, which leaves as Br⁻
- B1 the organic product is CH₃CH₂CH₂CN, butanenitrile
Evaluate a student's mechanism for hydroxide ions reacting with bromoethane, in which the first arrow starts on the δ+ carbon and ends on the oxygen of OH⁻, and the second starts on the bromine atom and ends on the carbon.
Mark scheme
- B1 a curly arrow shows the movement of an electron pair, so it must start where electrons are, on a lone pair or a bond
- B1 the first arrow is reversed: it should start on a lone pair on the oxygen of OH⁻ and end on the δ+ carbon
- B1 the second arrow is also reversed: it should start on the C–Br bond and end on the bromine, which leaves as Br⁻
- B1 the student has identified the correct atoms involved, but both arrows show electrons moving the wrong way, so neither is correct
Show that the percentage atom economy for making 2-bromopropane by adding hydrogen bromide to propene is 100%, whereas for the reaction CH₃CH(OH)CH₃ + HBr → CH₃CHBrCH₃ + H₂O it is about 87%. The table gives the relative molecular masses.
Substance Mr propene 42.0 hydrogen bromide 80.9 propan-2-ol 60.0 2-bromopropane 122.9 Mark scheme
- M1 atom economy = Mr of the desired product ÷ total Mr of the reactants × 100, with the reactants taken in the molar ratio of the equation
- A1 the addition has one product, so the reactant total 42.0 + 80.9 = 122.9 equals the Mr of 2-bromopropane and the atom economy is 122.9 ÷ 122.9 × 100 = 100%
- M1 for the substitution the reactant total is 60.0 + 80.9 = 140.9, and water, Mr 18.0, is the by-product
- A1 atom economy = 122.9 ÷ 140.9 × 100 = 87.2%, which is about 87%, to three significant figures
Identify each of the species Cl•, NH₃ and NO₂⁺ as a radical, a nucleophile or an electrophile.
Mark scheme
- B1 Cl• is a radical, since it has an unpaired electron
- B1 NH₃ is a nucleophile, since it has a lone pair on nitrogen to donate
- B1 NO₂⁺ is an electrophile, since it is positive and accepts an electron pair
Explain why ammonia can act as a nucleophile although it has no charge.
Mark scheme
- B1 the nitrogen atom in ammonia has a lone pair of electrons
- B1 it can donate that lone pair to an electron-deficient δ+ carbon, forming a new covalent bond
- B1 a nucleophile is defined as an electron pair donor, so a lone pair is required, not a negative charge
Explain why the carbon atom of the C–Br bond in bromoethane is attacked by nucleophiles.
Mark scheme
- B1 bromine is more electronegative than carbon
- B1 the bonding pair is drawn towards the bromine, so the carbon is δ+ and the bromine δ−
- B1 the lone pair of a nucleophile is attracted to the δ+ carbon and can form a bond to it
Explain why bromine, Br₂, which is not polar, can act as an electrophile towards ethene, and name the product.
Mark scheme
- B1 the high electron density of the C=C repels the electrons in the Br–Br bond as the molecule approaches, inducing a dipole
- B1 the bromine atom nearer the C=C becomes δ+ and can accept a pair of electrons from the double bond
- B1 the product with ethene is 1,2-dibromoethane, CH₂BrCH₂Br, one bromine atom added to each carbon of the C=C
Calculate the percentage atom economy for making chloromethane by the reaction CH₄ + Cl₂ → CH₃Cl + HCl. Use Ar values H 1.0, C 12.0, Cl 35.5.
Mark scheme
- M1 Mr of CH₃Cl = 50.5 and Mr of HCl = 36.5, so the total mass of products is 87.0
- M1 atom economy = 50.5 ÷ 87.0 × 100
- A1 atom economy = 58.0%, to three significant figures; allow 58%
Calculate the percentage atom economy for making ethanol by the reaction CH₃CH₂Br + NaOH → CH₃CH₂OH + NaBr. Use Ar values H 1.0, C 12.0, O 16.0, Na 23.0, Br 79.9.
Mark scheme
- M1 Mr of CH₃CH₂Br = 108.9 and of NaOH = 40.0, so the total mass of reactants is 148.9
- M1 Mr of CH₃CH₂OH = 46.0, and atom economy = 46.0 ÷ 148.9 × 100
- A1 atom economy = 30.9%, to three significant figures
Suggest the mechanism for the reaction of ethene with iodine monochloride, ICl, given that chlorine is more electronegative than iodine.
Mark scheme
- B1 the I–Cl bond is polar, Iδ+ and Clδ−, so ICl acts as an electrophile and the reaction is electrophilic addition
- B1 an arrow from the C=C to the δ+ iodine, and an arrow from the I–Cl bond to the chlorine, give a carbocation and Cl⁻
- B1 an arrow from a lone pair on Cl⁻ to the positive carbon gives the product CH₂ICH₂Cl, 1-chloro-2-iodoethane
Compare the action of the hydroxide ion as a nucleophile with its action as a base.
Mark scheme
- B1 in both, the hydroxide ion donates a lone pair of electrons from its oxygen
- B1 as a base it donates the pair to a hydrogen ion, whereas as a nucleophile it donates it to an electron-deficient carbon
- B1 it acts mainly as a nucleophile in warm aqueous solution, giving substitution, and mainly as a base in hot ethanol, giving elimination
Explain why the addition of bromine to ethene has a percentage atom economy of 100%, whereas a substitution reaction can never reach 100%.
Mark scheme
- B1 an addition reaction has a single product, 1,2-dibromoethane, and no by-product
- B1 so all the atoms, and the whole mass, of the reactants end up in the desired product
- B1 a substitution always forms a second product, such as HCl or a halide salt, so part of the mass of the reactants ends up outside the desired product
Calculate the percentage atom economy for making propene by dehydrating propan-1-ol: CH₃CH₂CH₂OH → CH₃CH=CH₂ + H₂O. Use Ar values H 1.0, C 12.0, O 16.0.
Mark scheme
- M1 Mr of CH₃CH₂CH₂OH = 3 × 12.0 + 8 × 1.0 + 16.0 = 60.0
- M1 Mr of CH₃CH=CH₂ = 42.0, and atom economy = 42.0 ÷ 60.0 × 100
- A1 atom economy = 70.0%, to three significant figures
Suggest, in terms of atom economy, why ethanol is made industrially by adding steam to ethene, CH₂=CH₂ + H₂O → CH₃CH₂OH, rather than by reacting bromoethane with sodium hydroxide.
Mark scheme
- B1 the reaction of ethene with steam is an addition with a single product, so its atom economy is 100%
- B1 the reaction of bromoethane with sodium hydroxide is a substitution that also forms sodium bromide, so its atom economy is only about 31%
- B1 the addition route turns all of the reactants into ethanol, wasting less material and producing no by-product to dispose of
State what happens to the electrons in homolytic fission, and name the type of species formed.
Mark scheme
- B1 the covalent bond breaks with one electron of the shared pair going to each atom
- B1 two radicals form, each a species with an unpaired electron
Describe the heterolytic fission of the C–Br bond in 2-bromo-2-methylpropane, (CH₃)₃CBr.
Mark scheme
- B1 both electrons of the C–Br bond go to the bromine, the more electronegative atom
- B1 forming a bromide ion, Br⁻, and a carbocation, (CH₃)₃C⁺
Name the type of reaction in each change: propene and hydrogen bromide forming 2-bromopropane, and ethanol forming ethene and water.
Mark scheme
- B1 propene and hydrogen bromide react by (electrophilic) addition
- B1 ethanol forming ethene and water is an elimination, also called dehydration
Suggest why bromoethane is hydrolysed much more slowly by water than by aqueous hydroxide ions, although both are nucleophiles.
Mark scheme
- B1 the hydroxide ion carries a full negative charge, whereas a water molecule is neutral
- B1 so the hydroxide ion is more strongly attracted to the δ+ carbon and donates its lone pair more readily
Practise reaction types, curly arrows and how a mechanism is written one question at a time