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Questions on oxidation and elimination of alcohols
Oxidation of primary, secondary and tertiary alcohols with acidified potassium dichromate(VI), the orange to green colour change, equations using [O], aldehydes by distillation and carboxylic acids by heating under reflux and the apparatus for each, required practical 5 in full, tests for aldehydes, ketones and carboxylic acids, the acid-catalysed elimination of water to give alkenes and its mechanism, the three alkenes from butan-2-ol, boiling temperature as a check on purity, Edexcel core practical 5 oxidising ethanol, and for other boards the conversion of alcohols into halogenoalkanes with hydrogen halides and phosphorus halides, including Edexcel core practical 6, 2-chloro-2-methylpropane from 2-methylpropan-2-ol and hydrochloric acid.
22 original questions · 74 marks · the oxidation and elimination of alcohols notes · Alkenes and alcohols · Flashcards for this unit
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Describe how a sample of propanal can be prepared from propan-1-ol and collected, including the apparatus and the conditions that keep the yield of propanal high.
Mark scheme
Indicative content
- acidified potassium dichromate(VI) is the oxidising agent, with the propan-1-ol in excess
- the mixture is heated in a flask with anti-bumping granules, using a heating mantle or water bath rather than a flame
- the flask is fitted with a still head, a thermometer with its bulb level with the side arm, and a sloping condenser with water in at the bottom and out at the top
- the alcohol is added slowly so that propanal distils off as soon as it forms, before it can be oxidised further
- the distillate is collected while the thermometer reads below about 60 °C, near propanal's boiling point of 49 °C
- the receiver is cooled in ice and water to reduce evaporation, and the apparatus is left open to the air, not sealed
Levels
- Level 3 (5–6 marks) the oxidising agent, the apparatus for distillation and all the conditions that keep the yield high are given: excess alcohol added slowly, propanal distilled off as it forms, collection below about 60 °C and a cooled receiver
- Level 2 (3–4 marks) acidified dichromate(VI) and distillation are described with the thermometer or condenser placed correctly; one condition for a high yield, such as adding the alcohol slowly or collecting at the right temperature, is not given
- Level 1 (1–2 marks) acidified dichromate(VI) is named, usually with heating under reflux, which turns the propanal into propanoic acid; no condition for keeping the aldehyde is given
Describe how a pure, dry sample of 2-chloro-2-methylpropane is prepared from 2-methylpropan-2-ol and concentrated hydrochloric acid and separated from the reaction mixture.
Mark scheme
Indicative content
- the alcohol is shaken with excess concentrated hydrochloric acid in a stoppered separating funnel, in a fume cupboard, at room temperature
- the funnel is inverted and the tap opened often to release the pressure of hydrogen chloride gas as the mixture is shaken
- the layers are allowed to separate and the lower aqueous layer is run off and discarded, the product being the upper layer
- the product is shaken with sodium hydrogencarbonate solution to neutralise the remaining acid, releasing the pressure of carbon dioxide, then washed with water
- the organic layer is run into a dry flask and swirled with anhydrous sodium sulfate until the cloudy liquid turns clear
- the dried liquid is decanted and distilled, and the fraction boiling between about 49 and 53 °C is collected, near the boiling point of 51 °C
Levels
- Level 3 (5–6 marks) every stage is in order and justified: shaking with the acid while releasing pressure, discarding the aqueous layer, neutralising with hydrogencarbonate and releasing carbon dioxide, drying with an anhydrous salt until clear, and distilling to collect the fraction near 51 °C; the product layer is identified correctly throughout
- Level 2 (3–4 marks) the separating funnel, removing the aqueous layer and drying are described; the hydrogencarbonate wash, venting the pressure, or the final distillation with its collection range does not appear
- Level 1 (1–2 marks) the alcohol and acid are shaken together or a separating funnel is named; the product is often placed in the lower layer, and the purification is largely missing
Deduce the class of each alcohol A, B and C, all C₅H₁₂O, from the results in the table, and give the structure of C.
Alcohol Warmed with acidified K₂Cr₂O₇ Distillate warmed with Tollens' reagent A Orange to green Silver mirror B Orange to green No change C Stays orange No change Mark scheme
- B1 A is a primary alcohol: it is oxidised, and its distillate is an aldehyde, which reduces Tollens' reagent
- B1 B is a secondary alcohol: it is oxidised, but to a ketone, which gives no silver mirror
- B1 C is a tertiary alcohol, with no hydrogen on the carbon bonded to the OH, so it is not oxidised
- B1 the only tertiary alcohol C₅H₁₂O is 2-methylbutan-2-ol, (CH₃)₂C(OH)CH₂CH₃
Outline the mechanism for the dehydration of propan-2-ol to propene in the presence of concentrated sulfuric acid.
Mark scheme
- B1 a curly arrow from a lone pair on the oxygen of the OH group to an H⁺ ion
- B1 the protonated alcohol, with the positive charge on the oxygen, and a curly arrow from the C–O bond to the oxygen as water leaves
- B1 the carbocation CH₃C⁺HCH₃, with the positive charge on the middle carbon
- B1 a curly arrow from a C–H bond on an adjacent carbon to the C–C bond, forming C=C and releasing H⁺
Explain why pentan-2-ol gives three alkenes when it is dehydrated, but pentan-3-ol gives only two.
Mark scheme
- B1 pentan-2-ol gives a carbocation on carbon 2, which can lose H⁺ from carbon 1 or from carbon 3
- B1 loss from carbon 1 gives pent-1-ene, and loss from carbon 3 gives pent-2-ene, which forms as E and Z isomers: three alkenes
- B1 pentan-3-ol gives a carbocation on carbon 3, whose two neighbours, carbons 2 and 4, are equivalent, so only pent-2-ene forms
- B1 pent-2-ene has E and Z isomers, so pentan-3-ol gives two alkenes; ignore mechanisms if not asked for
Give the reagents and conditions used to convert butan-1-ol into 1-bromobutane, an equation for the reaction of the alcohol, and the reason 50% concentrated sulfuric acid is used rather than fully concentrated acid.
Mark scheme
- B1 potassium bromide, or sodium bromide, with 50% concentrated sulfuric acid
- B1 the mixture is heated under reflux
- B1 CH₃CH₂CH₂CH₂OH(l) + HBr(aq) → CH₃CH₂CH₂CH₂Br(l) + H₂O(l); ignore state symbols
- B1 fully concentrated acid oxidises some of the bromide ions to bromine, wasting bromide and colouring the product orange
Calculate the percentage yield of 2-chloro-2-methylpropane from the results in the table.
Quantity Value Volume of 2-methylpropan-2-ol used 10.0 cm³ Density of 2-methylpropan-2-ol 0.775 g cm⁻³ Mr of 2-methylpropan-2-ol 74.0 Mr of 2-chloro-2-methylpropane 92.5 Mass of product after distillation 7.40 g Mark scheme
- M1 mass of alcohol = 10.0 × 0.775 = 7.75 g, so amount = 7.75 ÷ 74.0 = 0.1047 mol
- M1 the reaction is 1 : 1, so the maximum amount of product is 0.1047 mol
- M1 maximum mass = 0.1047 × 92.5 = 9.69 g; error carried forward from the amount of alcohol
- A1 percentage yield = 7.40 ÷ 9.69 × 100 = 76.4%, to three significant figures
Deduce the identity of alcohol X, C₅H₁₂O, from the results in the table, giving the reasoning at each stage.
Test Result Warmed with acidified potassium dichromate(VI) Orange to green Distillate warmed with Tollens' reagent No change Heated with concentrated phosphoric acid Two alkenes, neither of which has E/Z isomers Mark scheme
- B1 X is oxidised, so it is not tertiary, but its product gives no silver mirror, so the product is a ketone and X is a secondary alcohol
- B1 the secondary alcohols C₅H₁₂O are pentan-2-ol, pentan-3-ol and 3-methylbutan-2-ol
- B1 pentan-2-ol would give pent-1-ene and pent-2-ene, and pentan-3-ol pent-2-ene alone, and pent-2-ene exists as E and Z isomers, so both are ruled out
- B1 3-methylbutan-2-ol, (CH₃)₂CHCH(OH)CH₃, loses H⁺ from carbon 1 to give 3-methylbut-1-ene or from carbon 3 to give 2-methylbut-2-ene, and in each one carbon of the C=C carries two identical groups, so X is 3-methylbutan-2-ol
Give the reagents used to oxidise propan-2-ol in the laboratory, the colour change seen, and the name of the organic product.
Mark scheme
- B1 potassium dichromate(VI) acidified with dilute sulfuric acid; accept sodium dichromate(VI)
- B1 the solution changes from orange to green; both colours are needed for the mark
- B1 the product is propanone, CH₃COCH₃, a ketone
Explain why propanal is distilled out of the reaction mixture as soon as it forms when it is made by oxidising propan-1-ol.
Mark scheme
- B1 propanal can be oxidised further, to propanoic acid, by the acidified dichromate
- B1 propanal has a lower boiling point, 49 °C, than propan-1-ol or propanoic acid, because its molecules do not hydrogen bond to each other
- B1 so it vaporises and leaves the flask before it can be oxidised further
Explain why 2-methylpropan-2-ol is not oxidised by acidified potassium dichromate(VI).
Mark scheme
- B1 it is a tertiary alcohol: the carbon bonded to the OH is bonded to three other carbons
- B1 oxidation removes a hydrogen from the carbon bonded to the OH as well as the H of the OH, and this carbon has no hydrogen
- B1 oxidation would need a C–C bond to break, which acidified dichromate cannot do, so the solution stays orange
Describe how apparatus for heating under reflux is set up, and why it is used to make a carboxylic acid from a primary alcohol.
Mark scheme
- B1 a flask holding the mixture and anti-bumping granules, with a condenser fitted vertically above it, open at the top, water in at the bottom and out at the top
- B1 vapour from the boiling mixture condenses and runs back into the flask, so no volatile substance escapes
- B1 the aldehyde formed first stays in contact with the excess oxidising agent for as long as the mixture is heated, so it is oxidised to the carboxylic acid
Identify the three alkenes formed when butan-2-ol is heated with concentrated phosphoric acid.
Mark scheme
- B1 but-1-ene, CH₂=CHCH₂CH₃, from the loss of H⁺ from carbon 1
- B1 E-but-2-ene, from the loss of H⁺ from carbon 3
- B1 Z-but-2-ene, the other stereoisomer, because each carbon of the double bond in but-2-ene carries H and CH₃
Calculate the percentage yield when 6.00 g of propan-1-ol gives 2.90 g of propanal. Mr: propan-1-ol 60.0, propanal 58.0.
Mark scheme
- M1 amount of propan-1-ol = 6.00 ÷ 60.0 = 0.100 mol, so the maximum amount of propanal is 0.100 mol
- M1 maximum mass of propanal = 0.100 × 58.0 = 5.80 g
- A1 percentage yield = 2.90 ÷ 5.80 × 100 = 50.0%, to three significant figures; allow 50%
Calculate the minimum mass of potassium dichromate(VI), K₂Cr₂O₇ (Mr 294.2), needed to oxidise 1.38 g of ethanol (Mr 46.0) to ethanal. The half-equations are Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6e⁻ → 2Cr³⁺(aq) + 7H₂O(l) and CH₃CH₂OH(l) → CH₃CHO(l) + 2H⁺(aq) + 2e⁻.
Mark scheme
- M1 amount of ethanol = 1.38 ÷ 46.0 = 0.0300 mol
- M1 one Cr₂O₇²⁻ accepts six electrons and each ethanol gives two, so one dichromate oxidises three ethanol: 0.0300 ÷ 3 = 0.0100 mol
- A1 mass of K₂Cr₂O₇ = 0.0100 × 294.2 = 2.94 g, to three significant figures
Suggest why propanal collected in a school preparation is often contaminated with propan-1-ol and water, and suggest how a purer sample could be obtained.
Mark scheme
- B1 propan-1-ol is in excess and volatile, so some vaporises and distils over with the propanal, especially if the flask is heated too strongly
- B1 water from the reaction mixture also distils over
- B1 redistil the product and collect only the fraction boiling close to 49 °C, between about 46 and 50 °C
Compare the apparatus and the purpose of distillation and of heating under reflux in the oxidation of a primary alcohol.
Mark scheme
- B1 in distillation the condenser slopes down away from the flask and the condensed liquid is collected separately; in reflux the condenser is vertical and the liquid returns to the flask
- B1 distillation removes the aldehyde as it forms, so the product is the aldehyde; reflux keeps it in the flask with the oxidising agent, so the product is the carboxylic acid
- B1 distillation uses the alcohol in excess and reflux uses the oxidising agent in excess, and neither apparatus is sealed
Explain why the receiver is cooled in ice and water, and why the thermometer bulb is placed level with the side arm, when propanal is distilled.
Mark scheme
- B1 propanal boils at 49 °C and evaporates readily, so cooling the receiver reduces the loss of product
- B1 the thermometer bulb at the side arm measures the temperature of the vapour that is entering the condenser
- B1 so the reading shows which substance is distilling, and only the right fraction is collected
Give an equation, using [O] to represent the oxidising agent, for the oxidation of propan-1-ol to propanoic acid.
Mark scheme
- B1 correct formulae for propan-1-ol, CH₃CH₂CH₂OH, and propanoic acid, CH₃CH₂COOH
- B1 balanced with 2[O] on the left and H₂O on the right: CH₃CH₂CH₂OH(l) + 2[O] → CH₃CH₂COOH(l) + H₂O(l)
Suggest a test to show that the distillate from the oxidation of propan-1-ol contains an aldehyde and not a ketone, giving the result.
Mark scheme
- B1 warm the distillate with Tollens' reagent, or with Fehling's solution
- B1 an aldehyde gives a silver mirror, or a brick-red precipitate from the blue Fehling's solution, while a ketone gives no change
Suggest the organic product of heating cyclohexanol with concentrated phosphoric acid, and a test to identify it.
Mark scheme
- B1 cyclohexene, C₆H₁₀, formed by the elimination of water
- B1 shaking it with bromine water turns the bromine water from orange to colourless
Explain how measuring the boiling temperature of a distillate shows whether it is pure propanal.
Mark scheme
- B1 a pure liquid boils at one fixed temperature, which can be compared with the data-book value for propanal, 49 °C
- B1 a sample that boils over a range of temperatures, or well above 49 °C, contains impurities such as propan-1-ol or water
Practise oxidation and elimination of alcohols one question at a time