Questions on test-tube reactions for functional groups, and high-resolution mass spectrometry

Question 1

Describe how to carry out the Tollens' and the acidified dichromate tests on an unknown organic liquid safely, and how to interpret the results.

Accepted Answer

Indicative content

Tollens' reagent is made fresh: sodium hydroxide added to silver nitrate, then dilute ammonia until the brown precipitate just dissolves
a few drops of the unknown are added and the tube is warmed in a water bath, not over a flame, because the organic liquid is flammable
a silver mirror shows an aldehyde; no change rules an aldehyde out
for the second test the unknown is warmed in a water bath with potassium dichromate(VI) and dilute sulfuric acid
orange to green shows a primary or secondary alcohol only if the Tollens' test was negative, because aldehydes also turn it green
precautions: Tollens' residues are washed away at once because they can form explosive compounds, and gloves are worn because dichromate(VI) is toxic and carcinogenic

Levels

Level 3 (5–6 marks) both tests are described with the preparation of the reagent, water-bath warming and the safety reasons, the observations and what each shows, including the point that a positive dichromate result identifies an alcohol only once an aldehyde has been ruled out; all the precautions named are matched to their reasons
Level 2 (3–4 marks) both tests are described with observations and the water bath; preparing Tollens' reagent, disposing of its residues, or reading the two results together is not covered
Level 1 (1–2 marks) one test is described with its positive result; a positive dichromate test is typically taken to prove an alcohol, and the tubes are heated over a flame

Question 2

Calculate the precise relative molecular mass of a compound that, ionised by electrospray, gives an MH⁺ peak at m/z 89.0597 on a high-resolution mass spectrum, and show whether its formula is C₄H₈O₂ or C₅H₁₂O, using the data in the table and 1.0073 for the mass of a proton.

Accepted Answer

M1 MH⁺ carries one extra proton, so M = 89.0597 − 1.0073 = 88.0524; reject subtracting 1.0078, the mass of an H atom M1 C₄H₈O₂: 4 × 12.0000 + 8 × 1.0078 + 2 × 15.9949 = 88.0522 M1 C₅H₁₂O: 5 × 12.0000 + 12 × 1.0078 + 15.9949 = 88.0885 A1 M = 88.0524, no unit, which matches 88.0522 to within 0.0002, so the formula is C₄H₈O₂

Question 3

Deduce which of C₄H₆O, C₅H₁₀, C₃H₂O₂ and C₃H₆N₂ is the molecular formula of a compound whose molecular ion is at m/z 70.0419, using the precise masses in the table.

Accepted Answer

M1 C₄H₆O = 70.0417 and C₅H₁₀ = 70.0780 M1 C₃H₂O₂ = 70.0054 and C₃H₆N₂ = 70.0530 M1 only one value lies within 0.001 of the measured 70.0419; error carried forward from one wrong sum A1 the molecular formula is C₄H₆O, precise mass 70.0417

Question 4

Explain how fragment peaks in their mass spectra distinguish pentan-2-one, CH₃COCH₂CH₂CH₃, from pentan-3-one, CH₃CH₂COCH₂CH₃, which both have a molecular ion at m/z 86.

Accepted Answer

B1 the molecular ion breaks beside the C=O, giving a positive acylium ion and a neutral radical, and only the ion is detected B1 pentan-2-one gives CH₃CO⁺ at m/z 43, by loss of a propyl radical B1 pentan-2-one also gives CH₃CH₂CH₂CO⁺ at m/z 71, by loss of a methyl radical B1 pentan-3-one gives CH₃CH₂CO⁺ at m/z 57, by loss of an ethyl radical, and no CH₃CO⁺ at 43, since it has no CH₃CO group

Question 5

Suggest how test-tube reactions could distinguish pentan-1-ol, pentan-2-ol and 2-methylbutan-2-ol.

Accepted Answer

B1 warm each with acidified potassium dichromate(VI): 2-methylbutan-2-ol, a tertiary alcohol, leaves it orange B1 pentan-1-ol and pentan-2-ol both turn it from orange to green B1 oxidise each of these two in distillation apparatus and warm each distillate with Tollens' reagent; allow Fehling's solution B1 the distillate from pentan-1-ol is pentanal and gives a silver mirror; pentan-2-one from pentan-2-ol gives no change

Question 6

Evaluate a student's conclusion that a liquid is propanone, drawn from a low-resolution mass spectrum showing a molecular ion at m/z 58.

Accepted Answer

B1 m/z 58 fits propanone, C₃H₆O, but also other formulae with the same nominal mass, such as C₄H₁₀ and C₂H₆N₂ B1 a high-resolution spectrum would fix the formula, for example 58.0417 for C₃H₆O against 58.0780 for C₄H₁₀ B1 even with the formula C₃H₆O fixed, propanal and prop-2-en-1-ol are isomers with the same precise mass B1 the conclusion is therefore not justified until tests show no silver mirror with Tollens' and no change with bromine water or dichromate

Question 7

Outline a sequence of tests that would identify each of four unlabelled liquids: cyclohexene, propanal, propan-1-ol and ethanoic acid.

Accepted Answer

B1 sodium hydrogencarbonate gives effervescence only with ethanoic acid B1 bromine water turns from orange to colourless only with cyclohexene B1 Tollens' reagent, warmed, gives a silver mirror only with propanal B1 the remaining liquid, propan-1-ol, turns acidified dichromate(VI) from orange to green on warming; this test comes last because propanal would also turn it green

Question 8

Deduce which compound in the table is an unknown liquid, C₄H₈O, that gives an orange precipitate with 2,4-dinitrophenylhydrazine, and explain why the precipitate is recrystallised before its melting point is used. The crude precipitate melts over 108–113 °C; after recrystallisation from hot ethanol and drying it melts at 114–116 °C.

Accepted Answer

B1 the liquid is butanone: its derivative melts at 115 °C, inside the 114–116 °C measured for the purified crystals B1 butanal (123 °C) and 2-methylpropanal (187 °C) are ruled out because their derivatives melt well above the measured range B1 impurities, such as unreacted 2,4-DNP or the liquid itself, lower the melting point and spread it over a range, so the crude 108–113 °C matches no entry in the table B1 the recrystallised solid melts sharply over about 2 °C, which shows it is pure enough for its melting point to be compared with the data

Question 9

Explain why 1-chloropropane, C₃H₇Cl, gives molecular ion peaks at m/z 78 and 80 in the height ratio 3 : 1, and predict the molecular ion peaks of dichloromethane, CH₂Cl₂.

Accepted Answer

B1 chlorine is about 75% ³⁵Cl and 25% ³⁷Cl, two isotopes differing by two mass units B1 C₃H₇³⁵Cl⁺ gives the peak at 78 and C₃H₇³⁷Cl⁺ the peak at 80, in the ratio of the isotope abundances, 3 : 1 B1 dichloromethane gives three peaks, at m/z 84, 86 and 88, from ³⁵Cl³⁵Cl, ³⁵Cl³⁷Cl and ³⁷Cl³⁷Cl B1 their heights are in the ratio 9 : 6 : 1, from 3 × 3, 2 × 3 × 1 and 1 × 1

Question 10

Give what is observed when solid sodium hydrogencarbonate is added to aqueous ethanoic acid, and an equation, with state symbols, for the reaction.

Accepted Answer

B1 effervescence, or bubbles, of a gas that turns limewater milky; reject 'a gas is produced' on its own B1 CH₃COOH(aq) + NaHCO₃(s) → CH₃COONa(aq) + H₂O(l) + CO₂(g), with all species correct B1 the equation is balanced and every species carries a correct state symbol

Question 11

Describe how Tollens' reagent is prepared from silver nitrate solution, and the positive result it gives with an aldehyde.

Accepted Answer

B1 a drop of sodium hydroxide solution is added to silver nitrate solution, giving a brown precipitate B1 dilute ammonia solution is added drop by drop until the precipitate just dissolves B1 warmed with an aldehyde in a water bath, it gives a silver mirror on the inside of the tube

Question 12

Explain why propanal gives a positive result with Fehling's solution and propanone does not.

Accepted Answer

B1 propanal is an aldehyde and is oxidised to propanoic acid by the mild oxidising agent B1 copper(II) is reduced to copper(I), giving a brick-red precipitate of Cu₂O from the blue solution B1 propanone is a ketone, has no hydrogen on the carbonyl carbon and would need a C–C bond broken to be oxidised, so the solution stays blue

Question 13

Explain why high-resolution mass spectrometry can distinguish but-1-ene, C₄H₈, from propenal, C₃H₄O, although both give a molecular ion at m/z 56 on a low-resolution spectrum.

Accepted Answer

B1 a high-resolution spectrometer measures m/z to four decimal places B1 the precise masses of ¹H and ¹⁶O are not whole numbers; only ¹²C is exactly 12.0000 B1 so the precise relative molecular masses differ: C₄H₈ 56.0624 and C₃H₄O 56.0261; allow values within 0.0001

Question 14

Calculate the precise relative molecular masses of C₄H₈O and C₅H₁₂, and identify which formula fits a molecular ion at m/z 72.0574. Use ¹H = 1.0078, ¹²C = 12.0000 and ¹⁶O = 15.9949.

Accepted Answer

M1 C₄H₈O: 4 × 12.0000 + 8 × 1.0078 + 15.9949 = 72.0573 M1 C₅H₁₂: 5 × 12.0000 + 12 × 1.0078 = 72.0936 A1 72.0574 matches 72.0573 to four decimal places within 0.0001, so the formula is C₄H₈O

Question 15

Identify compound W, C₃H₆O, from the results in the table, and give the reasoning.

Accepted Answer

B1 decolourising bromine water shows that W contains a C=C double bond B1 no silver mirror shows that W is not an aldehyde, so the green dichromate result comes from a primary or secondary alcohol B1 W is prop-2-en-1-ol, CH₂=CHCH₂OH

Question 16

Compare what is observed when propanal and propanone are each warmed with Tollens' reagent and with Fehling's solution.

Accepted Answer

B1 with Tollens' reagent propanal gives a silver mirror and propanone gives no change B1 with Fehling's solution propanal turns the blue solution to a brick-red precipitate and propanone leaves it blue B1 both reagents give a positive result only with the aldehyde, because only the aldehyde is oxidised

Question 17

Suggest which of ethanol, ethanal, methanol, propan-2-ol and pentan-3-one give a pale yellow precipitate when warmed with iodine and aqueous sodium hydroxide, and why.

Accepted Answer

B1 ethanol, ethanal and propan-2-ol give the precipitate of triiodomethane, CHI₃ B1 ethanal is the one aldehyde with a CH₃CO group, and ethanol and propan-2-ol contain a CH₃CH(OH) group that the alkaline iodine oxidises to CH₃CO B1 methanol has no methyl group on its C–OH carbon, and the carbonyl of pentan-3-one, CH₃CH₂COCH₂CH₃, sits between two ethyl groups, so neither gives a precipitate

Question 18

Calculate the number of carbon atoms in a molecule whose molecular ion peak has a relative abundance of 18.0 and whose M+1 peak has a relative abundance of 0.99.

Accepted Answer

M1 n = (100 × height of M+1 peak) ÷ (1.1 × height of M peak) M1 n = (100 × 0.99) ÷ (1.1 × 18.0) = 99 ÷ 19.8 A1 n = 5.0, so the molecule has 5 carbon atoms, a whole number with no unit

Question 19

State the reagent used to test for a C=C double bond, and the observation when an alkene is present.

Accepted Answer

B1 bromine water is added and the mixture is shaken B1 the bromine water changes from orange to colourless

Question 20

Explain why a green colour when an unknown is warmed with acidified potassium dichromate(VI) does not show that the unknown is an alcohol.

Accepted Answer

B1 the green colour shows only that dichromate(VI) has been reduced to Cr³⁺ by something that is oxidised B1 aldehydes are also oxidised by acidified dichromate, so the unknown could be an aldehyde as well as a primary or secondary alcohol

Question 21

Suggest why a student testing propanal with Tollens' reagent sees a grey-black precipitate rather than a silver mirror, and state whether the result is positive.

Accepted Answer

B1 the test tube was not clean and grease-free, so the silver formed as a precipitate rather than a coating on the glass B1 silver has still been formed, so the result is positive for an aldehyde

Question 22

Explain how sodium hydroxide solution and sodium hydrogencarbonate together distinguish phenol from ethanoic acid.

Accepted Answer

B1 both are acids and react with sodium hydroxide, phenol forming sodium phenoxide B1 only ethanoic acid gives effervescence with sodium hydrogencarbonate; phenol is too weak an acid to release carbon dioxide

Test	Observation
Bromine water	Orange to colourless
Tollens' reagent, warmed	No change
Acidified potassium dichromate(VI), warmed	Orange to green
Sodium hydrogencarbonate	No effervescence