Suggested answers to in-text activities and unit-end exercises. Topic 16 Unit 54

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1 Suggested answers to in-text activities and unit-end exercises In-text activities Checkpoint (page 64) 1 a) Any one of the following: Add aqueous bromine to each compound. Propene turns the yellow-brown aqueous bromine colourless quickly but propan-1-ol does not. Add acidified dilute aqueous solution of potassium permanganate to each compound. Propene turns the purple permanganate solution colourless quickly but propan-1-ol does not. Add phosphorus pentachloride to each compound. Propan-1-ol gives steamy fumes but propene does not. (Extension) b) Treat each compound with the Lucas reagent. Propan-2-ol gives cloudiness in 5 minutes. Propan-1-ol shows no observable change. (Extension) c) Warm each compound with iodine and an aqueous solution of sodium hydroxide. Pentan-2-ol gives a bright yellow precipitate but pentan-3-ol does not. 2 Warm each compound with acidified aqueous solution of potassium permanganate. Pentan-1-ol is oxidized to pentanal and then pentanoic acid. The purple permanganate ion is reduced to colourless manganese(ii) ion. Pentan-2-one resists oxidation. Checkpoint (page 69) 1 a) Any one of the following: Treat each compound with 2,4-dinitrophenylhydrazine. CH 3 CH 2 CHO gives a yellow to red precipitate but CH 3 CH 2 COOH does not. Treat each compound with the Tollens reagent. CH 3 CH 2 CHO gives a silver mirror but CH 3 CH 2 COOH does not. Warm each compound with acidified aqueous solution of potassium dichromate. CH 3 CH 2 CHO turns the orange dichromate solution green but CH 3 CH 2 COOH does not. b) Any one of the following: Suggested answers to in-text activities and unit-end exercises 1 Jing Kung. All rights reserved.

2 Treat each compound with the Tollens reagent. CH 3 (CH 2 ) 3 CHO gives a silver mirror but C 2 H 5 COC 2 H 5 does not. Warm each compound with acidified aqueous solution of potassium dichromate. CH 3 (CH 2 ) 3 CHO turns the orange dichromate solution green but C 2 H 5 COC 2 H 5 does not. c) Any one of the following: Treat each compound with 2,4-dinitrophenylhydrazine. C 4 H 9 COCH 3 gives a yellow to red precipitate but C 4 H 9 CH(OH)CH 3 does not. Warm each compound with acidified aqueous solution of potassium dichromate. C 4 H 9 CH(OH)CH 3 turns the orange dichromate solution green but C 4 H 9 COCH 3 does not. 2 a) A, B and C are either aldehydes or ketones because they give an orange precipitate with 2,4-dinitrophenylhydrazine. B and C are aldehydes as they give a silver mirror with the Tollens reagent. b) A B and C Checkpoint (page 72) a) Test reagent Test result Deduction K 2 Cr 2 O 7 / H 3 O + positive functional group(s) that may be present: OH, CHO 2,4-dinitrophenylhydrazine negative functional group(s) absent: CHO, C=O Aqueous bromine positive functional group(s) that may be present: C=C, C C b) Any three of the following: H 2 C=CHCH 2 CH 2 OH H 3 CCH=CHCH 2 OH H 2 C=CHCH(OH)CH 3 H 3 CCH=C(OH)CH 3 Suggested answers to in-text activities and unit-end exercises 2 Jing Kung. All rights reserved.

3 Problem Solving (page 72) Treat each liquid with the Tollens reagent. Only ethanal gives a silver mirror. Treat the remaining four liquids with 2,4-dinitrophenylhydrazine. Only propanone gives a yellow to red precipitate. Mix the remaining three liquids with an aqueous solution of sodium hydrogencarbonate. Only ethanoic acid gives effervescence. Mix the remaining two liquids with phosphorus pentachloride. Only ethanol gives steamy fumes. The remaining liquid is pentyl ethanoate. Checkpoint (page 82) Solvent A The solubility of X in A is high at high temperatures but low at low temperatures. Checkpoint (page 86) 1 Substances 1, 2 and 4 are present. 2 a) Each amino acid distributes itself between the water in the paper fibres and the developing solvent. Amino acids that are more soluble in the developing solvent travel up more quickly. Thus, the amino acids separate. b) Spray a developing agent such as ninhydrin onto the paper. This converts the amino acids into coloured spots on the chromatogram. c) R f value for B = 5.5 cm 7.0 cm = 0.79 Checkpoint (page 91) 1 a) Carry out simple distillation. Collect the liquid boiling off between 70 C and 90 C. b) Cyclohexanol and phosphoric acid. c) Transfer the distillate to a separating funnel. Add saturated sodium chloride solution and shake. Allow the two layers to separate. Run off the lower aqueous layer. Run the top cyclohexene layer into a small conical flask. Add some anhydrous calcium chloride. Allow to stand until the cyclohexene becomes clear. Suggested answers to in-text activities and unit-end exercises 3 Jing Kung. All rights reserved.

4 Decant the cyclohexene into a clean flask and re-distil it, collecting the liquid which distils off between 81 C and 85 C. 2 a) Pour the reaction mixture over crushed ice and stir until the product solidifies. Collect the product by filtration under reduced pressure after all the ice melts. Wash the product with a little ice-cold ethanol (keep this wash liquid for (c)). b) To re-crystallize the product, dissolve it in the minimum amount of hot ethanol. Filter the mixture while hot. Allow the filtrate to cool. Collect the crystals by filtration under reduced pressure and dry them. c) Evaporate the wash liquid from (a) to 1 cm 3. Use a fine capillary table to put a spot of the solution on one end of a fine layer of silica coated onto a glass plate. After drying, place the glass plate in a jar lined with a piece of filter paper dipping in a solvent (an ethoxyethane-hexane mixture). Methyl 3-nitrobenzoate can be seen under ultraviolet light or by exposing the glass plate to iodine vapour. Methyl 2-nitrobenzoate, a minor product, may appear as a yellow spot on the silica layer. STSE Connections (page 94) 1 Pros of using distillation for water purification Distillation is often used as the preferred water purification method in developing nations or areas where the risk of waterborne disease is high, due to its unique capabilities to remove bacteria and viruses from drinking water. Distilled water will not cause scales in water-using appliances like coffee maker, iron or clothes steamer. Cons of using distillation for water purification Distillation is expensive. Distillation removes every possible impurity from water, including minerals and salts. The human body is incapable of surviving on distilled water for a prolonged period of time, as it does not contain the much needed composition of minerals. These minerals are very much needed by the body and a deficiency is bound to cause some ailments. The content of salts and minerals also give water a peculiar and welcoming 'taste'. 2 Two other water treatment methods: boiling; portable filters providing various degrees of protection. Suggested answers to in-text activities and unit-end exercises 4 Jing Kung. All rights reserved.

5 References Unit-end exercises (pages ) Answers for the HKCEE and HKALE questions are not provided. 1 Reagent Observation with A Observation with B 2,4-dinitrophenylhydrazine a yellow to red precipitate forms a yellow to red precipitate forms Warm with Tollens reagent silver mirror forms on the no observable change wall of reaction vessel (Extension) Warm with iodine and an aqueous solution of sodium hydroxide no observable change a bright yellow precipitate forms 2 Heat each compound with acidified aqueous solution of potassium dichromate under reflux. Pentan-1-ol is oxidized to pentanal and then pentanoic acid. Orange dichromate ion is reduced to green chromium(iii) ion. Pentan-2-one resists oxidation. 3 a) Warm each chemical with acidified aqueous solution of potassium permanganate. CH 3 CHO is oxidized to CH 3 COOH. Purple permanganate ion is reduced to colourless manganese(ii) ion. CH 3 CHO + [O] CH 3 COOH CH 3 COCH 3 shows no observable change. b) Add 2,4-dinitrophenylhydrazine to each chemical. CH 3 CH 2 COCH 3 gives a yellow to red precipitate but CH 3 CH 2 COOCH 3 does not. c) Mix each chemical with sodium hydrogencarbonate solution. Effervescence occurs for HOOCCH 2 CH 2 CH 3 but not for HCOOCH 2 CH 2 CH 3. HOOCCH 2 CH 2 CH 3 + NaHCO 3 + Na OOCCH 2 CH 2 CH 3 + H 2 O + CO 2 Suggested answers to in-text activities and unit-end exercises 5 Jing Kung. All rights reserved.

6 d) Any one of the following: Add 2,4-dinitrophenylhydrazine to each chemical. (CH 3 ) 2 CHCHO gives a yellow to red precipitate but (CH 3 ) 2 CHOH does not. Add Tollens reagent to each chemical and warm. (CH 3 ) 2 CHCHO gives a silver mirror on the wall of the reaction vessel but (CH 3 ) 2 CHOH does not. Add ethanoic acid to each chemical and warm. (CH 3 ) 2 CHOH gives a sweetish smell but (CH 3 ) 2 CHCHO does not. CH 3 COOH + (CH 3 ) 2 CHOH CH 3 COOCH(CH 3 ) 2 + H 2 O ester Add PCl 5 to each chemical. (CH 3 ) 2 CHOH gives steamy fumes but (CH 3 ) 2 CHCHO does not. (CH 3 ) 2 CHOH + PCl 5 (CH 3 ) 2 CHCl + POCl 3 + HCl steamy fumes e) Any one of the following: Add aqueous bromine to each chemical. (CH 3 ) 2 C=C(CH 3 ) 2 turns the yellow-brown aqueous bromine colourless quickly but does not. Add acidified dilute aqueous solution of potassium permanganate to each chemical. (CH 3 ) 2 C=CH(CH 3 ) 2 turns the purple permanganate solution colourless quickly but does not. Suggested answers to in-text activities and unit-end exercises 6 Jing Kung. All rights reserved.

7 4 a) Paper chromatography b) Sample A one black spot Sample B three colour spots (yellow, green and blue) 5 First place some of the dry solids in a glass capillary tube. Then tie the tube to a thermometer with a rubber band. The solids in the tube should be level with the bulb of the thermometer. Place this set-up together with a stirrer in an oil bath and heat gently with stirring. 6 a) CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH(OH)CH 3 (CH 3 ) 2 CHCH 2 OH (Extension) b) When treated with a solution of anhydrous zinc chloride in concentrated hydrochloric acid, cloudiness appears in 1 minute for alcohol X. Cloudiness appears in 5 minutes for the secondary alcohol CH 3 CH 2 CH(OH)CH 3. The reaction mixture of CH 3 CH 2 CH 2 CH 2 OH and (CH 3 ) 2 CHCH 2 OH remains clear. Suggested answers to in-text activities and unit-end exercises 7 Jing Kung. All rights reserved.

8 7 a) Reagent Aqueous solution of sodium hydrogencarbonate 2,4-dinitrophenylhydrazine Acidified aqueous solution of potassium dichromate Observation with X Effervescence occurs no observable change no observable change Observation with Y no observable change a yellow to red precipitate forms aqueous solution of potassium dichromate turns from orange to green b) i) ii) 8 a) Test reagent Test result Deduction K 2 Cr 2 O 7 / H 3 O + positive functional group(s) that may be present: OH, CHO Tollens reagent negative functional group(s) absent: CHO Aqueous bromine positive functional group(s) that may be present: C=C, C C b) 9 a) As Y is an oxidation product of X and gives a negative result when warmed with Tollens reagent, it can be deduced that Y is an aromatic ketone. Suggested answers to in-text activities and unit-end exercises 8 Jing Kung. All rights reserved.

9 Structure of Y: b) i) Hydroxyl group ii) As a ketone (Y) is obtained by the oxidation of X, thus X is a secondary alcohol. Structure of X: c) 10 a) Butanal gives a yellow to red precipitate when treated with 2,4-dinitrophenylhydrazine. b) Test Deduction(s) I Treat with 2,4-dinitrophenylhydrazine X and Y may either be aldehydes or ketones. Z is neither an aldehyde nor a ketone. II Warm with Tollens reagent Y is an aldehyde. X and Z are not aldehydes. III Shake with acidified aqueous solution of potassium dichromate Y and Z may be primary / secondary alcohols or aldehydes. X is a ketone or tertiary alcohol. From Tests I and III, it can be deduced that X is a ketone. The structural formula of X is CH 3 CH 2 COCH 3. From Test II, it can be deduced that Y is an aldehyde. The structural formula of Y is (CH 3 ) 2 CHCHO. From Tests I, II and III, it can be deduced that Z is a primary / secondary alcohol. Suggested answers to in-text activities and unit-end exercises 9 Jing Kung. All rights reserved.

10 The structural formula of Z is H 2 C=CHCH 2 CH 2 OH or or or 11 a) A gives positive results when treated with the Tollens reagent. It can be deduced that A is an aldehyde with the CHO functional group. A decolorizes aqueous bromine readily. It can be deduced that A contains either a C=C bond or C C bond. b) As the molecular formula of A is C 9 H 8 O, besides the aromatic ring ( C 6 H 5 / C 6 H 4 ) and the CHO functional group, the formula of the other group should be C 2 H 2 / C 2 H 3. Thus, A probably contains a HC=CH bond. The structural formula of A is either or. exhibits geometrical isomerism: exhibits position isomerism: Suggested answers to in-text activities and unit-end exercises 10 Jing Kung. All rights reserved.

11 a) Isolate the crude 1-bromobutane from the reaction mixture by distillation. b) The distillate collected has an organic and an aqueous layer. Use a teat pipette to remove the aqueous upper layer. Transfer the organic layer to a separating funnel and add concentrated hydrochloric acid. Stopper and shake well. This removes the unreacted butan-1-ol as it is soluble in the acid. Allow the two layers to separate. Run the organic layer into a conical flask. Shake the crude 1-bromobutane with sodium hydrogencarbonate solution in another separating funnel. This removes the excess hydrochloric acid and other acid impurities. Allow the layers to separate and run the organic layer into a conical flask. Add anhydrous sodium sulphate to dry the 1-bromobutane. Swirl the mixture until the liquid is clear. Decant the dried product into a pear-shaped flask. Carry out fractional distillation. Collect the fraction with the boiling point range of 1-bromobutane. This removes any remaining organic impurities. 15 a) Reflux condenser b) c) Add dilute sulphuric acid and filter. d) Dissolve the crude sample in the minimum amount of hot water. Filter the mixture while hot to remove the insoluble impurities. Suggested answers to in-text activities and unit-end exercises 11 Jing Kung. All rights reserved.

12 Allow the filtrate to cool. Collect the crystals by filtration and dry them. e) Determine the melting point of the product to see whether the product has a sharp melting point. Compare the melting point obtained with data book value a) Each component in an orange drink sample distributes itself between the water in the paper fibres and the developing solvent. Components that are more soluble in the developing solvent travel up more quickly. Thus, the components separate. b) Only the spot from D travels the same distance and have the same colour as the spot from A. c) i) Similarity Any one of the following: Spots are placed on a medium. Solvent is allowed to soak up the medium. Different components travel different distances. Separation is by differences in distribution between the mobile and stationary phases. Difference Any one of the following: The medium is different. The medium is spread on a piece of glass. ii) Permanent felt tip pen ink does not dissolve in water. iii) R f value for the felt tip pen ink = = 0.46 R f value for the prohibited dye = = 0.46 iv) To be certain whether the spots are the same / different. OR Suggested answers to in-text activities and unit-end exercises 12 Jing Kung. All rights reserved.

13 To allow more accurate comparison between the spots. 19 a) Band A Each component in the mixture has its own equilibrium between adsorption onto the surface of the adsorbent and solubility in the solvent. The better adsorbed ones travel more slowly. b) Band C It is the component that is the most soluble in the water. Thus, it moves the greatest distance from the origin relative to the solvent front Experiment title: Experiment A Testing methanal with Tollens reagent Hazardous chemical, procedure or equipment involved Nature of hazard Safety precautions Source of information Methanal flammable and wear safety glasses and Material irritant protective gloves carry out the experiment inside a fume cupboard keep the reagent bottle tightly closed keep the reagent bottle well away from naked flames dispose of into appropriate waste bottle inside the fume cupboard Safety Data Sheet (MSDS) Tollens reagent irritant and wear safety glasses and Material explosive protective gloves Tollens reagent becomes explosive on evaporation, therefore wash away the solution immediately after use dispose of into appropriate waste bottle inside the fume cupboard Safety Data Sheet (MSDS) and previous instruction Hot water may cause burns use water at 60 C previous Suggested answers to in-text activities and unit-end exercises 13 Jing Kung. All rights reserved.

14 if spilt instruction Experiment title: Experiment B Preparing 2-chloro-2-methylpropane Hazardous chemical, procedure or equipment involved Methylpropan-2-ol 2-chloro-2- methylpropane Concentrated hydrochloric acid Nature of hazard Safety precautions Source of flammable and harmful wear safety glasses and protective gloves avoid skin contact with the chemical keep the reagent bottle tightly closed keep away from naked flames dispose of into appropriate waste bottle inside the fume cupboard flammable wear safety glasses and protective gloves keep the reagent bottle tightly closed keep away from naked flames dispose of into appropriate waste bottle inside the fume cupboard corrosive wear safety glasses and protective gloves carry out this activity inside a fume cupboard as concentrated hydrochloric acid gives off harmful fumes wash affected areas with plenty of water if spilt on the skin or clothes; report the accident to the teacher wash the container of concentrated hydrochloric acid with great care; do not add water to the acid directly information International Chemical Safety Cards (ICSCs) Material Safety Data Sheet (MSDS) Material Safety Data Sheet (MSDS) Suggested answers to in-text activities and unit-end exercises 14 Jing Kung. All rights reserved.

15 dispose of into appropriate waste bottle inside the fume cupboard Separating funnel contents of the hold the stopper and tap previous separating funnel when inverting the funnel to instruction may spill if not mix the contents handled properly open the tap regularly to release any pressure when standing in the fume cupboard, keep the funnel in an upright position safely with stands and clamps 22 a) Test Procedure Observation Deduction 1 Add 2 cm3 of X to 10 cm3 of water. X is soluble in water. X may form hydrogen bonds with water. 2 Add 1 cm3 of dilute sulphuric acid to 2 cm3 of aqueous solution of potassium dichromate. Add 10 drops of X and warm gently. The dichromate solution changes from orange to green. The dichromate ions are reduced to chromium(iii) ions. X is either a primary / secondary alcohol, or an aldehyde. 3 Mix 2 cm3 of X with 2,4-dinitrophenylhydrazine An orange precipitate forms. X is either an aldehyde or a ketone. b) Based on the results of tests 2 and 3, X is probably an aldehyde. Thus, X contains group. c) Suggested answers to in-text activities and unit-end exercises 15 Jing Kung. All rights reserved.