4.5 The iodoform test. Task. How can methanol be distinguished from ethanol? (2) Science - Chemistry - Organic Chemistry - 4 Alcohols (P )

Transcription

1 Science - Chemistry - Organic Chemistry - 4 Alcohols (P772000) 4.5 The iodoform test Experiment by: Anouch Printed: Feb 25, 204 :5:07 PM intertess (Version 3.2 B24, Export 2000) Task Task How can methanol be distinguished from ethanol? (2) Carry out the iodoform test with methanol and ethanol. Use the space below for your own notes. Logged in as a teacher you will find a button below for additional information. - -

2 Addtional Information Learning objectives The two substances can be distinguished from each other by the fact that ethanol in contrast to methanol reacts with alkaline iodine/potassium iodide solution forming a yellow precipitate. The yellow precipitate consists of triiodomethane, which can be shown to be a halogenated alkane by the Beilstein test. Notes on setup and procedure Preparation: Iodine/potassium iodide solution can also be freshly prepared (20 g Kl and 0 g I 2 in 00 ml distilled water). Prepare dilute caustic soda (2 g NaOH to 00 ml water). The concentrations need not be exact. Cut out copper strips for the experiment or use some which were left over in previous experiments. Hazard and precautionary statements Ethanol: H225:Highly flammable liquid and vapour. P20: Keep away from heat/sparks/open flames/hot surfaces No smoking. Methanol: H225: Highly flammable liquid and vapour. H30: Toxic if swallowed. H3: Toxic in contact with skin. H33: Toxic if inhaled. H370: Causes damage to organs. P20: Keep away from heat/sparks/open flames/hot surfaces No smoking. P233: Keep container tightly closed. P280: Wear protective gloves/protective clothing/eye protection/face protection. P302+P352:IF ON SKIN: Wash with soap and water. P309+P30: IF exposed or you feel unwell: Immediately call a POISON CENTER or doctor/physician

3 Sodium hydroxide: H34: Causes severe skin burns and eye damage. H290: May be corrosive to metals. P280: Wear protective gloves/protective clothing/eye protection/face protection. P30+P330+P33:IF SWALLOWED: Rinse mouth. Do NOT induce vomiting. P309+P30: IF exposed or you feel unwell: Immediately call a POISON CENTER or doctor/physician. P305+P35+P338: IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses if present and easy to do continue rinsing. Hazards Ethanol and methanol are highly inflammable. Extinguish all open flames while handling them! Wear protective glasses! Methanol is poisonous. Carry out the experiment in the fume hood if possible and air the room well! Wash splashes on the skin off with copious water! Sodium hydroxide solution is corrosive. Solutions of iodine/potassium iodide in water attack the skin. Do not allow either of them to contact skin! Notes Iodine reacts in alkaline solution with methylene groups which contain a neighbouring hydroxyl or carbonyl group under the formation of triiodoalkanes. By the reaction with ethanol, one of the products is triiodomethane (iodoform), which precipitates out as a yellow solid: CH 3 CH 2 OH + 4 I NaOH CHI 3 + HCOONa + 5 NaI + 5 H 2 O As methanol does not contain such groups, it cannot react to triiodomethane under the given conditions. Remarks on the method The differences between methanol and ethanol will be carried over to the experiment with longer-chain alcohols which follow (P77200), so that the homologous series of the alkanes can be derived from the experimental data. Advanced courses: The iodoform reaction can be used as an example for the discussion of aliphatic electrophilic substitution reactions. Waste disposal Pour solutions into the waste container for combustible organic liquids. Put filters and residues into the container for solid organic waste

4 - 4 -

5 Material Material Material from "TESS advanced Chemistry Set Organic Chemistry, CH-4 " (Order No ) Position No. Material Test tube rack for 2 tubes, holes d = 22 mm, wood Order No Quantity 2 Test tube, 80x8 mm (2) 3 Test tube brush w. wool tip, d = 25 mm Test tube holder, up to d = 22 mm Lab. pencil, waterproof Graduated cylinder, 0 ml, plastic Funnel, plastic, dia. 50 mm Protecting glasses, clear glass Spoon spatula, special steel Pipette with rubber bulb Glass beaker DURAN, short, 250 ml Chemicals, Additional Material Position No. Material Bunsen burner /DIN/, nat.g., w. cock Order No Quantity Safety gas tubing, DVGW, sold per metre Ethanol, extra pure ab. 95%, 000 ml Aqueous potassium iodide/ iodine solution (Lugol's solution), 250 ml Copper foil, 00 g Methanol tech. gr., 000 ml Sodium hydroxide, flakes, 000 g Circular filter, d = 90 mm, 00 pcs Hot water Material required for the experiment - 5 -

6 - 6 -

7 Action Action Hazards Ethanol and methanol are highly inflammable. Extinguish all open flames while handling them! Wear protective glasses! Methanol is poisonous. Carry out the experiment in the fume hood if possible and air the room well! Wash splashes on the skin off with copious water! Sodium hydroxide solution is corrosive. Solutions of iodine/potassium iodide in water attack the skin. Do not allow either of them to contact skin! Procedure Number the test tubes from to 2 (Fig. ). Put ml of methanol in the first test tube, ml of ethanol in the second one (Fig. 2). Fig. Fig. 2 Add 2 ml of dilute sodium hydroxide solution to each of the alcohols in the test tubes. Carefully shake the tubes until the liquids are completely mixed. Add the solution of iodine in aqueous potassium iodide dropwise (Fig. 3) until each liquid has a weakly brown colour (Fig. 4)

8 Fig. 3 Fig. 4 Half-fill the beaker with hot water and place the test tubes in it (Fig. 5). Wait about 5 minutes, then put the test tubes next to each other in the test tube rack. Fig. 5 When they have cooled down, filter the content of test tube 2 into the emptied beaker (Fig. 6). Scratch some of the residue from the filter, put it on a strip of copper foil and carry out the Beilstein test (Fig. 7). Remove the test tubes containing alcoholic solutions before lighting the Bunsen burner)

9 Fig. 6 Fig. 7 Waste disposal Pour solutions into the waste container for combustible organic liquids. Put filters into the container for solid organic waste

10 Results Results Note the observations you make. Test tube : Test tube 2: Test tube : The brown-coloured methanol solution does not change. Test tube 2: A sweet-smelling, yellow precipitate forms in the ethanol solution after a short time. State the results of the Beilstein test

11 When the residue from the filter is heated on the copper strip, it colours the flame green. - -

12 Evaluation Evaluation Question : Draw conclusions from your observations. In contrast to methanol, ethanol reacts with alkaline iodine/potassium iodide solution forming a yellow precipitate whose sweet smell suggests that it is a halogenated alkane. This is confirmed by the positive result of the Beilstein test. Methanol and ethanol can therefore be distinguished from each other by the iodoform test. Question 2: Enter the properties of methanol and ethanol which have been observed in previous experiments in the "General data sheet", look up missing entries in your text book to fill it completely. See "General data sheet" - 2 -

13 General data sheet General data sheet - 3 -

14 Name of Methanol the substance: Chemical nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn formula: Colour: nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn State of nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn aggregation: Melting nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn point: Boiling point: nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn Other properties: Occurrence/ preparation: Use: - 4 -

15 Name of the substance:methanol Chemical formula: Colour: State of aggregation: Melting point: Boiling point: Other properties: Occurrence/ preparation: CH 3 OH colourless liquid -97 C 64.7 C very poisonous, causes blindness; combustible; borax test: positive (green flame); iodoform test: negative dry distillation of wood ("wood spirit"); catalytically from carbon monoxide; evolved during fermentation Use: solvent, fuel - 5 -

16 Name of Ethanol the substance: Chemical nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn formula: Colour: nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn State of nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn aggregation: Melting nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn point: Boiling point: nnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn Other properties: Occurence/ preparation: Use: - 6 -

17 Name of the substance:ethanol Chemical formula: Colour: State of aggregation: Melting point: Boiling point: Other properties: C 2 H 5 OH colourless liquid -4 C 78.3 C poisonous in large quantities; combustible; borax test: negative; iodoform test: positive (yellow precipitate) Occurrence/ preparation: alcoholic fermentation Use: in alcoholic beverages; solvent, fuel - 7 -