Preparation of Iodoform
Simple steps for preparing iodoform - common reaction of distinguishing methy ketones and aldehydes...
Preparation of Iodoform Objectives: 1. To prepare a pure sample of triiodomethyl (iodoform) from acetone under reflux 2. To learn how to use the reflux apparatus 3. To determine the melting point of iodoform crystals 4. To calculate the yield of iodoform crystals. Materials/Apparatus: Graduated cylinder, Beaker 250 mL, Erlenmeyer flask 250 mL, Pipette, Suction bulb, Beaker 25 mL, Filter paper, Funnel, Reflux condenser, Round Bottom Flask 500 mL, Heater, Watch Glass, Scale Reagents: Potassium iodide , KI, Potassium Carbonate, Iodine crystals, Water , H2O, Acetone , CH3COCH3, 5% aqueous sodium hypochlorite, 5% Sodium Hydroxide , NaOH, Ethanol , CH3CH2OH, . Background: Iodoform is produced when a methyl ketone, acetaldehyde, or an alcohol with the formula RCHOHCH3 is treated with sodium hydroxide and iodine. A positive result - the pale yellow precipitate of triiodomethane (iodoform) is given by an aldehyde or ketone .
Ethanol is the only primary alcohol to give the triiodomethane (iodoform) reaction. Lots of ketones give this reaction, but those that do all have a methyl group on one side of the carbon-oxygen double bond. These are known as methyl ketones. Compounds that are easily oxidized to acetaldehyde and methyl ketones also give a positive iodoform test. Only ethanol can be oxidized to acetaldehyde and secondary alcohols that have the general formula CH3CHOHR can be oxidized to methyl ketones For ethanol (CH3CH2OH), the following reactions show the formation of iodoform : CH3COCH3 + 3 NaClO + 3 KI → CH3COCl3 + 3 NaOH + 3 KCl CH 3COCl3 + NaOH → CH3COONa + CHI3 Reflux Condenser : Organic chemists often need to heat a mixture for a long time and to be able to leave it relatively untended. The "reflux apparatus" shown allows such heating; it allows the reaction to be carried out at the boiling point of the solvent and yet it prevents loss of solvent or reagent due to evaporation. A condenser is attached to the boiling flask and is clamped in an upright postion, the "reflux position", and cooling
water is circulated to cause the vapours to condense as they rise up the condenser and thus prevent them from escaping. The upper level of the vapours in the condenser can often be seen as a reflux line. It is a condenser such that vapour over a boiling liquid is condensed and flows back into the vessel to prevent its contents from boiling dry. The reflux condenser consists of a length of tubing connected directly to the head of a vessel and equipped with a water cooling jacket. Condensed vapors are returned directly to the vessel and any non-condensable gases can be released through a needle valve at the top of the condenser. A spiral wound inner packing in the condenser ensures maximum effectiveness in a rather short length.
Types of Reflux Condensers Heating Under Reflux
Determination of Melting Point Determining the melting point of a compound is one way to test if the substance is pure. A pure substance generally has a melting range (the difference between the temperature where the sample starts to melt and the temperature where melting is complete) of one or two degrees. Impurities tend to depress and broaden the melting range so the purified sample should have a higher and smaller melting range than the original, impure sample. Procedure: Preparation from Acetone: Dissolve 5 g of iodine in 5 ml acetone in a conical flask. Add 5 % sodium hydroxide solution slowly with shaking until the colour of iodine is discharged. Allow contents of flask to stand for 10 – 15 minutes 1. Weigh out about 10g of potassium iodide and add to 25 cm 3 of water in a 100 cm 3 conical flask. 2. Add 10 cm 3 of 2M sodium hydroxide solution. 3. Add 1.5 cm 3 of propanone and rinse the measuring cylinder with a little water (to ensure that all the propanone has been transferred) and add this to the flask. 4. Cork the flask securely.
5. Measure out 35 cm 3 of 2M sodium hypochlorite solution and transfer this to a dropping funnel. 6. Allow the hypochlorite to drop into the conical flask as you continually shake the mixture. 7. Replace the cork, shake the flask vigorously and then allow it to stand for five minutes 5. Iodoform is formed as a yellow precipitate. 6. To complete the precipitation waited for 10 minutes and the precipitate was filtered using filter paper. 3.2. Recrystallization 7. Set up a hot water bath and use this to heat about 5cm of ethanol. 8.Transfer the iodoform to a test tube and place this into the hot water bath. 9.Add hot ethanol dropwise and dissolve the iodoform in the minimum amount. 10. Allow the iodoform to crystallise out of solution and then filter 11. Allow the crystals to dry in a dessicator 12. Record the mass of iodoform and determine the percentage yield. Treatment of Data:
Record observations Calculate the percentage yield of iodoform Calculate theoretical percentage yield
Applications of iodoform test Purpose of reflux condenser. Uses of the different types of reflux condensers Compare the theoretical and measured yield of iodoform Discuss melting point and purity of a substance. How does impurities affect melting point Outline how melting apparatus can be used to determine purity