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Identify an unknown organic compound.

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Introduction

IDENTIFICATION OF AN ORGANIC UNKNOWN Introduction: In this assessed practical we have to plan a sequence of simple chemical test tube tests in order to identify an unknown organic compound containing a functional group. The functional group could be an alkene, primary alcohol, tertiary alcohol, aldehyde, ketone, carboxylic acid, ester or phenol. Safety: During all experiments Laboratory coats and safety glasses must be worn. When using a Bunsen burner tie long hair back and use a safety mat to protect the bench. Plastic gloves must be worn when handling the chemicals. There are some chemicals that need special attention when handling them: * 2, 4-DNP - is an irritant to skin and eyes. * Bromine water - is harmful if ingested or inhaled and prolonged skin contact can cause burns. It is an irritant to the eye and also make sure there is good ventilation. * Silver nitrate - is an irritant and can cause burns. * Ammonia - is an irritant and can cause burns. * Sodium hydroxide - is an irritant to skin and eyes. * Iron (III) Chloride - is toxic and can cause burns. Can also give off Hydrogen Chloride gas. * Sodium carbonate - is a low hazard, but still needs to be considered * Organic unknown - can be considered to be toxic and harmful if swallowed. If any of the above chemicals come into contact with skin or eyes, wash off immediately with cold water. ...read more.

Middle

A positive test will give a violet colour. Test 3 - 2,4-DNP Using a dropping pipette, place 5 drops of the organic unknown in a test tube. Measure out and add 2cm3 of 2,4-DNP solution. This is if the unknown is a liquid. If the unknown is a solid then measure out 20cm3 of de-ionised water and add 1g of the unknown solid and 2cm3 of 2,4-DNP in a test tube. Now do the same for the unknown if it is a liquid. Give the test tube a mix. A positive test will give an orange/yellow precipitate and a negative test will not give a precipitate. Test 4 - Tollens reagent (Ammoniacal Silver Nitrate solution) Take a 500cm3 beaker and fill it approximately half full with water. Place this on a tripod and gauze and heat the beaker of water using a Bunsen burner. Measure out 1cm3 of 0.05 mol dm-3 of silver nitrate in a measuring cylinder and place it in a test tube. Now add 5 drops of sodium hydroxide solution (2M) using a dropping pipette. Using a different dropping pipette, add 2M of ammonia solution drop wise until the precipitate has just dissolved. Then add 5 drops of the organic unknown and shake gently. Place the test tube in the beaker of warm water. A positive test will give a silver mirror on the inside of the test tube and a negative test will not give a mirror. ...read more.

Conclusion

Place this on a tripod and gauze and heat the beaker of water using a Bunsen burner. After the water has boiled, remove the flame and place the test tube into the beaker. Leave the test tube for about 5 - 10 minutes. A positive test will give a green coloured solution and a negative test will not give a green colour. Potassium Dichromate is a reagent, which oxidises alcohols when mixed with dilute sulphuric acid and warmed. The dichromate ion gives a colour change from orange to green, because it is reduced from an oxidation state of +6 to +3. However, this test does not oxidise a tertiary alcohol, therefore does not turn green. Alternative tests: For some of the tests that have been selected, there are alternatives that can be used. Firstly, to distinguish an aldehyde and a ketone, a test involving a solution called Fehling's can be used. This would involve mixing Fehling's A (aqueous copper sulphate), with Fehling's B (aqueous sodium hydroxide) and then adding the carbonyl. This test would give a brick red precipitate with an aldehyde but no precipitate with a ketone. However, Tollens will be used because it a test which is more familiar to me. An alternative test for an alkene that could be used is to add iodine in methylene chloride solution. This will produce a light tan solid, while retaining the purple colour of the iodine solution. However, this will not be used as it also gives a positive test for some alcohols and ketones. ...read more.

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