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Identification of an organic unknown

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Introduction

Identification of an organic unknown Here is the flow chart to determine the unknown organic. Start with the 2,4-DNP test and then follow the chart. DNP - 2,4-dinitrophenylhydrazine test The Safety: � DNP is harmful - avoid contact with skin, eyes etc. Wear safety glasses. � At this stage we have no idea what the organic chemical to be tested is, so wear gloves. The Instructions: To 1cm3 of 2,4-dinitrophenylhydrazine (DNP) solution, add a few drops of the unknown organic substance. If a yellow/orange precipitate is formed, it indicates the presence of a carbonyl group (aldehyde or ketone) (positive result). Move on to the Tollen's reagent test. If no precipitate is formed (negative result), move on to the Bromine water test. The Chemistry: 2,4-dinitrophenylhydrazine bonds to carbonyl compounds and makes an orange precipitate. The reaction also releases water. Tollen's reagent test The Safety: � Silver nitrate is poisonous and harmful, particularly to the eyes and nose. It will stain skin. � Sodium hydroxide is an irritant when dilute. � Aldehydes and ketones may be toxic. � Silver metal poses little threat. ...read more.

Middle

The Chemistry: Phenol undergoes electrophilic substitution with the bromine: C6H5OH + 3Br2 � C6H2(OH)Br3 + 3HBr Alkenes undergo electrophilic addition with the bromine: C2H4 + Br2 � C2H4Br2 PCl5 test The Safety: � PCl5 reacts violently with water - keep away from any moisture. Harmful, corrosive. � HCl fumes are toxic and corrosive. � Alcohols can be harmful and are probably flammable. � Esters may be harmful. � Carboxylic acids can be corrosive and harmful. � Wear gloves and goggles. Work in a fume cupboard. Keep away from water and fire. The Instructions: Place 0.5cm3 of the unknown into a dry test tube, and add 0.1g of phosphorus pentachloride. Blow gently across the top of the test tube to see whether a gas has been produced. If white fumes (HCl) are evolved, the organic compound contains an OH group (positive result). Move on to the acidified dichromate test. If no fumes are released (negative result), proceed to the phenolphthalein ester confirmation test. The Chemistry: PCl5 reacts readily with any OH group, producing HCl and replacing the OH group with a Cl. ...read more.

Conclusion

If the mixture doesn't change colour, the test is negative. Proceed to the sodium bicarbonate test. The Chemistry: Acidified potassium dichromate is an oxidising agent, and will oxidise a primary alcohol to an aldehyde or carboxylic acid. Tertiary alcohols and carboxylic acids won't be oxidised. In oxidising the alcohol, the orange-coloured chromium (+6) ions will be reduced to green chromium (+3) ions. R-CH2-OH + [O] � R-CHO + H2O Sodium bicarbonate test The Safety: � Sodium bicarbonate is a mild irritant to the eyes. � Lime water (Calcium Hydroxide solution) is corrosive and an irritant. � CO2 poses no risk. � Alcohols can be harmful and are probably flammable. � Carboxylic acids can be corrosive and harmful. � Wear safety glasses. The Instructions: Take roughly 1cm3 of the unknown in a test tube and add about one spatula of sodium bicarbonate powder, a little at a time. Effervescence (CO2 gas evolved) indicates a positive result - the unknown is a carboxylic acid. If no gas is produced (negative result), the unknown is a tertiary alcohol. If necessary, the gas can be bubbled through lime water to confirm that it is actually CO2, which will turn the lime water cloudy. The Chemistry: Sodium bicarbonate reacts with carboxylic acids to produce a salt, water, and carbon dioxide. ...read more.

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