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Identification of an unknown organic compound.

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Introduction

A2 Chemistry coursework: Identification of an unknown organic compound. Test wit Brady's reagent. Test identifies: the presence of a carbonyl group (aldehyde or ketone) Apparatus: * Test tube * Brady's reagent * Pipette Method: pipette a few drops of Brady's reagent into the test tube containing the unknown compound. Observation: * Colour change to a yellow/orange implies that a carbonyl group is present. * No colour change suggests that a C=O functional group is not present which implies that the unknown compound is any of the other six unknown organic compounds. Diagram: Explanation: * 2, 4 dinitrophenylhydrazine reacts with the carbonyl group to form products known as 2,4 dinitrophenylhydrazones. * This is a nucleophillic addition-elimination, as a water molecule is eliminated from the carbonyl group and 2 hydrogens from the hydrazine. * As this happens, the 2 compounds form a link to produce 2,4 dinitrophenylhydrazone. As the product forms an orange/yellow precipitate should form. * However if the carbonyl group is not present then no reaction takes place. This is because an oxygen atom needs to be present for a water molecule to be removed, which causes the formation of 2,4 dinitrophenylhydrazone. Safety: * Sulphuric acid is corrosive, avoid contact with skin. * Wear gloves at all times. * Keep goggles on at all times. * Brady's reagent is very harmful when inhaled or ingested, avoid contact with mouth and avoid inhalation of fumes. ...read more.

Middle

The other Br - ions bonds with the carbon with the + charge. It is due to this that bromine decolourises which shows the presence of a C=C bond. Phenol: Electrophillic substitution occurs as Phenol reacts with bromine. Because of instantaneous dipoles, the bromine atom with + is attracted to the delocalised ring from the phenol. A bond is formed with the phenol and Br leaving the other Br as an ion as it gains an electron. As this happens the ring becomes unstable, so it needs an electron to regain its stability. This is done by the phenol losing a hydrogen ion which bonds with the bromide ion to give HBr (hydrogen bromide). This occurs three times in total. 2,4,6 tribromophenol and 3 molecules of hydrogen bromide are produced when 3 molecules of bromine react with phenol. This happens as 4 hydrogen atoms are replaced by 3 bromine atoms on the aromatic ring. As this happens the bromine decolourises and a white precipitate is formed. Safety: * Wear goggles at all times. Bromine water is irritating to the eyes. * Bromine water is harmful when inhaled or ingested. Seek medical advice immediately if ingested. Test with sodium hydrogen carbonate. Test identifies: the presence of a carboxylic acid. Apparatus: * Conical flask * Limewater * Sodium hydrogen carbonate * Test tube * Gas jar * Delivery tube Method: Add sodium hydrogen carbonate to the solution of the unknown compound and collect gas, if any that is given off and then bubble it through limewater. ...read more.

Conclusion

Pop test- if hydrogen is present, when a lit splint comes in contact with the gas in the test tube; a squeaky noise (pop) is made. Observation: When the lit split enters the tube with the gas, if a squeaky pop noise is made then hydrogen is present. This presence of hydrogen comes from the reaction between the alcohol and sodium. This determines the presence of a tertiary alcohol. However if no hydrogen is given off, then no squeaky pop noise is made as a lit splint enters the tube. This means that a tertiary alcohol is not present; meaning that the unknown compound is an ester, because this is the only remaining unknown organic compound. Explanation: Hydrogen is produced when an alcohol and alkali metal react. The following reaction: Hydrogen being given off is an indication of sodium and the alcohol reacting. The gas which is collected is tested with a lit splint to determine whether the gas is hydrogen or not. Esters do not react with hydrogen, so no hydrogen is given off. If the pop test is negative, no pop noise is made; it means that a tertiary alcohol is not present, thus the unknown can only be an ester as all the other organic compounds have been eliminated. Safety: * Sodium is corrosive. Wear gloves at all times and avoid contact with skin. * Sodium is also harmful if ingested. * Wear goggles at all times. * Sodium is highly reactive when in contact with water and air. * Sodium is highly flammable, avoid contact with fires. ...read more.

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