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Identifying an Unknown Compound

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Introduction

Identification of an Unknown Organic Compound Ensure a lab coat, safety glasses and gloves are worn during the experiments and work in a ventilated area. For all experiments test tubes and pipettes will be required. ALKENES: Alkenes contain a double carbon-carbon bond. APPARATUS: Unknown compound, bromine water. METHOD: Add 5 drops of bromine water to a test tube containing 1cm3 of the unknown and shake. OBSERVATION: Bromine water will decolourise from orange. EXPLANATION: The alkene decolourises bromine water and produces 1,2-dibromoethane. The double bond allows electrophilic addition reactions to occur. HAZARDS: Bromine vapour released from the water is 1^harmful^1 (see appendix). ESTERS: Esters are sweet- smelling products from reactions between carboxylic acids and alcohols. APPARATUS: Unknown compound, phenolphthalein, water bath, NaOH. ...read more.

Middle

Phenol- 3^corrosive^3 and 4^toxic^4. CARBOXYLIC ACIDS: Carboxylic acids are products of the oxidation of a primary alcohol or an aldehyde. Their functional group is -COOH. APPARATUS: Unknown compound, water, limewater, NaHCO3. METHOD: 3Add water and NaHCO3 to the unknown compound.3 OBSERVATION: Effervescence occurs after NaHCO3 is added; test the gas with limewater and it will go cloudy. EXPLANATION: Carboxylic acids although weak acids react like any other acid and undergo the same reactions. So CO2 is produced when reacted with NaHCO3. HAZARDS: NaHCO3- 2^irritant^2. CARBONYL COMPOUNDS: Carbonyl compounds consist of the C=O bond, examples are aldehydes, ketones and esters. However the 2,4-DNPH test is only positive for aldehydes and ketones. APPARATUS: Unknown compound, 2,4-DNPH. METHOD: Add 5 drops of 2,4-DNPH to the unknown in a test tube. OBSERVATION: A deep yellow/ orange precipitate appears. EXPLANATION: 4See diagram below. ...read more.

Conclusion

The orange dichromate (VI) ion, Cr2O72-, is reduced to green chromium (III), Cr3+. If the solution has remained orange it proves the presence of a tertiary alcohol as they cannot be oxidised.6 Another test can be carried out to confirm the presence of a tertiary alcohol, it is known as Lucas' test. APPARATUS: Lucas' reagent: a solution of ZnCl2 in concn HCl. METHOD: Add 2cm3 of Lucas' reagent to 1cm3 of the unknown and shake. OBSERVATION: A cloudy emulsion may be seen rapidly or no reaction may occur after 5 minutes. EXPLANATION: 7The reagent dissolves the alcohol and removes the OH group leaving a carbocation. The cloudiness is caused as the carbocation reacts with Cl- forming a chloroalkane. Tertiary alcohol is present if the emulsion forms rapidly and a primary alcohol if no reaction occurs.7 HAZARDS: K2Cr2O7- 4^toxic^4 and 6^oxidising^6. H2SO4- 1^harmful^1 and 3^corrosive^3. Lucas' reagent- 4^toxic^4 and 3^corrosive^3. ...read more.

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