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Identification of an organic unknown

Categories: Other

In this chemistry coursework, I am supplied with an unknown organic compound containing one of the following functional group:

* Alkene

* Primary alcohol

* Tertiary alcohol

* Aldehyde

* Ketone

* Carboxylic Acid

* Ester

* Phenol

I am going to outline a sequence of simple chemical test that I could use to identify each group. Each test must be dependant on the result of previous test. I am going to write up a plan include a flow chat, and a procedure which provides fully details of each test with the expected observations and any necessary safety precaution.

Also I will need to consider the equipment and apparatus and chemicals use.

Primary and Tertiary alcohol

Oxidation can be used to decide whether it a primary or tertiary alcohol. Oxidation with acidified potassium dichromate(VI) solution results in a colour change from orange to green due to the formation of Cr3+ ions, as long as there is not an excess of dichromate present. A colour change will therefore only occur with primary and secondary alcohol.

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The Lucas test is another way of distinguishing between primary and tertiary alcohols. It depends on substitution of chlorine for the hydroxyl group. The alcohol is shaken hard in a stoppered flask with concentrated hydrochloric acid and anhydrous zinc chloride. The flask is then allowed to stand. For tertiary alcohols, substitution is rapidly becomes turbid or cloudy (or a separate layer develops) because the water-insoluble chloroalkane is formed.

Equations

Apparatus

Safety spectacles

Protective gloves

Ground-glass-joint

Spatula and teat-pipette

Sulphuric acid

Potassium dichromate(VI)

Measuring cylinder, 10cm3

Retort stands, bosses and clamps

Test tubes

Hazard Warning

The unknown compound could be is very flammable; therefore I must keep the stopper on the bottle as much as possible, keep the bottle away from flames and wear safety spectacles

Sulphuric acid is corrosive, protective gloves must wear.

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Potassium Dichromate(VI) is a powerful oxidant and can damage the skin, therefore wear safty spectacles and gloves.

Procedure

1. Into a pear-shaped flask, add 2 – 3 drops of sulphuric acid to acidified 5g of potassium dichromate(VI)

2. Swirl the flask gently until all the potassium dichromate(VI) has dissolved.

3. Set up the apparatus shown in the picture, so it is under reflux condition:

4. Drop by drop, add 1 cm3 of unknown down the condenser.

5. Boil gently under reflux for 20 minutes.

6. Note observations.

Positive results

The orange dichromate(VI) solution turns to the green of hydrated chromium(III) ions, shows there is a present of primary or secondary alcohol.

Negative results

Tertiary alcohols are not readily oxidized.

Aldehyde and Ketone

Reaction with 2,4 -dinitrophenylhydrazine

2,4-dinitrophenylhydrazine (1,4-DNPH) undergoes a condensation reaction with aldehydes and ketones. This gives a convenient analysis

test because the product has a characterists colour.

Apparatus

Safety spectacles

Protective gloves

2,4-dinitrophenylhydrazine

Test tubes

Hazards

The unknown compound could be is very flammable; therefore I must keep the stopper on the bottle as much as possible, keep the bottle away from flames and wear safety spectacles

2,4-dinitrophenylhydrazine

is corrosive, Safety spectacles and gloves must wear

Procedure

1. Put 1-2 drops of unknown in a test tube and add about 2 cm3 of 2,4-dinitrophenylhydrazine solution.

2. Note observations.

Positive results

The formation of orange precipitation confirms the presence of the carbonyl group.

Tollen Test

Tollen test could be use to distinguish aldehydes and ketones. The big difference between aldehydes and ketones is that aldehydes are very easily oxidized to carboxylic acids.

Equations

Apparatus

Safety spectacles

Protective gloves

Silver nitrate solution, 0.05M AgNO3

Ammonia solution, 2M NH3

Clean test tubes – a shiny silver will only form in clean test tubes

Beakers

Hazards

The unknown compound could be is very flammable; therefore I must keep the stopper on the bottle as much as possible, keep the bottle away from flames and wear safety spectacles

Tollens’ reagent becomes explosive on evaporation. Therefore wash away residues immediately after use.

Procedure

1. Put about 1cm3 of 0.05M AgNO3 into a very clean test tube and add three or four drops of sodium hydroxide solution.

2. Drop by drop, add ammonia solution until the precipitate of silver oxide nearly dissolves (do not try to get rid of all the little black specks of silver oxide).

3. Add one or two drops of unknown, shake the tube gently and place in a beaker of warm water.

4. Note observations and immediately rinse out the test tube.

Positive results

If a shinny silver mirror form, aldehydes presents.

Negative results

Ketones have no reaction with Tollens test.

Fehling’s test

Fehling’s test are similare to Tollens’ test, except that copper(II) is used instead of silver(I) as the oxidizing agent. Fehling’s regent must be freshly prepared before each used. Fehling’s solution A is awueous copper(II) sulphate; Fehling’s solution B contains alkali and tartrate ions. Equal volumes of Fehling’s A and B should be mixed to give a deep blue solution of copper(II) tartrate complex.

Equations

Apparatus

Safety spectacles

Protective gloves

Fehling’s solution A and B

Test tubes

Hazards

The unknown compound could be is very flammable; therefore I must keep the stopper on the bottle as much as possible, keep the bottle away from flames and wear safety spectacles

Fehling’s solution is corrosive, Safety spectacles and gloves must wear

Procedure

1. a test tube, put 1cm3 of Fehling’s solution A and then add Fehling’s solution 2 dropwise until the precipitate just dissolves

2. Add about 7 drops of unknown. Shake the tube gently and place on a beaker of boiling water for five to ten minutes – until no further colour change occurs.

3. Note the observations

Positive results

An aldehyde gives a red-brown precipitate of copper(I) oxide, Cu2O

Negative results

Ketones will have no reaction.

Carboxylic group

Reaction with sodium hydrogencarbomate solution

Carboxylic acid contains the carboxyl group, -COOH, which itself contains a carbonyl group and a hydroxyl group; hence the name.

Apparatus

Safety spectacles

Protective gloves

Sodium hydrogencarbonate solution, saturated, NaHCO3

Test tubes

Hazard Warning

The unknown compound could be is very flammable; therefore I must keep the stopper on the bottle as much as possible, keep the bottle away from flames and wear safety spectacles

Sodium hydrogencarbonate solution is corrosive, protective gloves must wear.

Procedure

1. Into a test tube, add about 1 cm3 of sodium hydrogencarbonate solution followed by a few drops of glacial ethanoic acid.

2. Shake gently and test any gas produced.

Positive results

Carbon dioxide produce. Ethanoic acid releases carbon dioxide from sodium hydrogencarbonate.

Ester

Hydrolysis of ester

Ester can be hydrolysis by both acids and alkalis. In hydrolysis, the overall result is that a water molecule is added for each easter linkage which is broken. There are two big advantages of doing this rather than using a dilute acid. The reactions are one-way rather than reversible, and the products are easier to separate.

Equations

First, hydrolysing ethyl ethanoate using sodium hydroxide solution:

and then hydrolysing methyl propanoate in the same way:

Mechanisms for hydrolysis of an ester

Procedure

1. Set up the apparatus shown in the picture, so it is under reflux condition:

2. Heat unknown under reflux with a dilute alkali like sodium hydroxide solution.

Positive results

Alkaline hydrolysis breaks up the ester completely.

Phenol

Reaction with bromine water

Equation

Apparatus

Safety spectacles

Protective gloves

Bromine water, Br2(aq)

Test tubes

Hazard Warning

Both phenol and bromine water attack the skin and give off irritating vapours. Therefore, safty spectacles and gloves must wear, avoid contact with skin, and keep stoppers on bottles as much as possible.

Procedure

1. Into a test tube pour about 5 cm3 of water and add a spatula-measure of unknown.

2. Cork and shake until the phenol has dissolved.

3. Pour half the solution into another test tube for the reaction with neutral Iron(III) chloride solution.

4. Add about six drops of bromine water to the aqueous phenol. Shaking the test tube after the addition of each drop.

5. Note your observation.

Positive results

White precipitate and brown colour disappeared means phenol presents.

Reaction with neutral Iron(III) chloride solution

I could use this reaction as a confirmation test to distinguish phenols. Phenols produce a colour complex with this reagent.

Apparatus

Safety spectacles

Protective gloves

Iron(III) chloride solution, 0.5M FeCl3

Test tubes

Hazard Warning

Both phenol and bromine water attack the skin and give off irritating vapours. Therefore, safty spectacles and gloves must wear, avoid contact with skin, and keep stoppers on bottles as much as possible.

Procedure

1. Into a test tube, pour about 1 cm3 of Iron(III) chloride solution. Add sodium carbonate solution, drop by drop, until a trace of the brown precipitate just remains after shaking

2. Add a few drops of neutral Iron(III) chloride solution to the solution prepared in the reaction with bromine water.

3. Note observation.

Positive results

Violet coloration means phenol presents. Also you can read about Ka of ethanoic acid

Cite this page

Identification of an organic unknown. (2020, Jun 02). Retrieved from http://studymoose.com/identification-organic-unknown-new-essay

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