analysing the unknown compound A

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Introduction

The aim of this coursework is to identify two unknown organic compounds which are mono-functional using wet tests we deem suitable and the spectroscopy of the unknown compounds.

Background

Wet Tests

Reaction with bromine

To test for alkenes bromine water can be used. If the unknown compound is an alkene it will decolourise the bromine water. This is because the π-bond which makes up part of the double bond in alkenes is relatively easily to break making alkenes react in electrophilic addition reactions. Although most of this information may seem relevant to the investigation, the only important part is that bromine water can be used to test for alkenes as alkenes decolourise the bromine water.

Reaction with acidified potassium dichromate

Acidified potassium dichromate is an oxidising agent which can be used to test for primary and secondary alcohols. This is because primary alcohols can be oxidised into aldehydes and secondary into ketones whereas tertiary alcohols cannot be oxidised at all. “In the reaction, the orange dichromate(VI) ions are reduced to green chromium(III) ions” as said by the Essential AS Chemistry book for OCR. In addition to this because aldehydes can be further oxidised into carboxylic acids, acidified potassium dichromate can also be used distinguish aldehydes from ketones as ketones cannot be further oxidised. The most important part of this information I need to consider for my investigation is to recognise that because primary alcohols can be oxidised and tertiary alcohols cannot, we can use acidified potassium dichromate to distinguish between the two. This is because if the unknown compound is a primary alcohol there will be a colour change from orange to green, and if it is a tertiary there will be no colour change.

Reaction with Fehling’s solution

“Fehling's solution is a solution used to differentiate between aldehyde or ketone functional groups” as said by . This is because it is an oxdising agent, which oxidises aldehydes but not ketones as they cannot be oxidised. The Fehling’s solution which is blue, when added to an aldehyde and heated oxidises the aldehyde into a caboxylic acid and forms a red cuprios oxide precipitate. The realevance of this information is where it tells us that when added to an adlehyde a colour change will occur from blue to red as aldehyde can be oxidised and not ketones.

Reaction with Brady’s reagent

Brady’s reagent which is 2,4-dinitrophenylhydrazine can be used to identify carbonyls. This is because it forms an 2,4-dinitrophenylhydrazones which is an orange crystalline precipitate. This is important of my investigation as the presence of this precipitate indicates that a carbonyl is present but whether its an aldehyde or ketone is unknown.

Reaction with sodium

Sodium is an alkaline metal which is very reactive and because of that it has to be stored in oil. Sodium can be used to test for molecules with an hydroxyl OH group because they form an ionic bond making the salt R-O-Na+ and hydrogen which can be seen as bubbles. More importantly we can visibly see these bubbles as it react vigorously so we are able to use it as a wet test for an OH group.  

Reaction with universal indictor

Universal Indicator is a pH indicator that has different colours to indicate the pH range of the solution it’s in” said by . It is a good indicator because of the fact it has a varied range of colour for different pH’s which is useful as more accurate in telling you the pH of the solution unlike phenolphthalein indicator which only has two different colours that are only able to indicate whether the pH is above or below a certain point. In my wet test I will be using the universal indicator to test for the carboxylic acid as it is an acid and should turn the indicator from yellow to red if it is in the solution. The most important part of this information is that universal indicator can be used as a test for carboxylic acids and if present you will see a colour change from yellow to red.

Iodoform Test

The iodoform test is a confirmatory test I plan to use to see if I have a methyl ketone as it tests for this. Because it only tests for methyl ketones you can only use this test when you are certain you have a ketone as it only distinguishes between methyl ketones from other ketones. The reagents I will use for this test is 10% Iodine in KI which I will mix with 2M NaOH and then add to the unknown compound. If the unknown compound is a methyl ketone a yellow crystalline precipitate will be formed. Even though this information may seem relevant to the investigation its relevance is in the method as I will have to prepare the iodoform myself. The important part of it that is relevant is that it is only a test for methyl ketones and it gives a positive result is a yellow crystalline precipitate is formed.

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Test for ester

Although there is very little information on testing for ester using information for Essential A2 Chemistry Book I found that esters can be hydrolysed with NaOH when heated in a water bath. Subsequently if I add phenylalanine indicator to the NaOH before I use it to hydrolyse the ester then the mixture will turn pink. Furthermore when esters are hydrolysed with NaOH they produce an alcohol and a salt. Therefore hydrolysing the ester with this mixture of NaOH and phenylalanine indicator the colour should change from pink to colourless as the alcohol and salt have a ...

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