Explanation:
- You can gently oxidise aldehydes to a carboxylic acid.
- However ketones cannot be gently oxidised; instead they are only oxidised through combustion. Reason being if a ketone was to be oxidised it would require enough energy to break a C-C bond as opposed to that in an aldehyde which requires less energy to break a C-H bond to be oxidised to a carboxylic.
- More energy is required because the C-C bond is stronger, it cannot be broken by gentle heating as this is does not provide sufficient amount of energy.
- As the aldehyde is oxidised, an electron is gained by the silver from silver nitrate; which means that it has been reduced. As a result the silver forms a metallic layer on the inside of the test tube, which is commonly known as a silver mirror.
- As this reaction takes place, the silver ions are reduced to form metallic ions; this occurs simultaneously as the aldehyde oxidises to a carboxylic acid. However ketones do not oxidise in the same way therefore no silver mirror would form if tollens reagent was added to the solution.
Safety:
- Ensure goggles are worn at all times
- Ammonium hydroxide solution is corrosive, avoid contact with skin; best prevented by wearing gloves and avoid any spillages.
- Do not heat as tollens reagent is explosive when dry. Discard the the contents immediately after experiment.
- Do not ingest any solution; if so seek medical advice immediately.
Test with bromine water:
Test identifies: the presence of an alkene or phenol.
Apparatus:
Method: Add bromine water to the solution of the unknown compound in a clean test tube. Take note to any colour change.
Observation:
- If the bromine water decolourises completely when added to the unknown compound solution, an alkene functional group is present.
- However if the bromine water decolourises and a white a precipitate forms, then a phenol is present.
Explanation:
- Alkenes have an electron dense area which is the C=C double bond. This is known as an area of high electron density; because two electrons are shared between the two carbons. Because of the C=C bond, the compound is susceptible to be attacked by electrophiles. Bromine although not an electrophile, can become one due to instantaneous dipoles; therefore react by electrophillic addition.
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Bromine has a + and - due to instantaneous dipoles. The electrons are attracted to the bromine atom with +. 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. Note any colour change to the limewater.
Observation:
If a milky emulsion appears in the limewater, this means that CO₂ is present which is initially given off from the carboxylic acid, in turn proves that a carboxylic acid is present. It there is no change CO₂ is not present.
Explanation:
When acids dissolve in water a proton is donated, so acids are known as proton donors. When a carboxylic acid dissolves in water the hydrogen atom from the COOH functional group dissociates.
As this reaction takes place CO₂ is given off. Carboxylic acid is the only possibility out of the unknown compounds that can be present because only a carboxylic acid is produced at the end. Therefore it CO₂ is given off during this reaction, a carboxylic acid is present.
CO₂ can be detected when bubbled through limewater. The following reaction:
Calcium carbonate (CaCO₃) is produced as a white precipitate. Therefore the presence of CaCO₃ supports the presence of CO₂; in turn further supports the presence of a carboxylic acid.
Safety:
- NaHCO₃ can be harmful if it comes in contact with the eye. So wear goggles at all times. May cause discomfort if inhaled.
- NaHCO₃ may cause irritation to the skin, if it comes in contact with skin wash immediately. Wear gloves to avoid contact with skin.
Test with acidified potassium dichromate.
Test identifies: presence of a primary alcohol.
Apparatus:
- Pipette
- Concentrated acidified potassium dichromate
- Test tube
- Beaker
- Hot water
Method: add acidified potassium dichromate to the test tube containing the unknown compound using a pipette. As this reaction requires heat, place the test tube in a beaker with hot water. Take note to any colour change.
Observation:
Colour change from orange to green shows that a primary alcohol is present; however if there is no colour change (stays orange) a primary alcohol is not present. This leaves two possible unknown compounds either a tertiary alcohol or an ester.
Explanation:
- Primary alcohols are oxidised to aldehydes by using the oxidising agent potassium dichromate.
- As the primary alcohol oxidises, the orange dichromate ions are reduced to chromium (iii) ions which are green. Therefore the colour change from orange to green clearly indicates the presence of a primary alcohol
- Tertiary alcohols however react differently, because it would require to break a C-C bond which needs a much higher amount of energy than that required for a C-H bond.
Safety:
- Potassium dichromate is toxic if swallowed or inhaled. It is also corrosive, so avoid contact with skin.
- Do not inhale or ingest any products or bi products of this or any experiment.
- Wear goggles at all times.
- If any chemical is ingested, seek medical advice immediately.
- Do not dispose of solution by pouring down the sink.
- Store the solution- when it has solidified; dispose of it as normal waste.
- Ensure the place of work is well ventilated.
- Clean any spillages immediately.
- Wear gloves at all times.
Test with Sodium.
Test identifies: the presence of a tertiary alcohol.
Apparatus:
- Sodium
- Conical flask
- Bung
- Tweezers
- Delivery tube
- Splint
- Gas jar
Method: Add sodium to the solution of the unknown compound in a test tube. Collect any gas given off, and test it to determine whether the gas collected is hydrogen or not by using the pop test.
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.
Bibliography:
- CGP Chemistry AS level revision guide.
- Ted Lister and Janet Renshaw ,Essential AS chemistry for OCR
- Ted Lister and Janet Renshaw, Essential A2 chemistry for OCR
