Aldehydes and ketones can undergo a variety of reactions, but they share many chemical properties with other related compounds such as acids and esters. It is possible to detect the presence of an aldehyde or ketone by reaction with 2,4 dinitrophenylhydrazine this test only gives a positive result with aldehydes and no reaction with ketones, nevertheless carry out the tollen’s reagent to find out whether the ketone is present.
Alcohols are hydrocarbons with -OH replacing one or more hydrogen atoms. Alcohols have physical properties intermediate between those of hydrocarbons and water.
Depending on where the – OH group appears, alcohols are classified as primary (1 o) [on a carbon attached to only one other carbon], secondary (2 o) [on a carbon attached to two other carbons], or tertiary (3o)[on a carbon attached to three other carbons].
It is important that test tube is dry in order to carry out a test of finding out the presence of OH group, since water contains an -OH group and can therefore produce fumes of HCl which would invalidate the test.
As the addition of phosphorus pentachloride is not a specific test for alcohols only. The usual oxidising agent is acid. K2Cr2O7 which turns from orange to green (Cr3+). The reactions involve the loss of the hydroxyl hydrogen together with a hydrogen atom from the adjacent alkyl group.
With primary alcohols, the initial aldehyde product may be oxidised further to a carboxylic acid, particularly if heat or excess oxidising agent is used, but ketones cannot be readily oxidised (involves breaking C-C bond).
carboxylic acids
Carboxylic acids are weak acids
CH3COOH + H2O CH3COO- + H3O+
Neutralized by a base
CH3COOH + NaOH CH3COO- Na+ + H2O
The reaction with sodium hydrogencarbonate is used as a test for carboxylic acids. All carboxylic acids tend to react with sodium hydrogencarbonate to release carbon dioxide, this then distinguishes them from weaker acids such phenols.
Esters are most often identified by their “fruity” odors but some are rather unpleasant smelling, so this is no sure guide. Esters react with hydroxylamine hydrochloride to yield a compound which can complex with Fe 3+ to give a colored species Esters react with water and acid catalyst Split into carboxylic acid and alcohol
O
?? H+
H – C-O-CH2CH3 + H2O O
??
H – C-OH + HO-CH2CH3
Aim: To identify 6 organic unknowns (Aldehyde, ketone, ester, alcohol, carboxylic acid, phenol).
General apparatus:
* 2,4-dinitrophenylhyrazine.
* Methanol.
* Pipette
* Test tubes
* Silver nitrate solution.
* Sodium hydroxide solution
* Ammonia solution
* Measuring cylinders.
* Phosphorus pentachloride
* Concentrated sulphuric acid
* Litmus paper
* Small beaker
* Stop watch
* Ethanoate
* Water bath
* Spatula
* Ethanoic acid
Safety considerations:
General safety precaution
* Lab coats must be worn.
* Approved safety goggles (NOT safety glasses) are to be worn continuously while you are in the laboratory. Safety goggles will protect your eyes against impact and splashes.
* If you should get a chemical into your eyes, wash with flowing water from the eyewash for approximately 15-20 minutes, and notify your teaching assistant.
* Never use an open flame when working with organic solvents.
* Take special care when working with strong acids or strong bases. Contact with these materials can cause severe chemical burns.
* Exercise great care in noting the odour of fumes and, whenever possible, avoid breathing fumes of any kind.
CHEMICAL RISK ASSESSMENT:
Chemical name
Chemical hazard
Safety precautions
2,4-DINITROPHENYLHYDRAZINE
TOXIC
Toxic by skin contact and if swallowed. Skin is stained yellow on contact which can be followed by dermatitis.
If dust or solution gets in eyes: Flood the eye with gently running tap water for 10 minutes. Seek medical attention.
If spilt on skin or clothes: Brush off as much solid as possible. Flood affected area with large quantities f water. Remove contaminated clothing, sock and rise repeatedly.
PHOSPHORUS (V) CHLORIDE
CORROSIVE
This solid causes burns. Severe irritations.
If dust or liquid gets in eyes: Flood the eye with gently running tap water for 10 minutes. Seek medical attention.
If spilt on skin or clothes: quickly brush off any solid. Flood affected area with large amount of water. Remove contaminated clothing.
SILVER NITRATE
CORROSIVE
Causes burns.
If spilt on skin or clothes: Flood the eye with gently running tap water 10 minutes. Seek medical attention.
IRON (iii) CHLORIDE
IRRITANT
Irritating to the eyes and skin. If left unattended can cause severe burns to the eyes.
If solution is spilt on skin or clothes: Remove contaminated clothing. Wash affected area thoroughly .
POTASSIUM DICHROMATE
TOXIC
If contact with the skin it causes irritation
If split on skin or clothes: Brush off as much solid as possible. Flood affected area with large quantise of water. Remove contaminated clothing, soak and rinse repeatedly until no yellow colour remains in water.
PHENOL
TOXIC
Toxic in contact with skin, causes burns
If spilt on skin or clothes: Flood the eye with gently running tap water 10 minutes. Seek medical attention. Remove all contaminated clothing as quickly as possible.
TEST FOR ORGANIC COMPOUNDS:
* Test 1 for aldehydes and ketones:
Detailed method:
Reaction with 2,4 dinitrophenylhydrazine solution:
If the unknown substance to be tested is a solid, dissolve approximate 0.5g in the minimum volume of methanol. To 1 cm3 of the reagent add several drops the test substance (or its solution in methanol)
Expected observation:
Yellow or yellow-orange precipitate shows that both aldehyde and ketones are present.
Test 2:Using tollen’s reagents
Preparation of the reagent:
* To about 5cm3 of silver nitrate solution (1M).
* Add a few drops of sodium hydroxide solution (2M).
* A grey precipitate is formed.
* Add ammonia solution (2M) drop wise till the precipitate just dissolves.
The test substance must either be a liquid or in aqueous solution. To the tollen’s reagents add one or two drops of the unknown and place the test tube in a beaker of hot water for a few minutes.
Expected observation
Observation with a positive test for the aldehyde: silver mirror on the inner surface, or even a dark grey precipitate of metallic silver produced.
* Test 3 for alcohol:
Balanced equation: (ethanol)
CH3CH2OH + PCl5 CH3CH2Cl + POCl3 + HCl
Detailed method:
Reaction with phosphorus halides:
* In a fume cupboard: to 1 cm3 of the unknown in a dry test-tube carefully add a spatula load of phosphorus pentachloride.
Expected observation:
Observation with a positive test: steamy fumes of HCL gas which turns damp blue litmus paper red.
To confirm:
* Put 1cm3 of the substance into a test tube.
* Add 1 cm3 of ethanoic acid and a few drops of concentrated sulphuric acid.
* Place the tube in the water bath for about 5 – 10 min.
* Pour the mixture into about 10cm3 of water into a small beaker.
* Fruity odour of ethyl acetate (ethanoate) should be apparent.
* Test 4 for carboxylic acid:
Balanced equation:
CH3COOH + NaHCO3 CH3COO- Na+ + CO2 + H2O
Sodium
Hydrogencarbonate
Detailed method:
* Add a load of the unknown into a test tube.
* Add 2cm3of sodium hydrogen carbonate to the unknown.
* Observe the reaction.
Expected observation
Observation with a positive test: CO2 gas which turns lime water milky.
* Test 5 for ester:
Detailed method:
* In a test tube add a spatula load of the unknown.
* Add 3 drops of concentrated sulphuric acid.
* Add 1cm3 of ethanoic acid.
* Place the test tube in a hot water bath for ten minutes.
* Pour out onto a beaker with cold water, then smell cautiously.
* Observe the reaction.
Expected observation:
Observation with a positive test: Insoluble layer which floats on water and sweet ‘gluey’ smell.
* Test 6 for phenol:
Reactions of neutral iron (iii) chloride
Preparing the reagent:
To a solution of iron (iii) chloride add sodium carbonate solution dropwise until a faint precipitate of iron (iii) hydroxide is obtained. This then becomes neutral iron (iii) chloride. If too much sodium carbonate is added, iron (iii) chloride solution may be added to the mixture until only a faint precipitate remains.
Detailed method:
* To about 1g of the unknown add the neutral iron (iii) chloride solution until no further colour change, if any, occurs.
Expected observation:
Observation with a positive test: a violet colour, this colour is probably due to a complex of Fe3+ with phenol.
Book title
Author
Chapter
Page number
Year published
Advanced chemistry (organic and inorganic)
Philip Mathew
116
786
1992
Chemistry 2 Advanced sciences
Brian Ratcliff
Helen Eccles
3
25
2001
Chemistry Palgrave foundation
Rob Lewis
Wynne Evans
18
332-339
2001
Chains/rings
Brian Ratcliff
1
2,51,67
1996
Organic chemistry, energetics and equilibrium
Brian Chapman
Alan Jarvis.
5
54
2000
BIBLIOGRAPHY:
Table to record observation:
Unknowns
Test 1: for Aldehydes and ketones
Test 2:Using tollen’s reagents
Test 3: for alcohol
Test 4: for carboxylic acid
Test 5: for ester
Test 6: for phenol
A
No observable
Reaction when 2,4 DNP was added.
Due to no observable reaction when the test for Aldehyde and ketone was carried out, the confirmation test isn’t necessary
there was an effervescent when potassium dichromate was added it also turned orange to green.
B
Yellow ppt formed when 2,4 DNP was added drop by drop.
Dark grey ppt was formed when silver nitrate was added. This happened with in 10 minutes in the water bath.
C
No observable
Reaction when 2,4 DNP was added.
Due to no observable reaction when the test for Aldehyde and ketone was carried out, the confirmation test isn’t necessary
Slight effervescent and no further observations
Effervescent of carbon dioxide and turns lime water milky
D
Yellow ppt formed when 2,4 DNP was added drop by drop.
No observable reaction
E
No observable
Reaction when 2,4 DNP was added.
Effervescence shows OH group present, turned universal indicator to purple.
No observable
Reaction
Positive insoluble layer of fat seen when poured out into a beaker and a smelly odour.
F
No observable
Reaction when 2,4 DNP was added.
A slight effervescent
No observable
Reaction.
No observable
Reaction
When neutral iron (iii) chloride added it turned violet colour. Also you can read about Ka of ethanoic acid
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