R.P 6 (Tests for alcohol, aldehyde, alkene and carboxylic acid) Flashcards
What is the test for alkenes
To 0.5 cm3 of bromine water in a test tube add a few drops of the unknown and shake.
Observation: alkenes should decolourise bromine water
What are the 2 tests for testing for an aldehyde
Tollens
Fehling’s
How do we make tollens
Add a few drops of NaOH to a solution of silver nitrate (forms a pale brown precipitate)
Add a few drops of dilute ammonia until the precipitate dissolves
This is tollens reagent
What are the practical steps in using tollens to distinguish between an aldehyde and ketone
- Add aldehyde/ketone to tollens in a test tube
- Place in a hot water bath
If an aldehyde is present, silver mirror appears
If ketone is present no silver mirror appears
Why do we not heat the test tube with a Bunsen burner when testing for aldehydes and ketones
Aldehydes and ketones are flammable
Write an equation for the reaction between aqeuous tollens and ethanal
State what occurs
CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+
Aldehydes only are oxidised by
Tollen’s reagent into a carboxylic acid and
the silver(I) ions are reduced to silver atoms
What are the conditions and positive test result for distinguishing between aldehydes and ketones using Fehling’s
Conditions: heat gently
Observation: Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.
Write an equation for the reaction between Fehling’s solution and ethanal
State what occurs
CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + Cu2O + 4H+
Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.
What are the practical steps in testing for carboxylic acids
To 0.5 cm3 of your unknown solution in a test tube add a small amount of sodium carbonate solid and observe.
Result carboxylic acids will fizz with sodium carbonate due to CO2 produced
Write an equation for the test for ethanoic acid
2CH3COOH + Na2CO3 —> 2CH3COONa + H2O + CO2
What is the reagent and positive result when testing for primary and secondary alcohol
Reagent:
Potassium dichromate and sulphuric acid
Result:
Orange to green colour change
What is the reagent and positive result when testing for a chloroalkane
Reagent:
Warm with silver nitrate
Result:
Slow formation of white
precipitate of AgCl
Which of these halogen precipitates are formed the quickest and which is formed from the most reactive halogenoalkane
AgI(s)- yellow precipitate
AgBr(s) – cream precipitate
AgCl(s) – white precipitate
AgI forms fastest
Then AgBr
Then AgCl
The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the haloalkane
What does the rate of the formation of a halide precipitate depend on
The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster
the reaction.
C-I is the weakest therefore AgI is produced the quickest
Write 2 equations for the formation of AgI through the hydrolysis of Iodoethane
CH3CH2I + H2O —> CH3CH2OH + I- + H+
Ag+(aq) + I-(aq) —> AgI(s)
