Aldehydes and Keytones Flashcards
What is Tollen’s reagent?
An ammoniacal solution of silver nitrate.
Give the method of using Fehling’s/Sandelle’s reagent.
Add 1cm3 if Fehling’s A and 1cm3 of Fehling’s B to a test tube
Add 10 drops of the test solution
Leave in water bath at 80•c for 10minutes
Name the mechanism of hydrogen cyanide and carbonyls.
Nucleophilic addition.
What functional group reacts positively in the iodoform test?
Methyl group attached to a carbonyl group.
What’s the difference between the structures of aldehydes and ketones?
The C=O group must be on the outer carbons, whereas a ketone has the C=O on any of the middle carbons.
Give the method of using Tollen’s reagent.
Add 1 drop of 0.4M NaOH to 1cm3 of 1.0M silver nitrate solution in a test tube. A precipitate will form.
Add 1M ammonia solution until the precipitate dissolves.
Add 4-5 drops of your test solution. Shake and leave in an 80•c water bath for 10 minutes. Do not shake again.
Give the equation for Fehling’s reagent test.
CH3COH + [O] (arrow) CH3COOH (works with alcohol too)
2Cu)+2 + 2e- (arrow) Cu2O(s
What’s the correct chemical name for Brady’s reagent?
2, 4 dinitrophenylhydrazine
Give the method of the iodoform test (haloform reaction).
Put 2cm3 of iodine solution into a test tube
Add 1M NaOH drop wise until the iodine colour discharges
Add 5 drops of the test solution and place into a water bath
Give the method to test for aldehydes/ketones using Brady’s reagent.
Add 2cm3 of Brady’s reagent to a test tube
Add 3-4 drops of the test solution and leave for several minutes
A precipitate or some crystallisation should form, dry them out
Calculate the melting points for each and look them up in a data booklet
What are the observations in the iodoform test?
Aldehyde - yellow precipitate/antiseptic smell
Ketone - slightly murky/different antiseptic smell
What are the reaction conditions for Tollen’s reagent?
Mild oxidising
What are the reaction conditions for Fehling’s reagent?
Mild oxidising
Name and describe the 3 chemical tests that allow you to distinguish between aldehydes and ketones.
Tollen’s reagent/silver mirror test
•aldehyde - silver mirror/black precipitate
•ketone - no change/negative result
Fehling’s reagent
•aldehyde - green precipitate/brick red precipitate
•ketone - negative result
Brady’s reagent
•aldehyde - red precipitate
•orange crystallisation
Note: this only proves that there’s a carbonyl group, to work out which is which you need to filter them, dry out the crystals and work out the melting point.
What is the name of the reaction mechanism between an aldehyde and hydrogen cyanide?
Nucleophilic addition
