Aldehydes and ketones Flashcards
Aldehyde test- tollens reagent I
- add solution of silver nitrate containing Ag+ ions to solution of excess ammonia to form a complex
Complex is added to aldehyde and is warmed gently - it is oxidised to the corresponding. Carboxylic acid
Ag+ ions are reduced to Ag - REDUCATION RXN
SILVER MIRROR RESULT SEEN
Doesn’t work for ketones
Aldehyde test- fehlings
It is a complex containing copper (II) sulfate containing Cu2+ ions, which are BLUE
Complex is added to the aldehyde which is oxidised to a carboxylic acid
Copper sulfate is reduced to copper oxide containing cu+ ions which are red
REDOX RXN- COLOIR CHANGE BLUE TO RED
doesn’t work for Ketones
Triiodomethane / iodoform
Add unknown compound to a solution of iodine and sodium hydroxide
A yellow ppt and antiseptic smell due to. Triiodomethane forming
A positive result is only seen if CH3CO is present
Ethanal is only aldehyde to give positive result
Ketones can have positive results
Name tests for to distinguish between aldehydes and ketones
And why?
Fehlings reagent
Tollens reagent
As they both don’t work for ketones only aldehydes
Give the observation of an aldehyde with tollens reagent ?
A silver mirror
What is the observation of an aldehyde with fehlings?
Blue to red colour change
What type of reaction is tollens test?
Redox
What type of reaction is fehlings test?
Redox reaction
What is 2-4-DNP also known as?
What is it a test for?
Brady’s reagent
Both aldehydes and ketones
2-4-DNP test to identify aldehyde and ketones
Add Unknown to 2,4-DNP In methanol or dilute acid
Orange crystals formed
Filter and wash cold solvent
Dry and take the MP and compare it with literature value
Describe 2-4-DNP
Aldehydes and ketones react to produce orange crystals (hydrazine deravatives)
It is a NUCLEOPHILIC addition + elimination RXN
Also called condensation as a molecule of water is eliminated
MPS of crystals are high due to more IMF (VDW+H bonds)- these MPS can be used to identify the original carbonyl compound by comparing it with literature value
If product is pure then MP is sharp
If product is impure then MP is over a range
How can the original carbonyl compound be identified in 2,4-DNP?
The MPS of the products can be used to identify original carbonyl compound by comparing it with the literature value
What type of reaction is aldehyde and ketone test 2,4-DNP?
NUCLEOPHILIC addition and elimination
What is a test for both aldehydes and ketones
2,4-Dinitrophenylhydrazine
What is formed in:
Tollens test
Fehlings test
Triiodomethane / iodoform
2,4-DNP
Tollens: silver mirror
Fehlings: COLOIR change blue to red
Iodoform: yellow ppt if positive result occurs
2,4-DNP: orange crystals
Give a test to distinguish between aldehyde and ketone
Fehlings reagent- add both to this complex which contains copper sulfate (Cu2+ ions which are blue), when aldehyde Is gently heated is oxidises to carboxylic acid- and there’s a blue to red colour change with aldehyde as the cooper sulfate reduced to Cu+ ions which are red. Ketones won’t react with fehlings reagent
What is a positive result when aldehyde or ketone reacts with a reagent
(Name reagent also)
Reagent is sodium hydroxide (aq) and iodine
Positive result is CH3CO
What produces orange crystals?
What type of reaction is this?
Aldehydes and ketones do with 2,4-DNP
This is a NUCLEOPHILIC addition / elimination reaction
What is produced in the addition of hydrogen cyanide and aldehyde or ketones
2-hydroxynitrile
Step by step of hydrogen cyanide adding to an aldehyde or ketone
HCN Ionises or disassociates into a H+ ion and cyanide ion - Heterolytic fission
The C on cyanide ion acts as a NUCLEOPHILE attacking the C on the carbonyl- NUCLEOPHILIC addition
A negative charge is produced in the oxygen and electron density increases - the oxygen becomes an electron pair donor attracting the H+ from the solvent eg water or the original HCN
A molecule of HCN has added across the C=O producing a hydroxy nitrile
Type of reaction occurring in 2,4-DNP?
NUCLEOPHILIC addition and elimination
What is the only aldehydes to produce a positive result with Triiodomethane/iodoform (NaOH(aq)/I2) ?
Ethanal
What is the positive result group for an aldehyde or ketone with iodine and sodium hydroxide (aq)?
CH3CO
Why is is better to use one step processes in synthesis ?
Better yield (in each stage there is waste products)
Why are 2 step processes carried out despite them being not as good as 1 step?
Because it’s cheaper
