Chapter 26 - Carbonyl compounds Flashcards
What symbol is used to represent oxidising agents (e.g. acidified dichromate (VI) ions)?
[O]
How can you distinguish between an aldehyde and a ketone?
An unknown carbonyl compound can be heated with acidified dichromate ions. If the colour changes from Orange to green, the compound has been oxidised and is therefore an aldehyde.
Why do aldehydes and ketones undergo nucelophilic addition ractions?
The polar C=O bond attracts nucleophiles.
δ+ δ-
C = O
What symbol is used to represent reducing agents (e.g. acidified dichromate (VI) ions)?
[H]
Aldehyde + 2[H] →
where [H] is sodium tetrahydridoborate (III), NaBH4
What is this type of reaction?
Aldehyde + 2[H] → Primary alcohol
Nucleophilic addition, the aldehyde is reduced by the NaBH4 via nucleophilic addiction, adding a hydrogen ion.
Ketone + 2[H] →
where [H] is sodium tetrahydridoborate (III), NaBH4
Aldehyde + 2[H] → Secondary alcohol
Aldehyde OR Ketone + HCN →
Aldehyde OR Ketone + HCN → Hydroxynitrile
Hydroxynitirile is an organic compound containing a -OH and -CN group.
What is the test used to detect a carbonyl group?
The compound is added to a solution of 2-4-dinitrophenylhydrazine (2,4.DNP).
In the presence of a C=O group, a yellow/orange precipitate forms.
How can the carbonyl compound from a carbonyl group test be identified?
The yellow/orange precipitate (the 2,4-DNP derivative) can be analysed to identify the carbonyl compound as follows.
- The impure solid is filtered and then recrystallised to produce a pure sample of the 2,4-DNP derivative.
- The melting point of the purified 2,4-DNP derivative is determined.
- The melting point is matched to a database.
What is the aldehyde group (-CHO) test?
The compound is warmed with Tollen’s reagent. In the presence of an aldehyde -CHO group, a silver mirror forms.
How can you distinguish between an aldehyde and a ketone using Tollen’s reagent?
Aldehydes form a silver mirror with Tollen’s reagent.
Ketones do not.
Describe the reaction of aldehydes with Tollen’s reagent.
Tollen’s reagent (ammoniacal silver nitrate) contains silver ions in aqueous ammonia.
- An aldehyde is oxidised by the Ag+ ions to a carboxylic acid:
RCHO + [O] → RCOOH - The Ag+ ions are reduced to form silver which shows up as a silver mirror:
Ag+(aq) + e- → Ag(s)
Describe the solubility of carboxylic acids.
Carboxylic acids are soluble in water.
The C=O and O-H bonds in carboxylic acids are polar allowing carboxylic acids to form hydrogen bonds with water molecules.
Describe the trend in solubility of carboxylic acids with increasing chain length.
Carboxylic acids are soluble in water.
The alkyl carbon chain is non-polar. Water solubility decreases as the carbon chain length increases and the non-polar alkyl group becomes more significant.
Carboxylic acids are ____ acids, ______ dissociating in water.
Carboxylic acids are weak acids, only partially dissociating in water.
They undergo the same neutralisation reactions as other acids.
