A - Aldehydes and Ketones Flashcards
What is the carbonyl functional group?
C=O
Where is the carbonyl group found in an aldehyde? How are aldehydes named?
C=O found at the end of the carbon chain.
Their names end in -al.
Where is the carbonyl group found in a ketone? How are ketones named?
In the somewhere in the middle of the chain.
Their names end in -one and often have a number to show which carbon the carbonyl group is on.
Aldehydes are readily oxidised to form what?
Carboxylic acids.
How is Tollen’s reagent used to distinguish between aldehydes and ketones?
Tollen’s reagent is a colourless solution of silver nitrate dissolve in aqueous ammonia.
When heated in a test tube with an aldehyde, the silver ions in Tollen’s reagent are reduced to silver atoms and a silver mirror forms.
A ketone cannot be oxidised by Tollen’s reagent so there’s no reaction and no colour change.
How is Fehling’s solution used to distinguish between an aldehyde and ketone?
Fehling’s solution is a blue solution of copper (II) ions dissolve in sodium hydroxide.
If it’s heated with an aldehyde, the copper(II) ions are reduced to a brick-red precipitate of copper(I) oxide.
Ketones don’t react with Fehling’s solution so there’s no reaction and no precipitate forms.
What is the name for NaBH4?
Sodium tetrahydridoborate(III) or sodium borohydride.
What are aldehydes and ketones reduced to?
What reducing agent is usually used?
What types of reactions are these?
Aldehydes are reduced to primary alcohols.
Ketones are reduced to secondary alcohols.
NaBH4 in aqueous solution.
Nucleophilic addition.
How can a reducing agent be indicated in equations?
[H]
What is the nucleophile from NaBH4?
H- ion.
Outline the nucleophilic addition reaction mechanism between an aldehyde and a reducing agent.
How does the mechanism differ with a ketone?
- H- ions from the reducing agent act as the nucleophile and attack the slightly positively charged carbon atom in the C=O bond in the aldehyde, donating a pair of electrons.
- This forms a bond between the carbon and hydrogen atom and a pair of electrons are transferred to the oxygen atom in the C=O bond.
- The oxygen donates a pair of electrons to a H+ ion which either comes from water or a weak acid that can be added as a source of H+ ions. This forms a hydroxyl group forming a primary alcohol.
The mechanism is the same with a ketone.
What is the overall balanced equation for the reaction between an aldehyde and hydrogen cyanide (HCN)?
What is the name of the product formed?
RCHO + HCN —> RCH(OH)CN
Hydroxynitrile.
Potassium cyanide dissociates in water to form what? Write an equation for this reaction.
K+ ions and CN- ions.
KCN —> K+ + CN-
Outline the nucleophilic addition mechanism for the reaction of KCN with an aldehyde or ketone followed by dilute acid.
- The CN- group attacks the partially positive carbon atom in the C=O bond and donates a pair of electrons.
- Both electrons from the double bond transfer to the oxygen.
- Dilute acid is added providing a source of H+ ions.
- The H+ ions add to the oxygen to form the hydroxyl group.
What are the overall reaction equations between KCN and an aldehyde, and KCN and a ketone?
Aldehyde:
RCHO + KCN —(H+)—> RCH(OH)CN + K+
Ketone:
RCOR’ + KCN —(H+)—> RCR’(OH)CN + K+
