Aldehydes and Ketones Flashcards
How are aldehydes oxidised to carboxylic acids?
Heat aldehyde under reflux to produce carboxylic acids.
What is the oxidising agent?
Acidified potassium dichromate (VI)
K2Cr2O7
Why is the reaction done under reflux?
Because aldehydes have a lower boiling point than carboxylic acids so would condense and leave the reaction before it can be fully oxidised into a COOH
Test to distinguish between aldehydes and ketones?
Tollens reagent- Aldehydes form silver mirror on side of test tube, ketones remain colourless.
Feelings reagent - Aldehydes form red precipitate, ketones remain blue in solution.
Reduction of aldehydes and ketones using aq NaBH4
H:- nucleophile from reagent is added. Frist a salt is formed so add a dilute acid providing another H:- nucleophile releases the alcohol from the salt.
Why is the reduction reaction carried out in a solvent of dry ether?
Because NaBH4 is very reactive so will react violently with water and alcohols.
What mechanism reaction are these?
Nucleophilic Addition
Reaction of carbonyl compounds with KCN and dilute acids to form hydroxy nitriles
CN:- nucleophile from reagent and H+ nucleophile from acid. 2 steps. Chemical synthesis as the carbon chain is extended by one carbon atom to form hydroxynitrile.
What is formed when KCN and dilute acid is reacted with aldehydes and unsymmetrical ketones?
Mixture of enantiomers.
The CN can attack from either above or below the planar double bond, causing different enantiomers to be produced.
Why is KCN used not HCN?
HCN is hard to store and its a toxic gas that produces dangerous by-products when it reacts.
Overall equation for reduction reaction of ketones and aldehydes
Aldehyde + 2[H] -> 1° Alcohol
Ketone + 2[H] -> 2° Alcohol
Mechanism for reduction of ketones and aldehydes
Overall equation for reaction with HCN to form hydroxynitrile
Mechanism for reaction with KCN and dilute acid
