Aldehydes and Ketones Flashcards
What polar group do aldehydes and ketones contain?
A carbonyl group (C=O).
Describe the C=O bond
It comprises a σ bond and a π bond above and below the plane of the sigma bond, caused by a p-p orbital overlap.
Describe an aldehyde
The carbon atom of the carbonyl group bonds to at least one hydrogen atom and has the suffix -al.
Describe a ketone
The carbon atom of the carbonyl group is directly bonded to two other carbon atoms and has the suffix -one.
What are two oxidising agents we use for oxidation reactions?
Acidified potassium dichromate (H+/Cr2O7^2-) and acidified potassium permanganate (H+/KmNO4).
What is the colour change seen in an oxidation reaction with acidified potassium dichromate?
Orange to green.
What is the colour change seen in an oxidation reaction with acidified potassium permanganate?
Purple to colourless.
Describe the oxidation of a primary alcohol
1° alcohol –> Aldehyde –> Carboxylic acid
Describe the oxidation of a secondary alcohol
2° alcohol –> Ketone
How is the oxidation reaction performed when requiring an aldehyde?
The aldehyde must be removed by distillation, otherwise all other reactions are performed under reflux.
What is the function of an ice bath in distillation?
It reduces the evaporation of volatile products.
How do you make Tollens reagent?
Add aqueous sodium hydroxide to a silver nitrate solution until a brown precipitate of silver (I) oxide is formed, then aqueous ammonia is added until the silver (I) oxide redissolves.
What does Tollens reagent test for?
Aldehydes.
What is the positive observation for Tollens reagent?
A silver mirror is produced, as the Ag+ ions are reduced to silver.
What does Fehlings reagent test for?
Aldehydes.
How do you make Fehlings reagent?
Using Fehlings A (aqueous copper sulphate) and Fehlings B (aqueous potassium sodium tartrate). On mixing, a deep blue solution of complex copper (II) ions are produced.
What is the positive observation for Fehlings reagent?
When warmed, the blue Cu2+ is reduced to a brick red precipitate of Cu+.
What is used as a substitute for Fehlings?
Benedict’s reagent as Fehlings decomposes easily, and it is safer and more stable as it is less alkaline.
What type of aldehyde does Fehlings not identify?
Aromatic aldehydes.
Describe the reduction of an aldehyde
Aldehyde –> 1° alcohol
Describe the reduction of a ketone
Ketone –> 2° alcohol
What are two reducing agents we use in reduction reactions?
Sodium tetrahyrdridoborate (III) (NaBH4) and lithium tetrahydridoaluminate (III) (LiAlH4).
Which reducing agent is preferred for reduction reactions?
Sodium tetrahydridoborate (III) as it is safer and can be used in aqueous conditions.
Describe the nucleophilic addition of aldehydes and ketones
The C≡N nucleophile attacks the electron deficient C=O and the C bonds to the Cδ+. The lone pair on the O attacks H+ and a nitrile is formed.
What are the conditions of nucleophilic addition of an aldehyde or ketone?
Aqueous potassium cyanide, sulphuric acid and heat.
What is Bradys reagent used to test for?
Aldehydes and ketones.
How do you make Bradys?
Dissolving 2,4-DNPH in acid.
What is the positive observation for Bradys?
Red-orange solid formed.
What can we do to further identify an aldehyde or ketone after using Bradys?
Comparing the melting temperature of the purified product with known values to identify the molecule present.
What is the function of iodoform?
An antiseptic.
What is iodoform used to test for?
A methyl carbonyl group (CH3C=O or CH3CH(OH)).
What is the positive observation for iodoform?
When the organic compound is warmed with iodine in aqueous sodium hydroxide, a yellow solid forms.
