4.4 aldehydes and ketones Flashcards
How are aldehydes formed
Oxidation of primary alcohols by using acidified potassium dichromate.
Heat and distill to prevent further oxidation
How are ketones formed
Oxidation of secondary alcohols using acidified potassium dichromate.
How to distinguish against aldehydes and ketones
Tollens reagent; positive for aldehydes forming a silver mirror
negative for ketones
What is fehlings test used for
Distinguish between aldehydes and ketones
Aldehydes reduce Fehlings from blue to red-brown ppt, whilst ketones do not.
Which reducing agent is used for aldehydes and ketones
NaBH4 (aq) dissolved in water and methanol
What functional group is identified by iodoform test
Methyl carbonyl group
CH3-CO-R or CH30-CHOH-R
What is the positive result of an iodoform test
Yellow ppt
what does 2,4DNPH test for? And what’s the positive test
Presence of carbonyl groups in aldehydes and ketones.
Forms an orange/red solid.
What is observed when an alcohol undergoes oxidation by acidified potassium dichromate
Orange dichromate ion is reduced to green chromium ion (Cr3+)
Goes from orange to green
What are the reagents and conditions to form a hydroxynitrile from aldehyde and ketones
Nucleophllic substitution
NaCN added to sulphuric acid to produce hydrogen cyanide.
20*c
Why is hydrogen cyanide not added directly to aldehyde or ketones Aldehydes reduce for a nucleophillic addition reaction
HCN is very poisonous gas and hard to store.
What type of reaction occurs when aldehydes and ketones react with 2-4dnph
Nucleophillic addition elimation or also known as a condensation reaction.
How can 2-4dnph be used to identify specific aldehydes and ketones
Add 2-4dnph so an orange/red solid forms.
Filer and purity via recrystallisation.
Compare melting point of crystals with 2-4dinitrophenylhydrazones of all aldehydes and ketones
What reagents needed for iodoform test
Warm solution with iodine in aqueous sodium hydroxide (ALKALINE IODINE)
How does the addition of HCN to ethanal effect chain length
C-C chain length increases by one carbon due to the Carbon in CN.
Why are hydroxynitriles important
Hydroxynitriles can be hydrolysed to form hydroxyacids which are very useful and are commonly used in the cosmetics industry.
