Aldehydes and Ketones Flashcards
(19 cards)
General Formula
CnH2nO
Aldehyde functional group
-CHO
Ketone functional group
-CO-
Minimum amount of carbons in Ketone
3 - Propanone
5 or more require number position C=O
Physical Properties
Short chain are liquids
-perm dipole of C=O causes perm d-d attractions that req more energy to break them than VdW alone
Short chain are miscible in water
-perm dipole of C=O bond allows molecules to break H bonds between H2O molecules and form H bonds w H2O molecules
Can they be oxidised or reduced
Aldehydes undergo mild oxidation
Ketones cannot
Both can be reduced
What do they form from
Aldehydes: oxidation of primary alcohols
Ketones: oxidation secondary alcohols
Common Mild Oxidising Agents
Acidified Potassium Dichromate
orange to green
Tollens Reagent
ammoniacal solution of Silver (I) Nitrate
colourless to silver mirror
Fehlings Solution
copper(II) sulfate and sodium tartrate and NaOH
blue solution to red ppt
Equation formation of ppt with Fehlings solution
Cu2+ + e- -> Cu+
Equation of Tolens Reagent
Ag+ + e- -> Ag
Equation Acidified Potassium Dichromate
Cr2O7 2- + 14H+ +6e- -> 2Cr3+ +7H2O
Aldehyde/Ketone with Hydrogen Cyanide
CnH2nO + HCN -> cyanohydrin
H goes to =O
CN goes on to C
Optically Inactive Product
aldehydes and unsymmetrical ketones = racemic mixture
Mechanism A/K with HCN
Nucleophilic Addition
arrow from=O to O (delta negative)
arrow from :CN- to C(delta +)
arrow from :O- to H+
cyanohydrin
2,4-dinitrophenylhydrazine
reacts w/ both a/k to form 2,4-dinitrophenylhydrazone and water
Reaction of 2,4-dnp
condensation reaction
used as a method of identifying aldehydes and ketones
solid formed that can be purified and mp determined
comparison of mp with known values could identify aldehyde or ketone
derivatives al orange/yellow solids - solution often called Bradys Reagant
Preperation of 2,4-dnp derivative
mix A/K with 2,4-dnp solution
orange/yellow ppt forms - if not add some H2SO4 and warm
suction filter to obtain solid
Recrystallisation
-dissolve solid in a minimum vol of hot solvent (usually ethanol)
minimum used to ensure as much solute obtained as possible
-filter solution through fluted filter paper while hot
remove insoluble impuities, prevents precip of solid on cold surface of ff
-allow solution to cool and crystallise
- suction filter - obtain solid crystals - wash with cold water
- dry crystals between 2 sheets of filter paper
- determine melting point to check purity
Reduction Reactions
Both reduced to alcohols using LiAlH4
-dissolved in dry ether
-Reaction heated under reflux
CH3CHO + 2[H] -> CH3CH2OH
CH3COCH2CH3 +2[H] -> CH3CH(OH)CH2CH3
Why is it important lithal is dry
explosion on reaction with water
