Aldehydes + Ketones
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What is a carbonyl group?
C=O
What is the functional group and general formula for an aldehyde ?
RCHO (Carbon with =O, -H and -R)
What is the functional group for a ketone ?
RCOR’ (C=O in the middle of chain)
How do you name aldehydes ?
-al suffix (C=O is the end of the chain)
How do you name ketones ?
-one suffix (designate what number C=O is on)
What kind of intermolecular forces do molecules with a carbonyl group have ?
VDW and permanent dipole-dipole forces due to the polarity of C=O (O is d-)
Are molecules with a carbonyl group water soluble ? Why?
Yes - they form hydrogen bonds between water molecules and oxygen of C=O As chain length increases, solubility decreases.
Which bond in carbonyl groups is typically involved in reactions ? Why?
C=O, due to polarity of the bond (large difference in electronegativity and electron density)
What is the strongest bond in carbonyl compounds ?
C=O
Draw / describe a mechanism for the nucleophilic addition of a carbonyl compound (Nu represents nucleophile)
• d+ on C, d- on O (C=O) • Arrow from lone pair of Nu to d+ C • Arrow from =O to O • This gives C-O- • Arrow from lone pair of O to H+
Why is the electrophilic addition of HCN to a carbonyl compound important ?
As it increases the length of the carbon chain by 1 - very useful
Will the product of HCN electrophilic addition to a carbonyl compound have optical isomers ? Why ?
• Yes they will • This is because in aldehydes / ketones the carbonyl group is planar, so the CN- can attack from either above or below • This results in two enantiomers.
What is the name of the compound when HCN is added to a carbonyl compound ?
Hydroxynitriles (Have both OH and CN groups)
What is Fehlings solution ? What colour is it ?
It is copper complex ions, blue.
What happens when an aldehyde is added to Fehlings solution ?
Cu is reduced to Cu+ ions -> colour change to brick red ppt
What happens when a ketone is added to Fehlings solution ?
No visible change -> stays blue
What is tollens reagant ?
Silver complex ions, colourless solution
What happens when an aldehyde is added to tollens reagent ?
Silver mirror forms as Ag+ is reduced to Ag(s)
What happens when a ketone is added to tollens reagent ?
No visible change
What is an example of an oxidising agent for alcohols and aldehydes ? What colour change does it undergo ?
• Acidified potassium dichromate (VI) - H2SO4 and K2Cr2O7 • Colour change from orange to green.
What is a reducing agent for aldehydes and ketones ? What ions does it release in solution ?
NaBH4, releases a H- ion
Draw / describe the mechanism for the reduction of an aldehyde
• Electrophilic addition • Arrow from H- lone pair to d+ of C on C=O • Arrow from C=O bond to d- O • This gives C-O- • Arrow from lone pair of O- to H+