Transcript reactions of the carbonyl group in aldehydes and ketones

REACTIONS OF AlDEHYDES AND
KETONES
L.O.:
 Outline the mechanism for nucleophilic addition
reaction of aldehydes and ketones with hydrides.
Nucleophile: is a atom or groups of
atoms attracted to an electron-deficient
centre, where it donates a pair of
electrons to form a new covalent bond
A curly arrow is a symbol used in reaction
mechanisms to show the movement of an
electron pair in the braking or forming of a
covalent bond
Mechanism:
Reduction of an aldehyde by nucleophilic addition
The reduction of an aldehyde
You get exactly the same organic product whether you use
lithium tetrahydridoaluminate or sodium tetrahydridoborate.
For example, with ethanal you get ethanol:
Reduction of an aldehyde leads to a primary alcohol.
Reaction summary:
Reduction:
Sodium borohydride (NaBH4) is a reducing agent. It provides a source of
hydrogen and also allows hydrogen to act as a nucleophile by creating H-.
It can be represented by [H].
Many reducing agents will reduce ketones and
aldehydes to alcohols.
NaBH4 (sodium tetrahydroborate(III) ) generates
the nucleophile H-, the hydride ion.
Write the mechanism of the reaction of a
ketone/aldehyde with H-.
Will NaBH4 react with an alkene?
NO! H- is repelled by the electron density of C=C.
CH2 = CHCHO
+
2[H]
———>
CH2 =CHCH2OH
CARBONYL COMPOUNDS - REDUCTION
Example
COMPOUND X
What are the products when Compound X is reduced?
H2
NaBH4
CARBONYL COMPOUNDS - REDUCTION
Example
What are the products when Compound X is reduced?
COMPOUND X
H2
NaBH4
C=O is polar so is attacked by the nucleophilic H¯
C=C is non-polar so is not attacked by the nucleophilic H¯
Page 43, Q5