RCH 2 MgX
C OMgX
RCH 2
O
C OH
RCH 2
O
H 3 O +
CO 2
_ +
Carboxylic acid
Addition of organometallic reagent to nitrile: preparation of ketones
Grignard or organolithium reagent attacks nitrile to generate the magnesium
or lithium salt of imine. Acid hydrolysis of this salt generates a ketone.
Since the ketone is not formed until the work-up, the organometallic reagent
does not have the opportunity to react with the ketone.
RC N
C N
R'
R
MgBr
C O
R'
R
R'MgBr
H 3 O +
+ NH 4
+
+
Ketone
Nitrile
Addition of acetylides and alkynides to carbonyl compounds
Acetylide (RC À À
À À
À À CNa) and alkynide (RC À À
À À
À À CMgX and RC À À
À À
À À CLi) are good
nucleophiles. They react with carbonyl group to from alkoxide, which under
acidic work-up gives alcohol. The addition of acelylides and alkynides
produces similar alcohols to organometallic reagents.
R'C CNa
C C C R'
R
OH
H
C C C R'
R
O
H
C
O
R
H
+
_
H 3 O +
..
..
:
:
: :
R'C CMgX
R'C CLi
C C C R'
R
O
R
C C C R'
R
OH
R
C
O
R
R
H 3 O +
+
_
+
_
or
..
:
..
:
: :
Addition of phosphorus ylide to carbonyl compounds: Wittig reaction
Georg Wittig (1954) discovered that the addition of a phosphorus ylide
(stabilized anion) to an aldehyde or a ketone generates an alkene, not an
alcohol. This reaction is known as Wittig reaction.
C P(Ph) 3
R'
R'
O
R
Y
R
R'
Y
R'
Y = H or R
+ Ph 3 P=O
_
Alkene
Phosphorus ylide
Triphenylphosphine
oxide
+
+
Preparation of phosphorus ylide Phosphorus ylides are produced from
the reaction of triphenylphosphine and alkyl halides. Phosphorus ylide is a
5.3 ADDITION REACTIONS
215