736
CHAPTER 19 Aldehydes and Ke tones: Nucleophilic Addition Reactions
SUMMARY AND KEY WORDS
acetal [R 2C{O R'bL 717
acyl group, 697
1,2- addition, 725
1,4-addition,725
aldehyde{RCHOl. 695
betaine, 720
Cannizzaro reaction, 724
carbanion, 708
conjugate addition, 725
cyanohydrin [RCH(OH)C
707
= NJ,
enamine (R2N- CR = CR 2),
710
hemiacetal,717
imine IR 2C=NR), 710
keton e (R2C= Ol. 695
Mcl.aftertv rearrangement,732
nucleophilic ad ditionreaction,
702
Schiff base, 710
Wittig reaction,720
Wolff-K ishner reaction,715
ylide, 720
Aldehyd es and keton es are among the m ost im po rtant of all compounds,
both in bio chemistry and in th e chemical inelust ry. Aldehyd es are normally prepa red in th e lab orat or y by ox ida tio n o f primary alcohols o r by partial reducti on
of es ters. Ket ones are sim ilarly prepared by ox idation o f seco ndary alcohols or
by addition of di organocopper reagents to acid chlorides.
Th e nucleophilic addition reaction is the m os t com mon reaction of aldehydes and ket ones. Many different kin d s of prod ucts can he prepared by nucl eoph ilic additi on s. Aldehyd es a n d ketones a re redu ced by Nal1 H4 or LiAIH 4 to
yield seco n da ry and primary a lco h o ls, respectively. Addition of Grignard
reagen ts to ald eh yd es and keto nes als o gives alcoh ols (tertiary and secondary,
respectively), and additi on of HCN yield s cyanohydrins. Prima ry am ines add to
carbonyl co m po un ds yieldin g im in es, and second ary arnin es yield enamines.
Reaction of an alde h yde or ket one with hyd ra zine and base gives an alkane (the
Wolff-Kishner reaction). Alco h ols acid to car bo nyl g roups to yield acetals,
wh ich are va luable as pr o tecting g ro u ps. Phosphorancs add to aldehy des and
ketones to give alkenes (th e Wittig reaction) in wh ich the n ew C=C bond in
the product is exactly where the C = O bond was in th e sta rti ng material.
Cl', {3-Uns a tura ted ald eh yd es and ketones oft en react with nucleop hiles to
give the product of conjugate addition, o r l,4-add ition. Parti cularly useful is
th e reaction with a dior gan ocopper reagent, whi ch results in th e addition or an
alkyl, ar yl, or alkenyl grou p to th e double bond.
IR spe ctroscopy is helpfu l for ide n tifyin g aldehydes and keto nes, Carbon yl
gro u ps ab sorb in the IR range 1660 to 1770 cm -I , with the exact position highly
dia gn osti c of the kind of car bo ny l gro up pr esent in th e m olecule. 13C NMRsp ectroscopy is also useful fo r ald ehyd es and ketones beca use their carbony l carbon s
sh ow resonances in the 190 to 2 15 8 ran ge. IH NMR is usefu l for aldehyde
- CHO protons, which absorb near 10 8. Aldehydes and ketones undergo two
ch aracte ristic kinds of fragmentation in the mass spect rometer: 0' cleavage and
McLafferty rearrangement .
SUMMARY OF REACTIONS
1. Preparation o f aldehyd es (Sect ion 19.2)
(a) Ox idati on o f primary a lco ho ls (Sec t ion 1 7. 7)
(h) Partial redu cti on of esters (Sectio n 19.2)
1. DISAH , tol uene
2. H30 +
+
R'O H
Summ ary of Reaction s
2 . Prepa ra tion of ke to n es (Sect io n 19 .2)
Diorgan o co pp er rea cti on with ac id ch lo rides
3. Reaction s o f aldeh yd es (Section 19 .3)
Oxida tion to give carboxylic acids
o
II
R'
/c~
~ OH
4. Nu cl eo phili c ad dit io n react ions of a ldehyde s and ketones
(a) Ad d ition of hyd rid e: alcohols (Sectio n \ 9.7)
H
1. NaBH 4 , ethanol
2. H30 +
R/
\ /
C
OH
' R'
(h) Addi tio n o f Grignard rea ge n ts: a lco ho ls (Sectio n 19 .7)
R" OH
1. R"M gX. et her
2. H30+
R/
\/
C
' R'
(e) Ad di tio n of He N: cyanohydrins (Sectio n 19.6)
HeN
CN OH
\ /
C
R/
' R'
(d ) Add itio n o f prim ary arnin es: im ines (Sectio n \9. 8)
o
II
>C,
R>
' R'
(e) Addi tion o f seco nd a ry am in es: en arn ines (Secti o n 19. 8)
o
R
"
/ C,
HNR '2
C
/ H
/ \
737
738
CHAPTE R 19 Aldehydes and Ket ones: Nucleophilic Addition Reactions
(f) Wo lff-Kishn er reaction (Sectio n 19 .9)
H H
\ /
R/
C
' R'
+
+
N2
H2 0
(g) Ad d it io n o f a lco h o ls: aceta ls (Sec tion 19.10)
o
R"O
II
R/
C
' R'
+
\ /
A cW
2 R"OH
C
R/
cat alys t
OR"
' R'
+
H20
(11) Ad d iti o n of phosph orus ylides : Wittig reaction (Sectio n 19.] I)
THF
S. Co n juga te ad d itions to O' ,.8-u nsa tu ra te d al de hyd es and keton es (Sectio n 19 .13 )
(a) Con jug ate ad ditio n of am ines
o
II
R/
/C ~
\/
/ C,
~ C/
/ 'H
NHR'
(b ) Conjuga te ad d ition of water
o
II
\ /
C
-: ' c/
j \
C
" OH
H
(c) Co n jug ate add ition of alky l groups: dio rgano copper reac tio n
1. R' 2CuLi , ether
2. H30 +