ll.l Aldehydes ond ketones

l3.l Aldehydesand Ketones
595
ll.l Aldehydesond ketones
AIM: To describethe corbon-oxygenbond of the corbonylgroup
of oldehydesond ketones.
Focus
The general structures of
aldehydes and ketones are
similar.
The functional group known as the carbonyl group ( )C-O)-a
carbon
ato m and an oxygen atonxj oined by a double bond-is fciund in comp ounds
called aldehydes arrd ketones.
Structures
of aldehydes
and ketones
Aldehydes are organic compounds in which the carbonyl carbon-the carbon to which the oxygenk bonded-is always joined to at least one hydrogen.The general formula for an aldehyde is
Carbonr.l.
ox'gen \
t
O
- ^ - L ^ - . | grouP
--^..1a---zCatbonvl
R-C-H
I
Carbonyl/
carbon
This structural formula is often abbreviated to RCHO.
Ketones are organic compounds in which the carbonyl carbon isjoined
to two other carbons:
Carbonrtl.
orygen
\
O
-CarA
^ * L ^on1'l
. ^ , , group
^r - ^ , . ^
!1<-_-R-C-R
,(
Carbonyl/
carbon
The abbreviated form for a ketone is RCOR.
Note the similarity in structure of aldehydesand ketones. Becausethey
both contain the carbonyl group, the chemistry of aldehydesand ketones is
similar. Both aldehydes and ketones are highly reactive, but aldehydes are
generally the more reactive of the two classes.
Namtng
aldehydes
and ketones
The IUPAC system may be used for naming aldehydes. We must first
identify the longest hydrocarbon chain that contains the carbonyl carbon. The -e ending of the hydrocarbon is replacedby -al to designatean
aldehyde. Using the IUPAC system, we name the aldehydes methanal,
ethanal, propanal, butanal, and so forth. In naming substituted aldehydes, the longest chain is counted starting from the carbon of the aldehyde group.
,94
13 Aldehydesand Ketones
CHAPTER
I X A M P L EI 5 . I
Naminga substitutedaldehydeby the IUPACsystem
\.4lhatis the IUPAC name for the following compound?
cHzcH3
o
cH3ccH2cH2cH2c-H
I
CHs
soruTroN
The longest continuous chain contains sevencarbons, making heptane the
parent alkane. In counting the carbons in the chain, we number the chain
so that the carbon of the aldehyde group is at position J. The carbonyl carbon is understood to be at position 1 and will not appearin the final name
ofthe aldehyde.
6;
.f"""'
o
cH3-c-cH2cH2cH2c1-H
CH:
Obtain the aldehyde name by dropping the -e ending from the alkane name
and adding the ending -al; orn aldehyde is a heptanal. The substituents are
named in the same way as for hydrocarbons. The molecule has two methyl
groups in the 5 position of the parent chain, which are designated
5,5-dimethyl. Combine this with the parent name to obtain the complete
name of the aldehyde: 5,5-dimethylheptanal.
PRACTICE
EXERCISE
II.I
Give the IUPAC names for each of the following aldehydes.
(a) CHgCHTCHO
(b)
f",
cH3cH2cHCH2CHO
(c) CHaCHzCH2CH2CHO
(d)
cl
I
cH3cHCH2CHO
Ketones also can be named by the IUPAC system.We show that a compound is a ketone by changing the ending of the longest carbon chain that
containsthe carbonyl group from -eto -one,as demonstratedinTable 13.1.
If there are severallocations in the chain where the carbonyl group could be
placed, its position is designatedby the lower number.
E X A M P L EI 3 . 2
Namingketonesby the IUPACsystem
Using the IUPAC system, name the following two structural isomers of
c5H160:
o
cH3-c-cH2cH2cH3
o
cH3cH2-c-cH2cH3
l3.l Aldehydesand Ketones
Table lI.l
595
Sone CommonAldehydesand Ketones
formula
Condensed
formula
Structural
IUPACname
Commonname
methanal
formaldehyde
ethanal
acetaldehyde
propanal
propionaldehyde
butanal
butyraldehyde
benzaldehyde
benzaldehyde
3-phenyl-2-propenal
cinnamaldehvde
propanone
acetone
(dimethyl ketone)
butanone
methyl ethyl ketone
diphenylmethanone
benzophenone
(diphenyl ketone)
cyclohexanone
cyclohexanone
Aldehydes
o
HCHO
H-C-H
o
CH3CHO
cH3-c-H
o
CH3CH2CHO
cH3-cH2-c-H
o
CH3CH2CH2CHO
cH3-cH2-cH2-c-H
o
c6H5cHo
/
(
\ll
c6H5cH:CHCHO
/
(
o
\ \FCH:CH-C-Hll
)-c-H
\:/
Ketones
cH3cocH3
o
ll
cH3-c-cH3
o
cH3coc2H5
c6H5coc6H5
c6H1oo
cH3-c-cHz-cHs
o
II
o'o
G"
SOLUTION
In both compounds the longest chains that contain the carbonyl group
have flve carbons, making pentane the parent alkane. Indicate that the
compounds are ketones by dropping the -e ending from pentane and
adding the ending -one.The compounds are pentanones.Give the position
of the carbonyl $oups ilr the carbon chain. The numbering that gives the
lowest position number to the isomer on the left is 2 and on the right, 3.
596
CHAPTER
13 Aldehydesand Ketones
These compounds are 2-pentanone and 3-pentanone, respectively.
o
tl
cH3-c-cI{2cH2cI{3
2-Pentanone
o
tl
cH3cH2-c-cH2cH3
3-Pentanone
== PRACTICE
EXERCISE
IT.2
-=
..:-=.
= Give the IUPAC name for each of the following ketones.
-a=
.a=
(a)
a=-a
=
tl
=
cl{3cHrcHrccHrcl{,
=':
?:.
=
=
=-=
(b)
o
tl
cI{3ccHrcH3
=j.:
=
=
.=
(c)
gn,
g
ttl
cH3cHCI{2CCHs
Common names for aldehydes and ketones are frequently used.
common names for mettranal and ethanal arc formaldehydeand acett
hyde, respectively. The common names of the ketones are obtained
naming each of the alkyl grogps attached to the carbonyl carbon
adding the word ketone. The sole exception is dimethyl ketone, which
almost everyone caTlsacetone,aversatile solvent.
Write the structure for ethylisopropyl ketone.
sotuTtoN
The common name of a ketone indicates the alkyl or aryl groups bonded to
either side of the carbonyl group.Write a carbonyl group and attach ethyl
and isopropyl groups to obtain the structure of the ketone. ,
o
tl
cI{3cH2ccHCH3
CHr
EXERCISE
II.I
= PRACflCE
-=,
Write
the structure of each of the following compounds.
.=
(a)
(c) diethyl ketone
=
acetaldehyde
O) acetone
=;
(d) 3-methylbutanal
4..: