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Aldehydes and ketones Chapter 15
Aldehydes and ketones
Chapter 15
The carbonyl group• Aldehydes and ketones are among the first examples of compounds that
possess a C-O double bond that we’ve seen (oxidation of alcohols section, Ch-14).
• This group is called a carbonyl group, and it has very different chemical properties than a C-C double bond in alkenes:
• Because oxygen is more electronegative than carbon, the bond is polar.• Bond angles are about 120o around the carbon atom (see VSEPR theory).
+ C O
The carbonyl group
• The local geometry around the carbonyl group is trigonal planar. The rest of the molecule doesn’t have to be planar:
HH
H C H 3C H 3
CH 3C H 3
CH 3
CC
O
CC
Local trigonal planar geometry
Compounds containing the carbonyl group
• The following classes of organic compounds involve the carbonyl group:– Aldehydes have a H-atom or a carbon substituent (alkyl,
cycloalkyl, aromatic) bound to a CHO group (carbonyl group bound to a H-atom):
C H
O
R
C H
O
C H
O
CH 3C H
O
H
General formula for aldehyde:
Compounds containing the carbonyl group
• Ketones have two carbon substituents (akyl, cycloalkyl, aromatic and not necessarily the same)
R'C
O
R
CH2CH3 C H
C H 3
C H 3
C
O
C
O
CH 3C
O
CH 3
General formula for ketones:
Compounds containing the carbonyl group
• Carboxylic acids have an OH (hydroxyl) group bound to the carbonyl carbon, in addition to either a H-atom or a carbon group (alkyl, cycloalkyl, aromatic):
C O H
O
R
C O H
O
C O H
O
H C H
C H 3
C H 3 C H 2 C O H
O
C O H
O
CH 3
General formula for carboxylic acids:
Compounds containing the carbonyl group
• Esters have a carbonyl group singly bound to an oxygen, which in turn is bound to a carbon group (alkyl, cycloalkyl, or aromatic). The other bond to the carbonyl is either to a H-atom or another carbon group:
R'OC
O
R
CH 2
CH 3
CH
CH 3
OC
O
CH 2 C H 3OC
O
CH 3O C H 3C
O
H
General formula for an ester:
Compounds containing the carbonyl group• Amides are the first nitrogen-containing organic compounds we’ve seen.
In these compounds, the carbonyl group is bound to a nitrogen (an amino group), in addition to either a H-atom or a carbon group (alkyl, cycloalkyl, aromatic). The R’ and R” groups of the amino group may either be H or carbon groups:
R"
R'
C N
O
R
C NC N C H 2CH 3 CH 2 C NH 2
O
C H 3
HO
C H 3
CH 3
CH 3O
H
General formula for an amide:
Aldehyde and ketone functional group
• As we saw, alcohols can be used to create aldehydes and ketones. Oxidation of a primary alcohol yields an aldehyde:
• And oxidation of a secondary alcohol yields a ketone:
[O]CH 2CH 3 C H
O
CH 2CH 3 C
H
H
O H
[O]CH 2CH 3 C CH 3
O
CH 2CH 3 C
H
CH 3
O H
Aldehyde and ketone functional group
• Cyclic aldehydes are not possible, because in order for the carbonyl group to be part of the ring structure, two bonds to carbon groups would be required.
• Aldehydes may incorporate ring structures, but not be part of the ring.• Also, note that cyclic ketones aren’t heterocyclic compounds.
a cyclic ketone
a cyclic diketone
an aldehyde incorporatinga cyclic compound
C H
O
O
OO
C H
C H 3
O
H
C H 3
C H
O
C H 2C H 2C H
O
C H 2C H 3
Nomenclature for aldehydes• IUPAC rules:
– Select as the parent chain the longest continuous chain that includes the carbonyl carbon
– Name the parent chain by changing the corresponding alkane name (ending with “e”) to an ending with “al”
– Number the parent chain assuming the carbonyl carbon is C-1– Identify substituents on the parent chain as before, at the beginning
of the compound’s name.
Propanal
4-Methylpentanal2-Ethylpentanal
Nomenclature for aldehydes• For aldehydes having short carbon chains, the following
common names are usually encountered:
• The following aromatic aldehyde is called benzaldehyde:
Formaldehyde(Methanal)
Acetaldehyde(Ethanal)
Propionaldehyde(Propanal)
Butyraldehyde(Butanal)
C H
O
C H 2C H 3C H
O
C H 3 C H
O
C H 2C H 2C H 3CH H
O
CH
O
4-Bromobenzaldehyde 4-Hydroxy-2-methylbenzaldehyde
OH
C H 3
CH
O
Br
CH
ODerivatives:
IUPAC
Benzaldehyde
Nomenclature for ketones• IUPAC:
– Select as the parent chain the longest continuous chain that involves the carbonyl carbon
– Name the parent chain by removing the “e” from the corresponding alkane name and adding “one”
– Number the chain to give the carbonyl group the lowest numbering. The number goes before the parent chain name
– Determine the number and location of substituents and number them accordingly
– For cyclic ketones, the carbonyl carbon is C-1 and the name begins with “cyclo”
3-Hexanone
4-Methyl-2-hexanone 3-Bromo-2-butanone
2-Methylcyclopentanone
CH 3 O
C H
C H 3
Br
C
O
C H 3
C H 2 C H 3
C H C C H 3
O
C H 2C H 3C H 2 C H 3C
O
C H 2C H 2C H 3
Nomenclature for ketones
• The common system of naming ketones is similar to what we saw for ethers:
Ethyl propyl ketone
Isobutyl methyl ketone 1-Bromoethyl methyl ketone
C H
C H 3
C C H 3
O
C H 2C H 3C H 2 C H 3C
O
C H 2C H 2C H 3 C H
C H 3
Br
C
O
C H 3
Isomerism for aldehydes and ketones
• Aldehydes and ketones that have a given number of carbon atoms are functional group isomers. (This is the third group of compounds we have seen that have this relationship; others were alcohols/ethers and thiols/thioethers)
Propanone Propanal
C3H6O
C H 2
C H 3C
O
HC
C H 3
O
C H 3
a ketone an aldehyde
Isomerism for aldehydes and ketones
• Positional isomers are possible for ketones (but not aldehydes)
• And skeletal isomers are possible for both
3-Pentanone
C5H10O
2-Pentanone
C H 2
C H 3C H 2
C
O
CH 3C H 2
C H 3
C H 2
C H 3 C
O
C5H10O
2-Pentanone3-Methyl-2-butanone
CH
CH 3
C H 3C
O
C H 3
C H 2
C H 3C H 2
C
O
C H 3
Common aldehydes and ketones
• Aldehydes are often recognizable by their “sweet” smells:
Vanillin(vanilla flavoring)
Benzaldehyde(almond flavoring)
Cinnamaldehyde(cinnamon flavoring)
C
O
HC HC HC
H
O
OC H 3
OH
CH
O
Common aldehydes and ketones
• Some ketones (e.g. acetone) have a “sweet” smell also). Other examples are:
2-Heptanone(clove flavoring)
Butanedione(butter flavoring) Carvone
(spearmint flavoring)
C H 3
O
C H 3 C H 2
C
C H 3
O
CC H 3
O
C H 3
C
O
CC H 3(CH2)4
Naturally occurring aldehydes and ketones
• A wide variety of biologically relevant molecules possess aldehyde and/or ketone functional groups:
Testosterone
Progesterone
CortisoneO HO
C H 3
CH 3
O
O
CH 3
O
C H 3
C H 3
O
O HC H 3
CH 3
O
CH2OH
D-Glucose
CH2OHC
H
C
H
C
H
C
H
O HO H
O H
O H
C
H
O
Physical properties of aldehydes and ketones
• Neither aldehydes nor ketones possess the ability to H-bond with other molecules like themselves. Consequently, boiling points for aldehydes and ketones are lower than for alcohols of similar molar mass.
• The C-O double bond in these molecules is polar, so dipole-dipole forces do exist. As a result, their boiling points tend to be higher than for alkanes of similar molar mass.
+ C O
Physical properties of aldehydes and ketones
Physical properties of aldehydes and ketones
• Water molecules can interact (H-bond) with the non-bonding pairs of the carbonyl group oxygen atom, enabling aldehydes and ketones that have small carbon chain components to be water-soluble.
• As we saw for alcohols, the greater the carbon chain length, the lower the water-solubility (makes the molecule less polar)
H-bond
..
....
..
H
OH
OC
Physical properties of aldehydes and ketones
Physical properties of aldehydes and ketones
Comparing an aldehyde and a ketone of a given number of C-atoms, the ketoneis generally more soluble. Why?
Preparation of aldehydes and ketones
• We saw already (in Ch-14) how alcohols can be oxidized to form aldehydes and ketones.
• Primary (1o) alcohols are oxidized to aldehydes (and subsequently to carboxylic acids)
• Secondary (2o) alcohols are oxidized to ketones
1o alcohol
[O]
[O]
2o alcohol
aldehyde
ketone
H
O
CR
H
H
O H
CR
O
CR
H
O H
CR R'R'
[O] = KMnO4 or K2Cr2O7
Oxidation and reduction of aldehydes and ketones
• Aldehydes can be oxidized easily to carboxylic acids
• Ketones are resistant to oxidation.
Oxidation reactions
aldehyde
[O]
[O]
ketone
no reaction
carboxylic acid
O
CR
O H
O
CRH
O
CR
R'
Oxidation and reduction of aldehydes and ketones
• There are several tests that have been developed to determine the presence of aldehydes, based on their oxidation to carboxylic acids:– Tollen’s test
– Benedict’s test
Oxidation reactions
Ag+
NH3, H2Oheat
Ag
silver metalcarboxylic acidaldehyde
H
O
CR O H
O
CR+ +
Cu2+ Cu2O
reddish solidcarboxylic acidaldehyde
H
O
CR O H
O
CR ++
Oxidation and reduction of aldehydes and ketones
• Both aldehydes and ketones are easily reduced to alcohols with H2 in the presence of a catalyst (Ni, Pt, Cu).
Reduction reactions
Reactions of aldehydes and ketones with alcohols
• When aldehydes and ketones react with alcohols in the presence of an acid, the resulting product is called a hemiacetal. Hemiacetals can further react with alcohols to form acetals:
aldehyde or ketone + alcohol hemiacetal
hemiacetal + alcohol acetal
acidcatalyst
acidcatalyst
Hemiacetals
Reactions of aldehydes and ketones with alcohols
hemiacetalaldehyde alcohol
hemiacetal
ketone alcohol
R
O
C
H
OOR
HO
CR O
HO
C
O
H
R
HO
CR
H
O
HO
C
H
OR H
O
C
R'R"
R'
R"R"
R'
R'R'R'
+
+
A hemiacetal is an organic compound that possesses a carbon atom that is bound toan OH (hydroxy) group and an OR (alkoxy) group
Reactions of aldehydes and ketones with alcohols
• Hemiacetal formation can also involve a carbonyl group and OH group on the same molecule. Here is an important process which involves this reaction:
See this again in Ch-18
Hemiacetals
Cyclic hemiacetals are stable,unlike non-cyclic hemiacetals
Reactions of aldehydes and ketones with alcohols
• Hemiacetals can be converted to acetals in the presence of an alcohol and a catalytic amount of acid:
Acetals
H2O
acetal
H2O
acetalhemiacetal(derived from
a ketone)
hemiacetal(derived froman aldehyde)
HO
R C O
O
R C O
O
R
H
C OR
H
OC
HO
HO
O
H
R
HO
C
R'
R" R"' OH
R'
R"'
R"
R'
R"
R" OH
R"
R'R' +
++
+
Reactions of aldehydes and ketones with alcohols
• Indicate whether each of the following structures is a hemiacetal, acetal, or neither:
OH
C H 2C H 3
OH
C H 2
C H 3
C
OH
O
C H 3
C C H 3C H 3
C
C H 3
C H 3O
C H 3
C HC H 3
C H 3
O
Reactions of aldehydes and ketones with alcohols
• Acetals can be isolated and used in subsequent chemical reactions. (Hemiacetals are less stable and generally can’t be isolated.)
• If an acetal is treated with acid in the presence of water, a hydrolysis reaction occurs
Acetals
Hydrolysis reaction: a reaction of a compound with water in which the compound splits intotwo or more fragments.
H2O
acidcatalyst
acetal (derivedfrom an aldehyde)
acetal (derivedfrom a ketone)
ketone
aldehyde alcohol 1 alcohol 2
H2O
acidcatalyst
alcohol 2alcohol 1
H
O
R C O O HO H
O HO H
O
CR
O
R C O
H
O
CR
R"'
R" R"R'
R"'R"R'
R"'
R'
R" + +
+
+
++
Reactions of aldehydes and ketones with alcohols
• Draw the aldehyde/ketone and alcohols that will result when the acetals below are treated with acid/H2O:
aldehyde/ketone alcohols
C H
C H 3
C H 2C H 3
CC H 3
C H 2C H 3 O
O
C H 3
C H 2C H 3
OCH 3
C
C HC H 3
C H 3
O
H
O C H 3
C
C H 2 C H 3O
C H 3C H 2C H 3
Sulfur-containing carbonyl groups• Sulfur analogues of aldehydes and ketones are known. The sulfur
atom can either replace the carbon or the oxygen of the carbonyl group.
• In the first case, the resulting compounds are called thioaldehydes or thioketones, and these are generally unstable:
• In the second case, sulfoxides result:
a thioaldehyde a thioketone
R'C
S
RC H
S
R
a sulfoxideR'S
O
R
Sulfur-containing carbonyl groups• The best known example of a sulfoxide is Dimethyl sulfoxide
(DMSO), which is a sulfur analogue of acetone:
• DMSO is an excellent solvent; it can dissolve a wide variety of polar and non-polar substances.
DMSO acetone
C C H 3
O
CH 3S CH 3
O
CH 3
-
.
.
.
... .
.
..
.
.. .C H 3
O
SCH 3 C H 3
O
SCH 3+
