Chapter 17 Aldehydes and Ketones
17.1 Nomenclature
Nomenclature of Aldehydes O
O H
H 4,4-dimethylpentanal O
5-hexenal O
HCCHCH 2-phenylpropanedial
Nomenclature of Aldehydes
O when named as a substituent formyl group
C
H
when named as a suffix carbaldehyde carboxaldehyde
Substitutive Nomenclature of Ketones
O
O
CH3CH2CCH2CH2CH3
CH3CHCH2CCH3 CH3
3-hexanone
H3C
O
4-methyl-2-pentanone
4-methylcyclohexanone
Functional Class Nomenclature of Ketones
O
O
CH3CH2CCH2CH2CH3
ethyl propyl ketone
CH2CCH2CH3
benzyl ethyl ketone
O divinyl ketone H2C
CHC CH
CH2
17.2 Structure and Bonding: The Carbonyl Group
Structure of Formaldehyde
planar
bond angles: close to 120° C=O bond distance: 122 pm
The Carbonyl Group
very polar double bond O 1-butene
propanal
dipole moment = 0.3D
dipole moment = 2.5D
Carbonyl group of a ketone is more stable than that of an aldehyde heat of combustion
O
2475 kJ/mol H 2442 kJ/mol O Alkyl groups stabilize carbonyl groups the same way they stabilize carbon-carbon double bonds, carbocations, and free radicals.
Spread is greater for aldehydes and ketones than for alkenes O H 2475 kJ/mol
Heats of combustion of C4H8 isomeric alkenes CH3CH2CH=CH2 2717 kJ/mol cis-CH3CH=CHCH3 2710 kJ/mol
trans-CH3CH=CHCH3 2707 kJ/mol O 2442 kJ/mol
(CH3)2C=CH2 2700 kJ/mol
Resonance Description of Carbonyl Group
••
•• –
O ••
•• O ••
C
C +
nucleophiles attack carbon; electrophiles attack oxygen
Bonding in Formaldehyde
Carbon and oxygen are sp2 hybridized
Bonding in Formaldehyde
The half-filled p orbitals on carbon and oxygen overlap to form a p bond
17.3 Physical Properties
Aldehydes and ketones have higher boiling than alkenes, but lower boiling points than alcohols.
boiling point –6°C O
49°C
OH
97°C
More polar than alkenes, but cannot form intermolecular hydrogen bonds to other carbonyl groups
17.4 Sources of Aldehydes and Ketones
Many aldehydes and ketones occur naturally
O
2-heptanone (component of alarm pheromone of bees)
Many aldehydes and ketones occur naturally
O H
trans-2-hexenal (alarm pheromone of myrmicine ant)
Many aldehydes and ketones occur naturally
O H
citral (from lemon grass oil)
Synthesis of Aldehydes and Ketones from alkenes
A number of reactions already studied provide efficient synthetic routes to aldehydes and ketones.
ozonolysis
from alkynes hydration (via enol) from arenes Friedel-Crafts acylation from alcohols
oxidation
What about..? aldehydes from carboxylic acids
R
O
O
C
C
OH
R
H
What about..? aldehydes from carboxylic acids
R 1. LiAlH4 2. H2O
O
O
C
C
R
OH
H PDC, CH2Cl2
RCH2OH
Example
benzaldehyde from benzoic acid O
O
COH
CH
1. LiAlH4 2. H2O (81%)
CH2OH
PDC CH2Cl2
(83%)
What about..? ketones from aldehydes
R
O
O
C
C
H
R
R'
What about..? ketones from aldehydes
R 1. R'MgX 2. H3O+
O
O
C
C
R
H OH
RCHR'
R' PDC, CH2Cl2
Example
3-heptanone from propanal O
O
C CH3CH2
CH3CH2C(CH2)3 CH3
H
(57%)
1. CH3(CH2)3MgX 2. H3
O+
OH CH3CH2CH(CH2)3 CH3
H2CrO4