Aldehydes and Ketones - Part 1 | Organic Chemistry 2
Nomenclature of Carbonyls
Name: the -ane ending of the corresponding alkane is replace by -al (aldehyde) or by -one (ketone)
The rules for naming alcohols also apply to carbonyls, except that it is not necessary to specify the position of the aldehyde ⇒ always carbon 1
Carbaldehyde: aldehyde functional group is attached to a ring and the carbon of -CHO is carbon 1
Cycloalkanones: ketone functional group is part of the ring and carbon of the CO is carbon 1
Aldehydes and ketones take precedence over alcohols, alkenes and alkynes. Aldehydes takes precedence over ketones. If aldehydes or ketones do not have priority, the prefixes formyl- (aldehyde) and acyl- (ketone) or oxo- (ketone in the presence of an aldehyde) must be used
Properties of Carbonyls
C=O bond of carbonyls:
short, strong and very polar
Geometry:
The carbon of the C=O bond of carbonyls are sp2 hybridized ⇒ trigonal planar geometry
Absorption spectroscopy:
NMR: 1H δ ~ 9-10 ppm (H of aldehyde); 13C δ ~ 200 ppm
IR: C=O stretching ⇒ intense band ~ 1690-1750 cm-1
Reactivity of Carbonyls:
Synthesis of Carbonyls
Oxidation of Alcohols (see Chapter 8 - Organic Chemistry 1):
Oxidation of Allylic Alcohols:
Ozonolysis of Alkenes (see Chapter 2 - Organic Chemistry 2):
Hydration of Alkynes (see Chapter 3 - Organic Chemistry 2):
Friedel-Crafts Acylation (see Chapter 5 - Organic Chemistry 2):
Mechanisms of Addition Reactions
Mechanisms:
Under basic conditions: nucleophilic addition-protonation (nucleophilic attack first)
Under acidic conditions: electrophilic protonation-addition (electrophilic attack first)
Additions of Hydrides and Organometallic Reagents
Additions of Water
Mechanisms:
Base-catalyzed hydration:
Acid-catalyzed hydration:
Additions of Alcohols
Formation of hemiacetal:
Mechanism:
Same as for the addition of water, H2O being replaced by ROH ⇒ catalyzed by acids or bases
Formation of acetal:
Mechanism:
- Hemiacetal formation under acidic conditions (see acid-catalyzed hydration - addition of H2O)
- Acetal formation via an acid-catalyzed SN1:
Synthetic Strategy:
Acetal is widely used as protecting group: acetal formation is reversible and carbonyl is transformed into a non-reactive ether-like moiety ⇒ carbonyl protection against nucleophilic attacks (base, LiAlH4, RMgX). 1,2-ethanediol is typically used to form a cyclic acetal:
Additions of Ammonia and its Derivatives
Formation of imine:
Mechanism:
Formation of enamine:
Mechanism: