Aldehydes and Ketones - Part 1 | Organic Chemistry 2

Aldehydes and ketones are studied in this chapter: name, properties and synthesis of carbonyls, addition reactions: addition of hydride and organometallic reagents, addition of water, addition of alcohols, addition of ammonia and its derivatives

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:

  1. Hemiacetal formation under acidic conditions (see acid-catalyzed hydration - addition of H2O)
  2. 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: