Download Chapter 19. Aldehydes and Ketones: Nucleophilic Addition Reactions

Chapter 13. Aldehydes
and Ketones
Based on McMurry’s Organic Chemistry, 6th edition
1
Aldehydes and Ketones
 Aldehydes and ketones are characterized by the carbonyl
functional group (C=O)
 The compounds occur widely in nature as intermediates in
metabolism and biosynthesis
 They are also common as chemicals, as solvents, monomers,
adhesives, agrichemicals and pharmaceuticals
2
The Carbonyl Group
3
The Carbonyl Group
4
Naming Aldehydes and Ketones
 Aldehydes are named by replacing the terminal -e of the
corresponding alkane name with –al
 The parent chain must contain the CHO group
 The CHO carbon is numbered as C1
 If the CHO group is attached to a ring, use the suffix -carbaldehyde
5
Naming Ketones
 Replace the terminal -e of the alkane name with –one
 Parent chain is the longest one that contains the ketone group

Numbering begins at the end nearer the carbonyl carbon
6
Common Names
 IUPAC retains well-used but unsystematic names for a few
ketones:
7
Ketones and Aldehydes as Substituents
 The R–C=O as a substituent is an acyl group is used with the
suffix -yl from the root of the carboxylic acid
 CH3CO: acetyl; CHO: formyl; C6H5CO: benzoyl; ArCO: aroyl
 The prefix oxo- is used if other functional groups are present
and the doubly bonded oxygen is labeled as a substituent on a
parent chain
8
Preparation of Aldehydes and Ketones
 Preparing Aldehydes
 Oxidize primary alcohols using pyridinium chlorochromate (PCC)
 Preparing Ketones
 Oxidize secondary alcohols using (PCC), or CrO3 or Na2Cr2O7
9
Preparing Ketones: Other Methods
 Friedel–Crafts acylation of an aromatic ring with an acyl chloride
in the presence of AlCl3 catalyst
 Hydration of terminal alkynes in the presence of Hg2+ catalyst
10
Oxidation of Aldehydes
 CrO3 in aqueous acid oxidizes aldehydes to carboxylic acids
efficiently
 Silver oxide, Ag2O, in aqueous ammonia (Tollens’ reagent)
oxidizes aldehydes (no acid)
11
Nucleophilic Addition Reactions of Aldehydes
and Ketones
 Nu- approaches 45° to the plane of C=O and adds to C,
pushing p electrons onto oxygen

A tetrahedral alkoxide ion intermediate is produced
12
Nucleophiles
 Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu)
at the reaction site
 The overall charge on the nucleophilic species is not considered
13
14
Relative Reactivity of Aldehydes and Ketones
 Aldehydes are generally more reactive than ketones in
nucleophilic addition reactions
 The transition state for addition is less crowded and lower in
energy for an aldehyde (a) than for a ketone (b)
 Aldehydes have one large substituent bonded to the C=O:
ketones have two
15
Electrophilicity of Aldehydes and Ketones
 Aldehyde C=O is more polarized than ketone C=O
 As in carbocations, more alkyl groups stabilize + character
 Ketone has more alkyl groups, stabilizing the C=O carbon
inductively
16
Reactivity of Aromatic Aldehydes
 Less reactive in nucleophilic addition reactions than aliphatic
aldehydes
 Electron-donating resonance effect of aromatic ring makes C=O
less reactive electrophilic than the carbonyl group of an aliphatic
aldehyde
17
Nucleophilic Addition of H2O: Hydration
 Aldehydes and ketones react with water to yield 1,1-diols
(geminal (gem) diols)
 Hydration is reversible: a gem diol can eliminate water
18
Relative Energies
 Equilibrium generally favors the carbonyl compound over
hydrate for steric reasons
 Acetone in water is 99.9% ketone form
 Exception: simple aldehydes

In water, formaldehyde consists is 99.9% hydrate
19
Base-Catalyzed Addition of Water
 Addition of water is catalyzed by
both acid and base
 In base-catalyzed hydration
nucleophile is the hydroxide ion,
which is a much stronger
nucleophile than water
20
Acid-Catalyzed Addition of Water
 Protonation of C=O makes it
more electrophilic
21
Nucleophilic Addition of Alcohols: Acetal Formation
 Two equivalents of ROH in the presence of an acid catalyst add
to C=O to yield acetals, R2C(OR)2
 These can be called ketals if derived from a ketone
22
Formation of Acetals
 Alcohols are weak nucleophiles but acid promotes addition
forming the conjugate acid of C=O
 Addition yields a hydroxy ether, called a hemiacetal (reversible);
further reaction can occur
 Protonation of the OH and loss of water leads to an oxonium
ion, R2C=OR+ (an oxygen bearing a positive charge) to which a
second alcohol adds to form the acetal
23
Uses of Acetals
 Acetals can serve as protecting groups for aldehydes and
ketones
 It is convenient to use a diol, to form a cyclic acetal (the reaction
goes even more readily)
24
Cyclic Structures of Monosaccharides:
Hemiacetal Formation
glucose
25
Hydride Addition: reduction
 Convert C=O to CH-OH
 LiAlH4 and NaBH4 react as donors of hydride ion
 Protonation after addition yields the alcohol
26
Reduction Reagent: Sodium Borohydride
 NaBH4 is not sensitive to moisture and it does not reduce other
common functional groups
 Lithium aluminum hydride (LiAlH4) is more powerful, less specific,
and very reactive with water
 Both add the equivalent of “H-”
27
End of chapter 13
28