chapter 19. aldehydes and ketones: nucleophilic … why this chapter? much of organic chemistry...
TRANSCRIPT
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Based on McMurry’s Organic Chemistry, 7th edition
Dr M. Mehrdad University of Guilan, Department of
Chemistry, Rasht, Iran
2شیمی آلی
Chapter 19. Aldehydes and Ketones: Nucleophilic Addition Reactions
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Aldehydes and Ketones
Aldehydes (RCHO) and ketones (R2CO) are
characterized by the the carbonyl functional
group (C=O)
The compounds occur widely in nature as
intermediates in metabolism and biosynthesis
coenzyme
involved in
a large
number of
metabolic
reactions
a steroid
hormone
secreted by
the adrenal
glands to
regulate fat,
protein, and
carbohydrate
metabolism
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Why this Chapter?
Much of organic chemistry involves the chemistry of
carbonyl compounds
Aldehydes/ketones are intermediates in synthesis of
pharmaceutical agents, biological pathways,
numerous industrial processes
An understanding of their properties is essential
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19.1 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
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19.1 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
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For cyclic aldehydes in which the -CHO group is directly
attached to a ring, the suffix -carbaldehyde is used.
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For cyclic aldehydes in which the -CHO group is directly
attached to a ring, the suffix -carbaldehyde is used.
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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
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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
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Ketones with Common Names
IUPAC retains well-used but unsystematic names for
a few ketones
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Ketones and Aldehydes as
Substituents The R–C=O as a substituent is an acyl group, used with
the suffix -yl from the root of the carboxylic acid CH3CO: acetyl; CHO: formyl; C6H5CO: benzoyl
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
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19.2 Preparation of Aldehydes and Ketones
Preparing Aldehydes
Oxidize primary alcohols using pyridinium chlorochromate
Alkenes with a vinylic hydrogen can undergo oxidative cleavage when treated with ozone, yielding aldehydes
Reduce an ester with diisobutylaluminum hydride (DIBAH)
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Preparing Ketones
Oxidize a 2° alcohol
Many reagents possible: choose for the specific
situation (scale, cost, and acid/base sensitivity)
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Ketones from Ozonolysis
Ozonolysis of alkenes yields ketones if one of the unsaturated carbon atoms is disubstituted
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Aryl Ketones by Acylation
Friedel–Crafts acylation of an aromatic ring with an
acid chloride in the presence of AlCl3 catalyst
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Methyl Ketones by Hydrating
Alkynes
Hydration of terminal alkynes in the presence of Hg2+
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19.3 Oxidation of Aldehydes
and Ketones CrO3 in aqueous acid oxidizes aldehydes to
carboxylic acids efficiently
Silver oxide, Ag2O, in aqueous ammonia (Tollens’
reagent) oxidizes aldehydes (no acid)
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Hydration of Aldehydes
Aldehyde oxidations occur through 1,1-diols (“hydrates”)
Reversible addition of water to the carbonyl group
Aldehyde hydrate is oxidized to a carboxylic acid by usual reagents for alcohols
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Ketones Oxidize with Difficulty
Undergo slow cleavage with hot, alkaline KMnO4
C–C bond next to C=O is broken to give carboxylic
acids
Reaction is practical for cleaving symmetrical ketones
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19.4 Nucleophilic Addition Reactions
of Aldehydes and Ketones
Nu- approaches 75° to the plane of C=O and adds to C
A tetrahedral alkoxide ion intermediate is produced
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Nucleophiles
Nucleophiles can
be negatively
charged ( : Nu)
or neutral ( : Nu)
at the reaction
site
The overall
charge on the
nucleophilic
species is not
considered
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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
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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
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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 electrophile than the carbonyl group of an aliphatic aldehyde
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19.5 Nucleophilic Addition of
H2O: Hydration Aldehydes and ketones react with water to yield 1,1-
diols (geminal (gem) diols)
Hyrdation is reversible: a gem diol can eliminate water
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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
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Base-Catalyzed Addition of Water
Addition of water is catalyzed by both acid and base
The base-catalyzed hydration nucleophile is the hydroxide
ion, which is a much stronger nucleophile than water
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Acid-Catalyzed Addition of
Water
Protonation of
C=O makes it
more electrophilic
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Addition of H-Y to C=O
Reaction of C=O with H-Y, where Y is
electronegative, gives an addition product (“adduct”)
Formation is readily reversible
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19.6 Nucleophilic Addition of
HCN: Cyanohydrin Formation Aldehydes and unhindered ketones react with HCN
to yield cyanohydrins, RCH(OH)CN
Addition of HCN is reversible and base-catalyzed, generating nucleophilic cyanide ion, CN-
Addition of CN to C=O yields a tetrahedral intermediate, which is then protonated
Equilibrium favors adduct
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Uses of Cyanohydrins
The nitrile group (CN) can be reduced with LiAlH4
to yield a primary amine (RCH2NH2)
Can be hydrolyzed by hot acid to yield a carboxylic
acid
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19.7 Nucleophilic Addition of Grignard
Reagents and Hydride Reagents:
Alcohol Formation Treatment of aldehydes or ketones with Grignard
reagents yields an alcohol
Nucleophilic addition of the equivalent of a carbon
anion, or carbanion. A carbon–magnesium bond is
strongly polarized, so a Grignard reagent reacts for all
practical purposes as R : MgX +.
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Mechanism of Addition of
Grignard Reagents Complexation of
C=O by Mg2+,
Nucleophilic
addition of R : ,
protonation by
dilute acid yields
the neutral alcohol
Grignard additions
are irreversible
because a
carbanion is not a
leaving group
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Hydride Addition
Convert C=O to CH-OH
LiAlH4 and NaBH4 react as donors of hydride ion
Protonation after addition yields the alcohol
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19.8 Nucleophilic Addition of Amines:
Imine and Enamine Formation
RNH2 adds to C=O to form imines, R2C=NR (after loss of HOH)
R2NH yields enamines, R2NCR=CR2 (after loss of HOH)
(ene + amine = unsaturated amine)
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Mechanism of
Formation of Imines
Primary amine adds to C=O
Proton is lost from N and
adds to O to yield a neutral
amino alcohol
(carbinolamine)
Protonation of OH converts
into water as the leaving
group
Result is iminium ion, which
loses proton
Acid is required for loss of
OH – too much acid blocks
RNH2
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Imine Derivatives Addition of amines with an atom containing a lone pair of
electrons on the adjacent atom occurs very readily, giving
useful, stable imines
For example, hydroxylamine forms oximes and 2,4-
dinitrophenylhydrazine readily forms 2,4-
dinitrophenylhydrazones
These are usually solids and help in characterizing liquid
ketones or aldehydes by melting points
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Enamine
Formation
After addition of
R2NH, proton is lost
from adjacent
carbon
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19.9 Nucleophilic Addition of
Hydrazine: The Wolff–Kishner Reaction
Treatment of an aldehyde or ketone with hydrazine,
H2NNH2 and KOH converts the compound to an alkane
Originally carried out at high temperatures but with
dimethyl sulfoxide as solvent takes place near room
temperature
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19.10 Nucleophilic Addition of
Alcohols: Acetal Formation
Alcohols are weak nucleophiles but acid
promotes addition forming the conjugate acid
of C=O
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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+ to which
a second alcohol adds
to form the acetal
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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)
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19.11 Nucleophilic Addition of
Phosphorus Ylides:
The Wittig Reaction
The sequence converts C=O is to C=C
A phosphorus ylide adds to an aldehyde
or ketone to yield a dipolar intermediate
called a betaine
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BETAINE
Pronounced bayta-ene, a special class of zwitterions in which the positive
site has no hydrogen atoms attached to it.
YLIDE
A neutral species whose structure may be represented as a 1,2-dipole
with a negatively charged carbon or other atom having a lone pair of
electrons, bonded to a positively charged heteroatom. The term is
usually preceded by the name of the hetero-atom.
(a) The phosphorus ylide
methylenetriphenylphosphorane
(b) the nitrogen ylide
(trimethylammonio)methylide
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The intermediate spontaneously decomposes
through a four-membered ring to yield alkene
and triphenylphosphine oxide, (Ph)3P=O
Formation of the ylide is shown below
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Mechanism of the Wittig Reaction
A Betaine
a betaine
The betaine
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Uses of the Wittig Reaction
Can be used for monosubstituted, disubstituted, and trisubstituted alkenes but not tetrasubstituted alkenes The reaction yields a pure alkene of known structure
For comparison, addition of CH3MgBr to cyclohexanone and dehydration with, yields a mixture of two alkenes
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19.12 The Cannizaro Reaction
The adduct of an aldehyde and OH can transfer hydride ion to another aldehyde C=O resulting in a simultaneous oxidation and reduction (disproportionation)
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19.13 Conjugate Nucleophilic Addition to
-Unsaturated Aldehydes and Ketones
A nucleophile can add
to the C=C double
bond of an ,-
unsaturated aldehyde
or ketone (conjugate
addition, or 1,4
addition)
The initial product is a
resonance-stabilized
enolate ion, which is
then protonated
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Conjugate Addition of Amines
Primary and secondary amines add to , -
unsaturated aldehydes and ketones to yield -amino
aldehydes and ketones
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Conjugate Addition of Alkyl
Groups: Organocopper Reactions Reaction of an , -unsaturated ketone with a lithium
diorganocopper reagent
Diorganocopper (Gilman) reagents form by reaction of 1
equivalent of cuprous iodide and 2 equivalents of
organolithium
1, 2, 3 alkyl, aryl and alkenyl groups react but not alkynyl
groups
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Mechanism of Alkyl Conjugate
Addition
Conjugate nucleophilic addition of a diorganocopper
anion, R2Cu, to an enone
Transfer of an R group and elimination of a neutral
organocopper species, RCu
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19.14 Spectroscopy of
Aldehydes and Ketones Infrared Spectroscopy
Aldehydes and ketones show a strong C=O peak 1660 to 1770 cm1
aldehydes show two characteristic C–H absorptions in the 2720 to 2820 cm1 range.
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C=O Peak Position in the IR
Spectrum
The precise position of the peak reveals the exact nature of the carbonyl group
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NMR Spectra of Aldehydes
Aldehyde proton signals are at 10 in 1H NMR -distinctive spin–spin coupling with protons on the neighboring carbon, J 3 Hz
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13C NMR of C=O
C=O signal is at 190 to 215
No other kinds of carbons
absorb in this range
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Mass Spectrometry –
McLafferty Rearrangement
Aliphatic aldehydes and ketones that have hydrogens
on their gamma () carbon atoms rearrange as shown
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Mass Spectroscopy:
-Cleavage
Cleavage of the bond between the carbonyl group
and the carbon
Yields a neutral radical and an oxygen-containing
cation