1 Aldehydes and Ketones: Nucleophilic Addition Reactions.
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Transcript of 1 Aldehydes and Ketones: Nucleophilic Addition Reactions.
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Aldehydes and Ketones: Nucleophilic Addition Reactions
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Aldehydes and Ketones
Aldehydes and ketones are characterized by the 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
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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
If the CHO group is attached to a ring, use the suffix See Table 19.1 for common names
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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 is 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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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19.2 Preparation of Aldehydes and Ketones
Preparing Aldehydes Oxidize primary alcohols using pyridinium
chlorochromate Reduce an ester with diisobutylaluminum
hydride (DIBAH)
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Preparing Ketones
Oxidize a 2° alcohol (see Section 17.8) Many reagents possible: choose for the specific
situation (scale, cost, and acid/base sensitivity)
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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Ketones from Ozonolysis
Ozonolysis of alkenes yields ketones if one of the unsaturated carbon atoms is disubstituted (see Section 7.8)
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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 (see Section 16.4)
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Methyl Ketones by Hydrating Alkynes
Hydration of terminal alkynes in the presence of Hg2+ catalyst
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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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Nucleophilic Addition Reactions of Aldehydes and Ketones
Nu- approaches 45° 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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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 electrophilic than the carbonyl group of an aliphatic aldehyde
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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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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Nucleophilic Addition of HCN: Cyanohydrin Formation Aldehydes and unhindered ketones react with HCN
to yield cyanohydrins, RCH(OH)CN
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Mechanism of Formation of Cyanohydrins 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
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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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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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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)
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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
Note that overall reaction is substitution of RN for O
Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003
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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
CC
O
C
C
O
H++ R2NH
NH
R
C
C
HON
R
C
C
H2ON
R
C
CN
R
+ H3O+
R
H
H HH
H H
R R R
H H H
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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
See Figure 19.11 for a mechanism
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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
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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+ 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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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 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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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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Mechanism of the Wittig Reaction
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The Cannizzaro Reaction: Biological Reductions 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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The Biological Analogue of the Canizzaro Reaction Enzymes catalyze the reduction of aldehydes and ketones
using NADH as the source of the equivalent of H-
The transfer resembles that in the Cannizzaro reaction but the carbonyl of the acceptor is polarized by an acid from the enzyme, lowering the barrier
Enzymes are chiral and the reactions are stereospecific. The stereochemistry depends on the particular enzyme involved.
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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 from 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, an enone Transfer of an R group and elimination of a neutral
organocopper species, RCu
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Biological Nucleophilic Addition Reactions Example: Many enzyme reactions involve pyridoxal phosphate
(PLP), a derivative of vitamin B6, as a co-catalyst PLP is an aldehyde that readily forms imines from amino groups
of substrates, such as amino acids The imine undergoes a proton shift that leads to the net
conversion of the amino group of the substrate into a carbonyl group
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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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Protons on Carbons Adjacent to C=O
Slightly deshielded and normally absorb near 2.0 to 2.3
Methyl ketones always show a sharp three-proton singlet near 2.1
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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
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Enantioselective Synthesis When a chiral product is formed achiral reagents, we get both
enantiomers in equal amounts - the transition states are mirror images and are equal in energy
However, if the reaction is subject to catalysis, a chiral catalyst can create a lower energy pathway for one enantiomer - called an enantionselective synthesis
Reaction of benzaldehyde with diethylzinc with a chiral titanium-containing catalyst, gives 97% of the S product and only 3% of the R
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Summary Aldehydes are from oxidative cleavage of alkenes, oxidation of 1°
alcohols, or partial reduction of esters Ketones are from oxidative cleavage of alkenes, oxidation of 2°
alcohols, or by addition of diorganocopper reagents to acid chlorides. Aldehydes and ketones are reduced to yield 1° and 2° alcohols ,
respectively Grignard reagents also gives alcohols Addition of HCN yields cyanohydrins 1° amines add to form imines, and 2° amines yield enamines Reaction of an aldehyde or ketone with hydrazine and base yields an
alkane Alcohols add to yield acetals Phosphoranes add to aldehydes and ketones to give alkenes (the
Wittig reaction) -Unsaturated aldehydes and ketones are subject to conjugate
addition (1,4 addition)
6 electron Transition States
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