chapter 19 aldehydes and ketones: nucleophilic addition reactions
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Chapter 19 Aldehydes and Ketones: Nucleophilic Addition Reactions. Aldehydes and Ketones. Aldehydes (RCHO) and ketones (R 2 CO) are characterized by the carbonyl functional group (C=O) The compounds occur widely in nature as intermediates in metabolism and biosynthesis. Why this Chapter?. - PowerPoint PPT PresentationTRANSCRIPT
John E. McMurry
www.cengage.com/chemistry/mcmurry
Paul D. Adams • University of Arkansas
Chapter 19Aldehydes and Ketones:
Nucleophilic Addition Reactions
Aldehydes (RCHO) and ketones (R2CO) are characterized by the carbonyl functional group (C=O)
The compounds occur widely in nature as intermediates in metabolism and biosynthesis
Aldehydes and Ketones
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
Why this Chapter?
1. Carboxylic Acids (3 O bonds, 1 OH)
2. Esters (3 O bonds, 1 OR)
3. Amides
4. Nitriles
5. Aldehydes (2 O bonds, 1H)
6. Ketones (2 O bonds)
7. Alcohols (1 O bond, 1 OH)
8. Amines
9. Alkenes, Alkynes
10. Alkanes
11. Ethers
12. Halides
The parent will be determined based on the highest priority functional group.
O OH
OH
2-ethyl-4-hydroxybutanoic acid
Functional Group Priority
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 carbaldehyde
Naming Aldehydes
O
HCH3
pentanal
Cl
O
H
5-chloropentanalH O
CH3CH3
2-ethylbutanal
CH3
O
H
CH3
CH3
3,4-dimethylhexanal
Cl
CH3 O
H
5-chloro-2-methylbenzaldehyde
Cl
O
H
m-chloro-benzaldehyde
CH3
O
CH3 O
H
5-methoxy-2-methylbenzaldehyde
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
Naming Ketones
IUPAC retains well-used but unsystematic names for a few ketones
Ketones with Common Names
Naming Ketones
2-pentanone 5-chloro-2-pentanone 1-chloro-3-pentanone
4-ethylcyclohexanone 2-fluoro-5-methyl-4-octanone
O
CH3 CH3
ClO
CH3
O
Cl
CH3
O
CH3CH3
F O
CH3
CH3
Naming Ketones
O
butan-2-one
O
Cl
O
O
but-3-en-2-one
OHO
4-hydroxybutan-2-one
O
O
octane-2,7-dione
O
3-methylcyclopentanone
O
cyclohex-2-en-1-one
Cl
O
Br
7-bromo-3-chloro-4-methylcyclooctanone
3-chloro-5-ethylheptan-4-one3,5-dimethylheptan-4-one
Cl
O
3-chloro-5-ethyloct-7-en-1-yn-4-one
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 (lower priority functional group)
Ketones and Aldehydes as Substituents
O O
H
3-oxopentanal
O O
OH
O
3,4-dioxopentanoic acid
O O
H
O
3,4-dioxopentanal
O
H
O
OH
2-formylbenzoic acid
H
O
benzaldehyde
ClH
O
3-chlorobenzaldehyde
O O
HO
methyl 3-oxopropanoate
O O
O
methyl 3-oxobutanoate
O O
HH
O
3-oxopropanoic acid
Ketones/Aldehydes as minor FGs, benzaldehydes
Solubility of Ketones and Aldehydes
Good solvent for alcohols Lone pair of electrons on oxygen of carbonyl can accept a
hydrogen bond from O—H or N—H. Acetone and acetaldehyde are miscible in water.
Preparing Aldehydes Oxidize primary alcohols using PCC (in dichloromethane) Alkenes with a vinylic hydrogen can undergo oxidative
cleavage when treated with ozone, yielding aldehydes Reduce an ester with diisobutylaluminum hydride (DIBAH)
Like LiAlH4
19.2 Preparing Aldehydes and Ketones
Oxidize a 2° alcohol (See chapter on alcohols) Many reagents possible: choose for the specific
situation (scale, cost, and acid/base sensitivity)
Preparing Ketones
Ozonolysis of alkenes yields ketones if one of the unsaturated carbon atoms is disubstituted
Ketones from Ozonolysis
Friedel–Crafts acylation of an aromatic ring with an acid chloride in the presence of AlCl3 catalyst
Aryl Ketones by Acylation
+
O
Cl
OAlCl3Heat
Limitations of FC Acylation
Does not occur on rings with electron-withdrawing substituents or basic amino groups
+
O
ClAlCl3Heat No Rxn
NH2
Mechanism of FC Acylation
Not subject to rearrangement like FC alkylation.
Hydration of terminal alkynes in the presence of Hg2+ (catalyst: Section 9.4) Markovnikov addition Produces enol that tautomerizes to ketone Works best with terminal alkyne
Methyl Ketones by Hydrating Alkynes
CrO3 in aqueous acid oxidizes aldehydes to carboxylic acids efficiently
Silver oxide, Ag2O, in aqueous ammonia (Tollens’ reagent) oxidizes aldehydes
19.3 Oxidation of Aldehydes
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
Oxidation of Ketones
Nu- approaches 75° to the plane of C=O and adds to C
A tetrahedral alkoxide ion intermediate is produced
19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones
Nucleophiles can be negatively charged ( :Nu) or neutral ( :Nu) at the reaction site
The overall charge on the nucleophilic species is not considered
Nucleophiles
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
Relative Reactivity 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
Electrophilicity of Aldehydes and Ketones
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
Reactivity of Aromatic Aldehydes
Aldehydes and ketones react with water to yield 1,1-diols (geminal (gem) diols)
Hyrdation is reversible: a gem diol can eliminate water
19.5 Nucleophilic Addition of H2O: Hydration
Aldehyde hydrate is oxidized to a carboxylic acid by usual reagents for alcohols
Relatively unreactive under neutral conditions, but can be catalyzed by acid or base.
Hydration of Aldehydes
Hydration occurs through the nucleophilic addition mechanism, with water (in acid) or hydroxide (in base) serving as the nucleophile.
Acid catalyzed
Base catalyzed
• The hydroxide ion attacks the carbonyl group. • Protonation of the intermediate gives the hydrate.
Hydration of Carbonyls
Reaction of C=O with H-Y, where Y is electronegative, gives an addition product (“adduct”)
Formation is readily reversible (unstable alcohol)
Addition of H–Y to C=O
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
19.6 Nucleophilic Addition of HCN: Cyanohydrin 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+.
19.7 Nucleophilic Addition of Grignard Reagents and Hydride Reagents: Alcohol Formation
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
Mechanism of Addition of Grignard Reagents
Convert C=O to CH-OH LiAlH4 and NaBH4 react as donors of hydride ion Protonation after addition yields the alcohol
Hydride Addition
RNH2 adds to R’2C=O to form imines, R’2C=NR (after loss of HOH)
R2NH yields enamines, R2NCR=CR2 (after loss of HOH) (ene + amine = unsaturated amine)
19.8 Nucleophilic Addition of Amines: Imine and Enamine Formation
Primary amine adds to C=O
Proton is lost from N and adds to O to yield an amino alcohol (carbinolamine)
Protonation of OH converts it 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
Optimum pH = 4.5
Mechanism of Formation of Imines
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
Imine Derivatives
After addition of R2NH and loss of water, proton is lost from adjacent carbon
Enamine Formation
Treatment of an aldehyde or ketone with hydrazine, H2NNH2, and KOH converts the compound to an alkane
Can be conducted near room temperature using dimethyl sulfoxide as solvent
Works well with both alkyl and aryl ketones
19.9 Nucleophilic Addition of Hydrazine: The Wolff–Kishner Reaction
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
19.10 Nucleophilic Addition of Alcohols: Acetal Formation
Mechanism of Acetal Formation
Mechanism of Acetal Formation
Acetals can serve as protecting groups for aldehydes and ketones Aldehydes more reactive than ketones It is convenient to use a diol to form a cyclic acetal (the reaction goes
even more readily)
Uses of Acetals
The sequence converts C=O 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
19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction
Mechanism of the Wittig Reaction
Product of Wittig Rxn
The Wittig reaction converts the carbonyl group into a new C═C double bond where no bond existed before.
A phosphorus ylide is used as the nucleophile in the reaction. Ylide usually CH2 or monosubstituted, but not disubstituted. Unstabilized ylide tends to favor Z-alkene.
+(Ph)3P
+-CHCH3
THF(Ph)3P=OO
H
Z-alkene
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
19.13 Conjugate Nucleophilic Addition to -Unsaturated Aldehydes and Ketones
Primary and secondary amines add to , -unsaturated aldehydes and ketones to yield -amino aldehydes and ketones
Conjugate Addition of Amines
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
Conjugate Addition of Alkyl Groups: Organocopper Reactions
Conjugate nucleophilic addition of a diorganocopper anion, R2Cu, to an enone
Transfer of an R group and elimination of a neutral organocopper species, RCu
Mechanism of Alkyl Conjugate Addition: Organocopper Reactions
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.
19.14 Spectroscopy of Aldehydes and Ketones
The precise position of the peak reveals the exact nature of the carbonyl group
C=O Peak Position in the IR Spectrum
Aldehyde proton signals are near 10 in 1H NMR - distinctive spin–spin coupling with protons on the neighboring carbon, J 3 Hz
NMR Spectra of Aldehydes
C=O signal is at 190 to 215 No other kinds of carbons absorb in this range
13C NMR of C=O
Aliphatic aldehydes and ketones that have hydrogens on their gamma () carbon atoms rearrange as shown
Mass Spectrometry – McLafferty Rearrangement
Cleavage of the bond between the carbonyl group and the carbon
Yields a neutral radical and an oxygen-containing cation
Mass Spectroscopy: -Cleavage