chapter 13. aldehydes and ketones - portale …docenti.unicam.it/tmp/3500.pdfaldehydes and ketones...
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Chapter 13. Aldehydes
and Ketones
Based on McMurry’s Organic Chemistry, 6th edition
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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
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The Carbonyl Group
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The Carbonyl Group
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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
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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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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; 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
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Preparation of Aldehydes and Ketones
Preparing Ketones
Oxidize secondary alcohols using (PCC), or CrO3 or Na2Cr2O7
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Preparing Aldehydes
Oxidize primary alcohols using pyridinium chlorochromate (PCC)
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Preparing Ketones: Other Methods
Hydration of terminal alkynes in the presence of Hg2+ catalyst
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Friedel–Crafts acylation of an aromatic ring with an acyl chloride
in the presence of AlCl3 catalyst
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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)
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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
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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 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)
Hydration 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
In 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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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+ (an oxygen bearing a positive charge) 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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Cyclic Structures of Monosaccharides:
Hemiacetal Formation
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glucose
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Hydride Addition: reduction
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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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-”
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End of chapter 13