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Aldehydes and Ketones:
Nucleophilic Addition to the
Carbonyl Group
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Longest chain that contains the aldehyde group is the parent
chain. Aldehyde C is C-1.
Nomenclature
4,4-Dimethylpentanal 5-Hexenal or Hex-5-enal 2-Phenylbutanedial
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Nomenclature
Cyclopentanecarbaldehyde2-Naphthalenecarbaldehyde
or Naphthalene-2-carbaldehyde
When a formyl group (CH=O) is attached to a ring, the ringname is followed by the suffix -carbaldehyde.
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Acceptable common names.
Nomenclature
Formaldehyde
(methanal)
Acetaldehyde
(ethanal)
Benzaldehyde
(benzenecarbaldehyde)
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Nomenclature
With oxygen-containing groups a higher oxidation state
substituent takes precedence so the –OH is a hydroxy
substituent.
5-Hydroxypentanal
trans-4-Hydroxycyclohexanecarbaldehyde
p-Hydroxybenzaldehyde
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With ketones, the -e ending of an alkane is replaced by -one
in the longest continuous chain containing the carbonyl group.
The chain is numbered to give a lower number for this group.
Nomenclature
3-Hexanone
or Hexan-3-one
4-Methyl-2-pentanone
or 4-Methylpentan-2-one
4-Methylcyclohexanone
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Ketone has preference over alcohols, halides, alkenes, and
alkyl groups in name and numbering.
Nomenclature
4-Methyl-3-penten-2-one
or 4-Methylpent-3-en-2-one
Aldehydes have preference over ketones in name and
numbering – ketone is an oxo substituent.
2-Methyl-4-oxopentanal
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17.2
Structure and Bonding:The Carbonyl Group
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The carbonyl is flat with sp2 hybridization of carbon and oxygen
with bond angles about 120o.
Structure and Bonding
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Structure and Bonding
The C=O comprises a s-bond and a p-bond (shown) similar to
ethylene.
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The carbonyl group makes aldehydes and ketones polar.
Carbonyl and Polarity
CH3CH2CH=CH21-Butene
Dipole moment: 0.3 D
CH3CH2CH=O
Propanal
Dipole moment: 2.5 D
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Ways to show this polarization include.
Carbonyl and Polarity
Or using resonance structures.
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Electrostatic potential maps show positive potential aspurple and negative potential as red.
Carbonyl and Polarity
1-Butene (CH3CH2CH=CH2) Propanal (CH3CH2CH=O)
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The polarity and structure dictate the reactivity of carbonyls.
Structure, Bonding and Reactivity
electrophiles, especially
protons, bond to oxygen
nucleophiles
bond to carbon
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Heats of combustion reveal that 2-butanone is more stablethan its aldehyde isomer butanal.
Structure and Stability
Butanal
2475 kJ/mol (592 kcal/mol)
2-Butanone
2442 kJ/mol (584 kcal/mol)
Heats of combustion
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17.3
Physical Properties
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Higher boiling points than alkanes and alkenes due to
stronger dipole-dipole forces. Lower boiling points thanalcohols which have stronger hydrogen bonding.
Physical Properties
Name
bp (1 atm)
Solubility in
water (g/100 mL)
1-Butene
-6°C
Negligible
Propanal
49°C
20
1-Propanol
97°C
Miscible in all
proportions
More water soluble than alkanes and alkenes since the
carbonyl can hydrogen bond to water but less than
alcohols.
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17.4
Sources of Aldehydes and Ketones
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Naturally Occurring Ketones and Aldehydes
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Reactions that Form Ketones and Aldehydes
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Reactions that Form Ketones and Aldehydes
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Strategy. Reduction then oxidation.
Combinations of Reactions to Form Aldehydes
Example.
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Strategy. Grignard reaction then oxidation.
Combinations of Reactions to Form Ketones
Example.
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Formaldehyde, acetaldehyde and acetone can be prepared
by dehydration of an alcohol.
Industrial Syntheses
Hydroformylation of alkenes yields aldehydes.
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17.5
Reactions of Aldehydes and Ketones:A Review and a Preview
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Review of Reactions of Ketones and Aldehydes
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A negatively polarized atom or group bonds to the positively
polarized carbon of the carbonyl group in the rate-
determining step of these reactions.
Nucleophilic Addition Reactions
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17.6
Principles of Nucleophilic Addition:Hydration of Aldehydes and Ketones
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Aldehydes and ketones react with water in a rapid equilibrium.
The product is a geminal diol, also called a hydrate.
An addition reaction.
Effects of Structure on Equilibrium
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Effects of Structure on Equilibrium
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Electronic effect:
Alkyl substituents stabilize ketones more than aldehydes.
Effects of Structure on Equilibrium
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Electronic effect:Trifluoromethyl substituents are electron withdrawing and
therefore destabilize the carbonyl. Opposite effect to
alkyl substituents: hydrate favored!
Effects of Structure on Equilibrium
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Steric Effects:
Bond angles at C shrink from ≈120 in the reactant to ≈109.5o
in the hydrate. This introduces crowding and with more alkyl
groups Khydr decreases.
Effects of Structure on Equilibrium
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Mechanism of Hydration in Basic Solution
Step 1. Nucleophilic attack.
Reaction equation.
Step 2. Protonation.
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Mechanism of Hydration in Basic Solution
Potential energy diagram for base catalyzed hydration.
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Mechanism of Hydration in Acidic Solution
Step 1. Protonation.
Reaction equation.
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Mechanism of Hydration in Acidic Solution
Step 2. Nucleophilic attack.
Step 3. Deprotonation.
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Mechanism of Hydration in Basic Solution
Potential energy diagram for base catalyzed hydration.