1

Ketone & Aldehyde

Jully TanSchool of Engineering

EP101 / EG101 2

Learning Outcome At the end of this chapter, students are able to:

Provide nomenclature of the ketone and aldehyde Physical properties of ketone and aldehyde Synthesis and reaction of ketone and aldehyde

EP101 / EG101 3

Chapter 16Aldehydes and Ketones I

Nucleophilic Addition to the Carbonyl Group

The general structure of the carbonyl group is

C=OH

R

::

OC

The general formula for an aldehyde is

And that for a ketone is C=OR'

R:

:

C=OH

H

::

There is also the unique aldehyde, formaldehyde, in which there are two hydrogen atoms attached to the carbonyl group.

The R groups can be either alkyl or aryl.

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Nomenclature of Aldehydes

Common names are derived from those of the corresponding carboxylic acids by dropping the "(o)ic acid" and adding "aldehyde."

OH

O

H

O

Acetic acid Acetaldehyde

O

OH

O

HBenzoic acid Benzaldehyde

IUPAC (systematic) names are based on the following rules when the aldehyde function has priority and is named by use of a suffix.

(1) Select the longest continuous chain containing the -CHO and use as the parent the name of the alkane of that chain length.(2) Replace "e" in the alkane name with "al."

(3) Number from the end where the carbonyl group is located and follow all the other rules for locating substituent groups.

4-Phenylpentanal2,2-Dimethylbutanal

H

O

H

OEXAMPLES:

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IUPAC systematic names for ketones are based on these rules:

(1) The name of the longest alkane chain present that contains the carbonyl group is used as the parent.

(2) Replace the "e" in the alkane name with the suffix "one" and indicate the carbonyl position by a number. Number from the end that gives the lower number to the ketone position.(3) Designate the substituents and their positions in the usual way.

2-Pentanone

2-Methyl-4-phenyl-3-pentanone

O

O

Note use of alphabetical order.

O

(E)-3-Penten-2-one

Note provision for C C bond.

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Physical Properties of Aldehydes and KetonesBecause of the polar carbonyl group ( ~ 2.3-2.8 D), aldehydes and ketones have higher boiling points than hydrocarbons of comparable size. However, they have lower boiling points than alcohols of comparable size because only the latter have intermolecular hydrogen bonds.

CH3CH2CH2CH3 CH3CH2CH CH3CCH3 CH3CH2CH2OHO= O=

Butane Propanal Acetone 1-PropanolMW 58 58 58 60

BP -0.5 oC 49.0 oC 56.1 oC 97.2 oC

Aldehydes and ketones form hydrogen bonds to water molecules and consequently the lower MW ones are soluble in water up to about C6. The smallest ones (formaldehyde and acetone) are miscible with water.

O

HO H

HO H

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Synthesis of Ketones & Aldehydes

Oxidation of alcohol Ozonolysis of alkene Friedel-crafts acylation Mercuric catalysed hydration of alkynes Ketone from nitriles

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Synthetic Methods for AldehydesBecause aldehydes are between 1o alcohols and carboxylic acids in the oxidation-reduction sequence, they can be synthesized by either selective oxidation of 1o alcohols or by selective reduction of carboxylic acid derivatives.

RCH2OH RCHO= RCOOH[O]

[H]

[O]

[H]or

RCOX

Selective Oxidation with Pyridinium Chlorochromate (PCC)PCC

CH2Cl21-Heptanol Heptanal (78%)

PCC is

It is a unique Cr +6 oxidant. Other oxidants carry oxidation on to the carboxylic acid stage because aldehydes are more easily oxidized than alcohols.

OH H

O

N H Cl CrO3

Reduction of Carboxylic Acids

Aldehydes can be prepared by controlled reduction of carboxylic acids. Acids can be reduced with lithium aluminum hydride (LAH), a powerful reducing agent, but the process goes all the way to 1o alcohols.

RCOHO= LiAlH4

ether solvent RCHO= LiAlH4

facileRCH2OH

Lithium aluminum hydride (lithium tetrahydridoaluminate) and other metal hydride reducing agents, transfer hydride ion (H:-) to the electropositive carbon of the carbonyl group.

Al

H

HH

H

-Li+

C=O+ -

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Ozonolysis of alkenes.

CH

RC

R'

R''

1)

2)

O3

(CH3)2SC

H

RO + CO

R'

R''

Friedel-Crafts acylation Acid chloride/AlCl3 + benzene ketone

Ketone SynthesesOxidation of Secondary Alcohols

H OH

Cyclohexanol

K2Cr2O7 or CrO3

H2SO4

O=Cyclohexanone

These oxidations generally proceed well with few side reactions.

Aryl Ketones by Friedel-Crafts Acylation

+Butanoyl chloride

AlCl3

Phenyl propyl ketoneCl

OO

Ozonolysis of Alkenes

1,2-Dimethylcyclopentene

O3

CH2Cl2, -78 oCZn

H2O

2,6-Heptanedione

OO

EP101 / EG101 11

Ketones by Hydration of AlkynesAlkynes, like alkenes, add water in the presence of electrophilic catalysts such as H+ or Hg2+. Hydration of alkynes is conducted in aqueous solutions of sulfuric acid with mercuric sulfate as catalyst. It follows Markovnikov's rule, with the hydrogen attaching to the carbon with the greater number of hydrogens.

-C C-Alkyne

+ H2OHgSO4

H2SO4

-C=C-H

OH"Enol"

(or vinylic alcohol)

fast -C-C-H

H

O=

The "enol" rapidly rearranges to the ketone. The enol and ketone constitutional isomers actually are connected by an equilibrium which usually lies heavily on the side of the ketone. This process called tautomerization is catalyzed by acid.

-C=C-H

:OH:O

HH+ H2O + -C C-

H

H :OH

=

+Conjugate acid of ketone

Enol form(less stable)

+H

-C C-H

H :O:

= H3O++

Keto form(more stable)

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Ketones from NitrilesReaction of a nitrile with either a Grignard or organolithium reagent, followed by hydrolysis, yields a ketone.

R-C N:

Nitrile(R = alkyl or aryl)

+ R'-M ether R-C-R'N-M=

Imine saltR-C-R'

O=H3O+

H2O

The nitrile is a polar function similar to a carbonyl. The carbanionic center of an organometallic reagent adds to the electropositive carbon of the nitrile producing the salt of an imine.

-+

During aqueous workup, the imine is hydrolyzed to a ketone.

R-C=N-M

:

+ H-O-H R-C=N-R'

H

:

Imine

hydrolysis R-C=O:R'

:

R' - +

- +

EP101 / EG101 13

Examples

Butanenitrile(1) ether

(2) H3O++

O

Butyrophenone (Phenyl propyl ketone)

MgBrN

Benzonitrile

+N:Li

(1) ether(2) H3O+

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Characteristic Reactions of Aldehydes and Ketones

Reduction reactions Alcohol formation Alkane formation

Oxidation reactions Nucleophilic addition reactions

Grignard additions to form alcohols Addition of water (hydration) to form gem-diols Addition of alcohols to form acetals/ketals

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Reduction Reactions of Reduction Reactions of Aldehydes and KetonesAldehydes and Ketones

Alcohol formation Hydrogenation Hydride reduction

Alkane formation Clemmensen reduction Wolff-Kishner reduction

R C

OH

H2 / PtR CH2OH

R CO

H R CH2OHLiAlH4ether

H2OH+

R CO

H R CH3conc. HClZn(Hg)

R CO

H R CH3NH2NH2

OH- / H2O

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Oxidation of Aldehydes Oxidation of Aldehydes and Ketonesand Ketones

Conversion of Aldehydes to Carboxylic acids Oxidation of Aromatic Aldehydes/Ketones to Benzoic acid derivatives Haloform reaction of methyl carbonyls

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Aldehyde / Ketone Oxidations HIO4

C

O

C

O

HR RCOOH + HCOOH +HIO3

R C

O

H RCOOHAg(NH3)2+

(Tollens reagent)

C

O

H

or

C

O

R

COOHKMnO4 or K2Cr2O7

²

CH3 C

O

RX2

OH- / H2OHCX3 + RCOO-

Ar C

O

Hor (ArCOOH)

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Aldehyde / Ketone Nucleophilic Aldehyde / Ketone Nucleophilic Addition ReactionsAddition Reactions

R C R'O

R"MgXH+

H2OR C

OHR'

R"

R C R'O

R C R'O

R C R'O

H2OC

R

R'

OH

OH

dry HClR''OH C

R

R'

OH

OR"C

R

R'

OR"

OR"hemiketal ketal

H+ or OH-gem diol

HCN CR

R'

OH

CNcyanohydrin

R C R'O

H2N G ( H2O)- C N GRR'

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Ketals: Acetals of KetonesAcetal (ketal) formation with ketones is generally not a favorable process:

CR'

R

OR''

OR''+ H2O

H+

C=OR'

R+ 2 R''OH

KetalKetone H+

As in the hydration reaction, the equilibrium heavily favors the ketone.

Cyclic ketals, formed from 1,2- or 1,3-diols, are an exception:

C=OR'

R+ HOCH2CH2OH

1,2-Ethanediol(ethylene glycol)

H+

H+C

R'

R

O-CH2

O-CH2+ H2O

Cyclic ketal (stable)

Cyclic acetals or ketals hydrolyze easily in aqueous acid solution:

C=OR'

R+ HOCH2CH2OHH+

CR'

R

O-CH2

O-CH2+ H2O

excess