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Reactions of aldehydes and Ketones Aldehydes and Ketones undergo many reactions to give a wide variety of

useful derivatives. There are two general kinds of reactions that

aldehydes and ketones undergo:

1- Reaction at the carbonyl carbon (Nucleophilic addition reactions).

2-reaction at the α carbon.

A second general reaction of aldehydes and ketones involves reaction at

the α carbon. A C–H bond on the α carbon to a carbonyl group is more

acidic than many other C–H bonds, because reaction with base forms a

resonance-stabilized enolate anion.

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1- Nucleophilic addition reaction.

Two general mechanisms are usually drawn for nucleophilic addition,

depending on the nucleophile (negatively charged versus neutral) and the

presence or absence of an acid catalyst. With negatively charged

nucleophiles, nucleophilic addition follows the two-step process first

(nucleophilic attack) followed by protonation.

Absence of an acid catalyzed nucleophilic addition

Step [1]: The nucleophile attacks the carbonyl group, cleaving the π bond

and moving an electron pair onto oxygen. This forms a sp3 hybridized

intermediate with a new C–Nu bond.

Step [2]: protonation of the negatively charged O atom by H2O forms the

addition product.

Acid-catalyzed nucleophilic addition

The general mechanism for this reaction consists of three steps (not two),

but the same product results because H and Nu- add across the carbonyl π

bond. In this mechanism protonation precedes nucleophilic attack.

Step [1] Protonation of the carbonyl group

In Step [2], the nucleophile attacks, and then deprotonation forms the

neutral addition product in Step [3].

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Steps [2]–[3] Nucleophilic attack and deprotonation

a) Addition of Alcohols (Acetal Formation): Aldehydes and ketones react with two equivalents of alcohol to form

acetals. In an acetal, the carbonyl carbon from the aldehyde or ketone is

now singly bonded to two OR" (alkoxy) groups.

b) Nucleophilic Addition of H- (Reduction reaction)

Treatment of an aldehyde or ketone with either Sodium borohydride

(NaBH4) or Lithium hydride (LiAlH4) followed by protonation forms a 1°

or 2° alcohol.

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c) Reduction of Ketones and Aldehydes

Clemmensen reduction: The Clemmensen reduction is most commonly used to

convert acylbenzenes (from Friedel-Crafts acylation) to alkylbenzenes,

but it also works with other ketones or aldehydes that are not sensitive to

acid. The carbonyl compound is heated with an excess of amalgamated

zinc (zinc treated with mercury; Zn (Hg), and concentrated hydrochloric

acid (HCl). The actual reduction occurs by a complex mechanism on the

surface of the zinc.

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The Clemmensen reduction uses zinc and mercury in the presence of

strong acid.

Wolff– Kishner reduction: Compounds that cannot survive treatment with hot acid can be

deoxygenated using the Wolff–Kishner reduction. The ketone or

aldehyde is converted to its hydrazone, which is heated with Hydrazine

(NH2NH2), and strong base such as KOH. Ethylene glycol, diethylene

glycol, or another high-boiling solvent is used to facilitate the high

temperature (140-200°C) needed in the second step.

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d) Nucleophilic Addition of CN–

Treatment of an aldehyde or ketone with NaCN and a strong acid such as

HCl adds the elements of HCN across the carbon–oxygen π bond,

forming a cyanohydrin.

O

i) NH2NH2

ii) base

HH

CyclopentaneCyclopentanone

C

H

ONaCN

NaHSO3

C

H

CN

OH

MandelonitrileBenzaldehyde

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e) Addition of Bisulfate.

Sodium bisulfate adds to most aldehydes and to many ketones (especially

methyl ketones and cycloketones) to form bisulfate addition products:

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f) Addition of Organo Metallic reagents

The addition of Grignard reagents to aldehydes and ketones yields

alcohols. The organic group, transferred with a pair of electrons from

magnesium to carbonyl carbon, is a powerful nucleophile.

Prepration of Grignard reagents

C

O

+ R: MgX

C

R

OMgXH2O

C

R

OH + Mg(OH)X

H+

Mg++ + X- + H2O

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g) Addition of derivatives of Ammonia:-

Treatment of an aldehyde or ketone with a 1° amine affords an imine

(Schiff base). Nucleophilic attack of the 1° amine on the carbonyl

group forms an unstable carbinolamine, which loses water to form an

imine. The overall reaction results in replacement of C=O by C=NR.

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Aldehydes are readily oxidized to yield carboxylic acids; but

ketones are generally inert toward oxidation.

The difference is a consequence of structure: aldehydes have a –

CHO proton that can be abstracted during oxidation, but ketones do

not.

Oxidation reaction

RC

H

OHydrogen here

RC

OH

O[O]

An aldehyde Carboxylic acid

RC

R

O No hydrogen here

A ketone

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Many oxidizing agents, including KMnO4 and hot HNO3, convert

aldehydes into carboxylic acid.

Tollen's reagent In the laboratory, oxidation of an aldehyde can be carried out using a

solution of silver oxide (Ag2O) in aqueous ammonia, the so-called

Tollen's reagent. Oxidation of aldehyde is accompanied by reduction of

silver ion to free silver (in the form of a mirror under the proper

conditions).

RCHO or ArCHOKMnO4

K2Cr2O7

RCOOH or ArCOOH

hot HNO3

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Methyl ketones:

Oxidation of ketones required breaking of C–C bond next to the

carbonyl group and takes place only under vigorous conditions, except

for methyl ketones which oxidized smoothly by mean of hypohalite

(NaOX) to form Haloform (Haloform reaction).