[PDF] Chapitre CO-PC1. Seconde période Plan du cours Oxydoréduction

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Chapitre CO-PC1.Seconde périodePlan du cours

Oxydoréduction en

chimie organique lycée Jean DautetPCSI

Lycée Jean DautetPCSI D.Lecorgne

mardi 24 mai 16

I-Nombred'oxyda/onenchimieorganique

2 2s 2 2p 2 Ladé8initiondunombred'oxydationetso ncalcul serontétudiésdanslechapitre d'oxydoréductionensolutionaqueuse. Nous nouscontenteronsicidequelques résultatsutilesenchimieorganique. Complétantsacoucheexternea vec4éle ctronsa8ind'acquér irlacon8iguration isoélectroniquedunéon(2s 2 2p 6 danscecas-IV. soit 9 valeurs différentes... mardi 24 mai 16 - IV CH 4 + IV CO 2

C a pris 4

électrons de

valence à 4 atomes H parce que C est plus

électronégatif

que H mardi 24 mai 16 - IV CH 4 + IV CO 2

C a donné 4

électrons de

valence à 2 atomes parce que C est moins

électronégatif

que O mardi 24 mai 16 - IV CH 4 + IV CO 2

C a pris 4

électrons de

valence à 4 atomes H

C a donné 4

électrons de

valence à 2 atomes parce que C est plus

électronégatif

que H parce que C est moins

électronégatif

que O mardi 24 mai 16 OH O H O OH O OH O

H : on prend +1 (I)O : on prend - 2 (-II)

la somme des n.o d'un édifice neutre doit faire 0 mardi 24 mai 16 mardi 24 mai 16 atomesdecarbone.

Uneréductionen chimieor ganiquesetradui tpar

unediminutiondunombred'oxydationmoyend es atomesdecarbone. mardi 24 mai 16

Enchimieorganique:

Lesoxydationscorrespondentleplussouvent:

o àunediminutiondunombred'atomesd'hydrogène o ouuneaugmentationdunombred'atomesd'oxygène

Etdemême,àl'inverse:

o àunediminutiondunombred'atomesd'oxygène o ouuneaugmentationdunombred'atomesd'hydrogène mardi 24 mai 16 subituneoxydation).

Exemples:

mardi 24 mai 16 subituneoxydation).

Exemples:

Demi-équation électronique de l'oxydation de l'éthanol en éthanal : CH 3 CH 2

OH = CH

3

CHO + 2 H

+ 2 e Demi-équation électronique de l'oxydation de l'éthanol en acide carboxylique : CH 3 CH 2

OH + 2 H

2

O = CH

3

COOH + 4 H

+ 4 e Demi-équation électronique de l'oxydation de l'éthène en éthane-1,2-diol : CH 2 =CH 2 + 2 H 2

O = OH-CH

2 -CH 2 -OH + 2 H + 2 e Et pour l'éthène " transformé » en éthanol ? CH 2 =CH 2 + H 2

O = CH

3 -CH 2 -OH CE N'EST PA S DE L'OXYDORÉD UCTION, C'EST UNE RÉACT ION D'ADDITION D'EAU, UNE RÉACTION

D'HYDRATATION

mardi 24 mai 16

IIIOxyda(ondesalcools

L'oxydationdite"ménagée »:iln'yapasdanscecasdemodi8icationdu squelettecarboné. carboneCO 2 etd'eauH 2

O,ils'agitdanscecasd'unecombustion.

mardi 24 mai 16 combustionduglucoseC 6 H 12 O 6 enCO 2(g) etH 2 O (l) C 6 H 12 O 6 + 6 O 2 = 6 CO 2 + 6 H 2 O mardi 24 mai 16 oxydé en oxydé en oxydé en aldéhydecétoneacide pas oxydé en en pas oxydé en en mardi 24 mai 16 RCH 2

OH = RCHO + 2 H

+ 2 e R 2

CHOH = R

2

CO + 2 H

+ 2 e RCH 2

OH + 2 H

2

O = RCOOH + 4 H

+ 4 e mardi 24 mai 16

Quelsoxydantsutiliser?

plusutiliséss ontledichromatedepotassiumK 2 Cr 2 O 7 ,oudesodiumNa 2 Cr 2 O 7 ,ou l'oxydedechromeCrO 3 2 CrO 4

LepermanganatedepotassiumKMnO

4 :lesionspermanganateMnO 4 sontréduits enionsMn 2+ sont réduitsenCl mardi 24 mai 16

Quel cocktail !

mardi 24 mai 16 peracids do not react with alcohols: they are chemoselective oxidants of C=C double bonds only. Later in the book, you will meet more oxidizing agents, such as osmium tetroxide (OsO 4 ) and ozone (O 3 ) - these are also chemoselective for double bonds, because they react with the C=C !bond, and

we shall leave them until Chapter 35. In this section we will be concerned only with oxidizing agents

that oxidize alcohols and carbonyl compounds. The most commonly used methods for oxidizing alcohols are based around metals in high oxida- tion states, often chromium(VI) or manganese(VII), and you will see that mechanistically they are quite similar - they both rely on the formation of a bond between the hydroxyl group and the metal.

Another class of oxidations, those that use halogens, sulfur, or nitrogen in high oxidation states, we

will deal with relatively briefly.

How to oxidize secondary alcohols to ketones

We start with this, because overoxidation is difficult. Provided the alcohol is not acid-sensitive, a good method is sodium dichromate in dilute sulfuric acid. This is usually added to a solution of the alcohol in acetone, and is known as the Jones oxidation.

The mechanism starts with the formation of HCrO

4 ions, that is, Cr(VI), from dichromate ion in solution. In acid, these form chromate esters with alcohols. The esters (boxed in black) decompose by elimination of the Cr(IV) HCrO 3 , which subsequently reacts with a Cr(VI) species to yield 2 " Cr(V). These Cr(V) species can oxidize alcohols in the same way, and are thereby reduced to Cr(III)

(the final metal-containing by-product). Cr(VI) is orange and Cr(III) is green, so the progress of the

reaction is easy to follow by colour change. Chromic acid is best avoided if acid-sensitive alcohols are to be oxidized, and an alternative reagent for these is PCC (pyridinium chlorochromate), which can be used in dichloromethane.

How to oxidize primary alcohols to aldehydes

Aqueous methods like the Jones oxidation are no good for this, since the aldehyde that forms is fur-

ther oxidized to acid via its hydrate. The oxidizing agent treats the hydrate as an alcohol, and oxidizes

it to the acid. 63824
Chemoselectivity: selective reactions and protection

Oxidizing agents

Chemoselective for C=C double bonds

a

Chemoselective for alcohols or carbonyl compounds

peracids, RCO 3

H (Chapter 20)Cr(VI) compounds

osmium tetroxide, OsO 4 (Chapter 35)Mn(VII) compounds ozone, O 3 (Chapter 35)some high oxidation state Hal, N, or S compounds a not dealt with in this chapter. RR OH RR O Na 2 Cr 2 O 7 H 2 SO 4 , acetone Cr 2 O 7 + H 2 O H Cr OH O O O Cr O OO RR OH H RH O Cr O O OH R RR O Cr OH O

HOCr(III)

chromate ester oxidation of alcohols with Cr(VII) 2- N H Cr Cl O O O pyridinium chlorochromate, PCC

ROHR OROH RO

OHOHH oxidizeoxidize H 2 O aldehydehydrate overoxidation of aldehydes alcool primaire aldéhydeacide mis en jeu dans l'alcootestmis en jeu dans le dosage de l'éthanol dans le vin oxydationoxydation mardi 24 mai 16 l'aldéhyde.

CitonsparexempleCrO

3 ,av ecl'acidechl orhydriqueHCl,etenp résencede pyridine,danslediclorométha ne:ilenrés ulteuna gentoxydantappelé chlorochromatedepyridinium,pyH CrO 3 Cl ,notéePCC. peracids do not react with alcohols: they are chemoselective oxidants of C=C double bonds only. Later in the book, you will meet more oxidizing agents, such as osmium tetroxide (OsO 4 ) and ozone (O 3 ) - these are also chemoselective for double bonds, because they react with the C=C πbond, and

we shall leave them until Chapter 35. In this section we will be concerned only with oxidizing agents

that oxidize alcohols and carbonyl compounds. The most commonly used methods for oxidizing alcohols are based around metals in high oxida- tion states, often chromium(VI) or manganese(VII), and you will see that mechanistically they are quite similar - they both rely on the formation of a bond between the hydroxyl group and the metal.

Another class of oxidations, those that use halogens, sulfur, or nitrogen in high oxidation states, we

will deal with relatively briefly.

How to oxidize secondary alcohols to ketones

We start with this, because overoxidation is difficult. Provided the alcohol is not acid-sensitive, a good method is sodium dichromate in dilute sulfuric acid. This is usually added to a solution of the alcohol in acetone, and is known as the Jones oxidation.

The mechanism starts with the formation of HCrO

4 ions, that is, Cr(VI), from dichromate ion in solution. In acid, these form chromate esters with alcohols. The esters (boxed in black) decompose by elimination of the Cr(IV) HCrO 3 , which subsequently reacts with a Cr(VI) species to yield 2 × Cr(V). These Cr(V) species can oxidize alcohols in the same way, and are thereby reduced to Cr(III)

(the final metal-containing by-product). Cr(VI) is orange and Cr(III) is green, so the progress of the

reaction is easy to follow by colour change. Chromic acid is best avoided if acid-sensitive alcohols are to be oxidized, and an alternative reagent for these is PCC (pyridinium chlorochromate), which can be used in dichloromethane.

How to oxidize primary alcohols to aldehydes

Aqueous methods like the Jones oxidation are no good for this, since the aldehyde that forms is fur-

ther oxidized to acid via its hydrate. The oxidizing agent treats the hydrate as an alcohol, and oxidizes

it to the acid. 63824
Chemoselectivity: selective reactions and protection

Oxidizing agents

Chemoselective for C=C double bonds

a

Chemoselective for alcohols or carbonyl compounds

peracids, RCO 3

H (Chapter 20)Cr(VI) compounds

osmium tetroxide, OsO 4 (Chapter 35)Mn(VII) compounds ozone, O 3 (Chapter 35)some high oxidation state Hal, N, or S compounds a not dealt with in this chapter. RR OH RR O Na 2 Cr 2 O 7 H 2 SO 4 , acetone Cr 2 O 7 + H 2 O H Cr OH O O O Cr O OO RR OH H RH Oquotesdbs_dbs44.pdfusesText_44

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