Tautomeria y Oxido Reduccion

8/3/2019 Tautomeria y Oxido Reduccion

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Acidity of -Hydrogens

Hydrogens alpha to acarbonyl group aremore acidic than

hydrogens of alkanes,alkenes, and alkynesbut less acidic than the

hydroxyl hydrogen ofalcohols

Type of Bond pK a

16

20

25

44

51

O

CH3 C C- H

CH3 CH2 O- H

CH3 CCH2 - H

CH2 = C H - H

CH3 CH2 - H


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Acidity of -Hydrogens

-Hydrogens are more acidic because theenolate anion is stabilized by

1. delocalization of its negative charge

2. the electron-withdrawing inductive effect ofthe adjacent electronegative oxygen

:

:

O

CH3 -C-CH2 -H :A-

O-

CH3 -C=CH2 H- A

Enolate anion

+

+

O

CH3 -C CH2


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Keto-Enol Tautomerism

protonation of the enolate anion on oxygengives the enol form; protonation on carbongives the keto form

Enolate anion

Enol formKeto form

-O

CH3 -C-CH2 CH3 -C=CH2

CH3 -C=CH2

H- A

CH3 -C-CH3 + A-

-

+

H- A OH

O-

O


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Keto-enolequilibriafor simple

aldehydesandketones lie

far towardthe ketoform

OH

O

O

CH3 CH CH2 = CH

CH3 CCH3

Keto form Enol form

% Enol at

Equilibrium

6 x 10-5

OH

CH3 C=CH2 6 x 10-7

O

O OH

OH

1 x 10-6

4 x 10-5


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For certain types of molecules, however,the enol is the major form present atequilibrium

for -diketones, the enol is stabilized byconjugation of the pi system of the carbon-carbon double bond and the carbonyl group

H

H

HH

HO H

O

conjugated

system

1,3-Cyclohexanedione

O

O

O

OH


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Otro ejemplo de Tautomera


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En este par de tautmeros, la fenol-imina se convierte fotoqumicamente, osea, por absorcin de luz , en ceto-enamina.

La irradiacin de la fenol-imina causa que el hidrgeno unido al oxgeno vayaal nitrgeno de la ceto-enamina. En la oscuridad la ceto-enamina, cambia a la

fenol-imina ms estable


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Oxidation of Aldehydes

Aldehydes are oxidized to carboxylic acidsby a variety of oxidizing agents, includingH2CrO4

CHO H2 CrO4 COOH

Hexanal Hexanoic acid


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Chapter 18

Ejemplos Oxidacin de Aldehidos

Facilmente se oxidan a cidos carboxlicos


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Aldehydes are oxidized by O2 in a radicalchain reaction

liquid aldehydes are so sensitive to air that

they must be stored under N2

Benzoic acidBenzaldehyde

+CH

O O

COH2O22


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Aldehydes are also oxidized by Ag(I)

in one method, a solution of the aldehyde inaqueous ethanol or THF is shaken with a

slurry of silver oxide

Vanillic acid

Vanillin

+

+

CH

HO

CH3 O

O

O

CH3 O

HO

COH

Ag2 OTHF, H2 O

NaOH

Ag

HCl

H2 O


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Test Tollens

Adicionar solucin de amonaco a solucin de AgNO3hasta que el precipitado se disuelva.

Reaccin con aldehido forma un espejo de plata.

R C

O

H + 2 + 3Ag(NH3)2+

OH_ H2O

+ 2+ 4+2 Ag R C

O

O_

NH3 H2O


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Oxidation of Ketones

ketones are not normally oxidized by chromicacid

they are oxidized by powerful oxidants at high

temperature and high concentrations of acidor base

Hexanedioic acid(Adipic acid)

Cyclohexanone(keto form)

Cyclohexanone(enol form)

HNO3

O

HOOH

OO OH


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En el organismo los oxidantes y reductores utilizados son diferentes en suestructura a los usados comnmente en el laboratorio.

La estructura indicada corresponde NAD+, un oxidante en el organismo.

En la diapositiva que sigue aparece NAD+ con la palabra ad unida alanillo heterocclico para indicar la ribosa unida a ADP


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Reduction

aldehydes can be reduced to 1 alcohols

ketones can be reduced to 2 alcohols

the C=O group of an aldehyde or ketone can

be reduced to a -CH2- group

AldehydesCan BeReduced to Ketones

Can BeReduced to

O O

OH

RCHRCH2 OH

RCH3

RCR'RCHR'

RCH2 R'


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Hidrogenacin Catalitica

Ampliamente usada en la industria.

Niquel Raney: Ni en polvo finamente divididosaturado con hidrgeno gas.

Pt y Rh tambien se usan como catalizadores.

O Raney Ni

OH

H


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Catalytic Reduction

Catalytic reductions are generally carriedout at from 25 to 100C and 1 to 5 atm H2

+25 oC, 2 atm

Pt

Cyclohe xanone Cyclohe xanol

O OH

H2

1-Butanoltrans-2-Butenal

(Crotonalde hyde)

2 H2

NiH

O

OH


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Catalytic Reduction

A carbon-carbon double bond may also bereduced under these conditions

by careful choice of experimental conditions, itis often possible to selectively reduce acarbon-carbon double in the presence of analdehyde or ketone

1-Butanoltrans-2-Butenal

(Crotonalde hyde)

2 H2

NiH

O

OH


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Metal Hydride Reduction

The most common laboratory reagents forthe reduction of aldehydes and ketonesare NaBH4 and LiAlH4 both reagents are sources of hydride ion, H:-,

a very powerful nucleophile

Hydride ionLithium aluminum

hydride (LAH)

Sodium

borohydride

H

H H

H

H-B-H H-Al-HLi +Na+

H:


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Reduccin

Borohidruro de sodio, NaBH4, reduceC=O, pero no C=C.

Hidruro de Litio y aluminio, LiAlH4,mucho ms fuerte, difcil de manejar

Hidrgeno gaseoso con catalizador

tambien reduce el enlace C=C.


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NaBH4 Reduction

reductions with NaBH4 are most commonlycarried out in aqueous methanol, in puremethanol, or in ethanol

one mole of NaBH4 reduces four moles ofaldehyde or ketone

4RCH

O

NaBH4

( RCH2O) 4B-

Na+ H2O

4RCH2OH

A tetraalkyl borate

boratesalts

+

+methanol


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NaBH4 Reduction

The key step in metal hydride reduction istransfer of a hydride ion to the C=O groupto form a tetrahedral carbonyl addition

compound

from

water

from t he h ydride

reducing agent

+

H

H O O BH3

OH

H

H

H-B-HNa+

R-C-R' R-C-R'

Na+

H2 O

R-C-R'


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LiAlH4 Reduction

unlike NaBH4, LiAlH4 reacts violently withwater, methanol, and other protic solvents

reductions using it are carried out in diethyl

ether or tetrahydrofuran (THF)4RCR LiAlH4

(R2CHO)4Al

-

Li

+ H2O

4RCHR

OH

+

+ aluminum salts

ether

A tetraalkyl aluminate

O


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Clemmensen Reduction

refluxing an aldehyde or ketone withamalgamated zinc in concentrated HClconverts the carbonyl group to a methylene

group

Zn( Hg) , HCl

OH O OH


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Wolff-Kishner Reduction in the original procedure, the aldehyde or

ketone and hydrazine are refluxed with KOHin a high-boiling solvent

the same reaction can be brought about usinghydrazine and potassium tert-butoxide in

DMSO

+

diethylene glycol(reflux)

KOH

N2

Hydrazine

+ H2 NNH2

+ H2 O

O


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A continuacin en la siguiente diapositivase describe la reduccin en el organismo,mediante el reductor NADH, diferente en

estructura a los utilizados en ellaboratorio.


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Formacin de hemiacetalescclicos en Carbohidratos

C li h i l f i


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Cyclic hemiacetal formationFigure 25.3


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Haworth Projections

Haworth projections five- and six-membered hemiacetals are

represented as planar pentagons orhexagons, as the case may be, viewed

through the edge most commonly written with the anomeric

carbon on the right and the hemiacetal

oxygen to the back right the designation - means that -OH on the

anomeric carbon is cis to the terminal -CH2OH; - means that it is trans


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Haworth Projections

CH2OH

CHO

OH

H

OH

H

HO

H

H OH

H

H OH

HHO

H OHOH

H

CH2OH

O

C

H OH

HHO

HOH

H

CH2OH

OHO

H

OH

H OH

HHO

H HOH

H

CH2OH

O

D-Glucose

-D-Glucopyranose(-D-Glucose)

()()

-D-Glucopyranose(-D-Glucose)

anomericcarbon

+

anomericcarbon

5

5

12

3

4

6

1

23

4

6

redraw


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Haworth Projections

six-membered hemiacetal rings are shown bythe infix -pyran-

five-membered hemiacetal rings are shown by

the infix -furan-

OO

PyranFuran


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Conformational Formulas

five-membered rings are so close to beingplanar that Haworth projections are adequateto represent furanoses

O

OH()

H

H

HO OH

H H

HOCH2O

H

OH()

H

HO OH

H H

HOCH2

-D-Ribofuranose(-D-Ribose)-D-Ribofuranose(-D-Ribose)


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for pyranoses, the six-membered ring is moreaccurately represented as a strain-free chairconformation

-D-Glucopyranose(chair conformation)

OCH2 OH

HOHO

OHOH ()


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if you compare the orientations of groups oncarbons 1-5 in the Haworth and chairprojections of -D-glucopyranose, you will see

that in each case they are up-down-up-down-up respectively

O

CH2OH

HOHO

OHOHH

H OH

HHO

H

OH

OH

H

CH2OH

O

-D-Glucopyranose(chair conformation)

-D-Glucopyranose(Haworth projection)

()()


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Mutarotation

Mutarotation: the change in specificrotation that occurs when an or form ofa carbohydrate is converted to an

equilibrium mixture of the two

+80.2

+80.2

+52.8

+150.7

-D-galactose-D-galactose

[] afterMutarotation

(degrees)

[]Monosaccharide

% Present at

Equilibrium

28

72

64

36-D-glucose

-D-glucose+112.0

+18.7

+52.7

+52.7

(degrees)

Haworth formulas for and pyranose forms


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Haworth formulas for and -pyranose formsof D-glucoseFigure 25.4

Species present in aqueous solution of D ribose


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Species present in aqueous solution of D-riboseFigure 25.5


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Mutarotation

[] D25 + 18.7

-D-Glucopyranose

-D-Glucopyranose

Open-chain form

()

()

[] D25 + 112

OHOH

HOHO CH

2 OH

OCH2 OH

O

HOHO

HOOH

OH

OC

CH2 OHHO

HO

HO H

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