Tautomeria y Oxido Reduccion
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Transcript of 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-Enol Tautomerism
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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Keto-Enol Tautomerism
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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Oxidation of Aldehydes
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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Oxidation of Aldehydes
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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Conformational Formulas
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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Conformational Formulas
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