Other Reactions of Ketones and Aldehydes. Relative Reactivity of Carboxylic Acid Derivatives.
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Transcript of Other Reactions of Ketones and Aldehydes. Relative Reactivity of Carboxylic Acid Derivatives.
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Other Reactions of Ketones and Aldehydes
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R Cl
O
R O
O
R
O
R OR'
O
R
O
NH2
Increasing
Reactivity
Acid Chloride
Anhydride
Ester
Amide
Relative Reactivity of Carboxylic Acid Derivatives
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R Cl
O
R O
O
R
O
R OR'
O
R
O
NH2
Increasing
Reactivity
Acid Chloride
Anhydride
Ester
Amide
R H
O
R R'
O
Aldehyde
Ketone
Relative Reactivity of Carbonyl-containing Compounds
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Formation of hydrates (gem-diols) from aldehydes and ketones
R1 R2
O
+
-
R1 R2
HOH2O
OH
gem-diol
cat. acid or base
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R1 R2
O
H+
R1 R2
OH+
R1 R2
OH
HO H
R1 R2
HO OH2+
R1 R2
HO OH- H+
+ H+
+ H+
- H+
Mechanism of acid-catalyzed hydration of a ketone
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Formation of Acetals and Ketals
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Formation of hemiketals and ketals from ketones
R1 R2
O
+
-
R1 R2
HOR3OH, cat. H+ OR3
hemiketal
R1 R2
R3O OR3
ketal
R3OH, cat. H+
+ H2O
The equilibrium can be driven to the right by removal of water from the reaction (through a Dean-Stark trap, or addition of a drying agent)
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Full ketals and acetals are quite stable (including stability to strong base), but hemiketals and hemiacetals are in equilibrium with the aldehyde and ketone, with the C=O group usually favored, except in the case of cyclic hemiacetals, like carbohydrates, which exist primarily as cyclic hemiacetals as shown above. Note that the six-membered ring size is favored.
Hemiacetal
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The equilibrium between the open and closed (glucopyranose) form of glucose results in epimerization of the anomeric carbon (the aldehyde carbonyl carbon in the open chain form)
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Notice that there are two C=O’s, but only the aldehyde (and not the carbamate) carbonyl is affected by these reaction conditions.
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Note the use of a diol to facilitate transformation to the FULL acetal, through formation of a five-membered ring.
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Note that only the ketone carbonyl is converted to the ketal (since ketones are more reactive than amides).
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This (ketalization) reaction is reversible under acidic
conditions (and upon the addition of water)
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Use (of ketal) to ‘protect’ the ketone during reduction of the esters
Note that the ketal is formed in step 1, the reduction is performed in step 2, and the ketal is converted back to the ketone in step 3, above.
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Formation of Oximes and Hydrazones from Aldehydes
and Ketones
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Compared with other imines (C=N), oximes and hydrazones are more stable,due to resonance from the adjacent heteroatom through the C=N.
R1 R2
OH2NOH - HCl
NaOAc R1 R2
NHO
Oxime(usually forms as mix of E and Z isomers)
+ H2O
R1 R2
OH2NNH2
NaOAc R1 R2
NH2N
Hydrazone(usually forms as mix of E and Z isomers)
+ H2O
hydroxylamine hydrochloride
hydrazine
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R1 R2
O
H2NOH
H+
R1 R2
HOH2
+
NOH
R1 R2
H2O+ HN
OH
R1 R2
NHO
+ H2O- H+
Mechanism of oxime formation
Hydroxylamine(usually added as hydroxylamine hydrochloride)
Oxime(frequently solid)
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Use of hydrazine (H2N-NH2) leads to formation of the corresponding hydrazone.
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Formation of Imines
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Imines form rapidly from aldehydes and primary amines, but are not stable to hydrolysis (back to the aldehyde and amine), and are rarely isolable.
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Combination of Imine Formation with Hydride (NaBH3CN) Reduction of Intermediate Imine, to Produce
Amine (Reductive Amination)
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R1 R2
OH2N-R3
R1 R2
NR3
Imine
+ H+
R1R2
N+
R3H
H
-BH2CN
Iminium Ion
R1R2
N+
R3H
-BH2CN
H R1R2
NHR3
H
workup
Mechanism of Reductive Amination Procedure
Notice that it is the N-protonated iminium ion which is reduced by the hydride reagent. To do this requires a hydride reagent which is stable under slightly acidic conditions. The two most commonly used reagents are:
sodium cyanoborohydride, NaBH3CN, and sodium triacetoxyborohydride, NaBH(OAc) 3
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