18.8 Oxidation Oxidation by silver ion requires an...
Transcript of 18.8 Oxidation Oxidation by silver ion requires an...
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18.8 Oxidation Oxidation by silver ion requires an
alkaline medium
to prevent precipitation of the insoluble silver oxide, a complexing
agent is added: ammonia Test for detecting aldehydes
does not attack carbon-carbon double bonds.
occurs on either side of the carbonyl group (mixtures of carboxylic acids)
hypohalite
Methyl ketones
Tollens’ reagent
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18.9 Reduction
Ketone Alcohol 2O
Aldehyde
Alcohol 1O
Red
uct
ion
9-Borabicycio[3.3. 1]nonane (9-BBN) As dimer
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Aldehydes and ketones can be reduced to hydrocarbons by the action
(a) Of amalgamated zinc and concentrated hydrochloric acid, the Clemmensen reduction;
or (b) of hydrazine, NH2NH2 , and a strong base like KOH or potassium tert-butoxide,
the Wolff-Kishner reduction.
reduction by the action of such compounds as lithium aluminum hydride, LiAlH4
the nucleophile is hydrogen transferred with a pair of electrons as a hydride ion, H:- from the metal to carbonyl carbon
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18.10 Addition of cyanide
strongly basic
cyanide ion
Nitril group
hydrolysis
unsaturated acids
a-hydroxyacids
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18.11 Addition of derivatives of ammonia
for the characterization and identification of aldehydes and ketones
elimination of a molecule of water
these derivatives of ammonia are basic The salts are less easily oxidized by air than the free bases the basic reagents are liberated from their salts by addition of a base, usually sodium acetate.
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the solution must be acidic enough for an appreciable fraction of the carbonyl compound to be protonated, but not so acidic that the concentration of the free nitrogen compound is too low. The exact conditions used depend upon the basicity of the reagent, and upon the reactivity of the carbonyl compound.
PH adjustment
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18.12 Addition of alcohols. Acetal formation
Alcohols + the carbonyl group of aldehydes acetals Anhydrous acids
water is often removed as it is formed by means of the azeotrope of water, benzene, and ethyl alcohol
alcoholic solution an aldehyde exists in equilibrium with a compound called a hemiacetal
too unstable to be isolated
alcohol
ether
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SN1 route
Acetal formation thus involves
(a) nucleophilic addition to a carbonyl group,
and (b) ether formation via a carbocation.
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acetals rapidly converted even at room temperature into the aldehyde and alcohol by dilute mineral acids.
“ carbonium” ion
oxonium ion
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18.13 Cannizzaro reaction
aldehydes containing no a-hydrogens concentrated alkali
Alcohol + salt of carboxylic acid self-oxidation-and-reduction
Crossed Cannizzaro reaction
If one of the aldehydes is formaldehyde yields almost exclusively sodium formate
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addition of hydroxide ion to give intermediate I
addition of a hydride ion from I to a second molecule of aldehyde.
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18.14 Addition of Grignard reagents
R= alkyl(1O, 2O, 3O); allylic; aralkyl( benzyl); aryl
(ArMgCl must be made in the THF)
C-Mg (highly polar band)
Gelatinous material
Organolithium compounds
Li Mg electropositivity
C-Li C-Mg polarity
More negative
organolithium Grignard reagents reactivity
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18.15 Products of the Grignard synthesis
The Grignard synthesis is so important, - Formation a C-C bond - C nucleophile + C electrophile C=O( nucleophilic addition) - Formation of –OH group for further synthesis of bigger and more complicated structures
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to make primary alcohols containing two more carbons than the Grignard reagent. (utilizes ethylene oxide) breaking of a carbon-oxygen s bond in the highly strained three-membered ring
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18.16 Planing a Grignard synthesis
It depends upon which rectant are
more readily available
Simple alcohol
two-C
four-C
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And prepare larger alcohols
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And synthesis of aromatic alcohols
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Target molecule
Alcohol 3O Grignard reagent + ketone
Two possibilities
4C 3C
Let us suppose that we have available all alcohols of four carbons or fewer
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Synthesis of aromatic alcohol
Alcohol 2O Grignard reagent + aldehyde
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Lithium acetylide
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18.17 syntheses using alcohols
Synthesis of other compounds from alcohols
Target molecule
dehydrohalogenation
dehydration
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rearrangement
dehydrohalogenation Pure form
No rearrangement
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1 C more than our largest available alcohol
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18.18 Limitations of the Grignard synthesis
any compound containing hydrogen attached to an electronegative element oxygen, nitrogen, sulfur, or even triply-bonded carbon is acidic enough to decompose a Grignard reagent.
We cannot prepare a Grignard reagent from a compound (e.g., HOCH2CH2Br)
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18.20 Analysis of aldehyde and ketones
Aldehydes or ketones 2,4-dinitrophenylhydrazine
an insoluble yellow or red solid
aldehydes Tollens' reagent
ketones Tollens' reagent
phenols and amines
Schiff test aldehyde fuchsin-aldehyde reagent
magenta color
Aliphatic aldehydes and ketones (having a-hydrogen)
Br2 / CC14 too slow (- HBr)
Aldehydes and ketones identified m.p. of derivatives like
2,4-dinitrophenylhydrazones, oximes, and semicarbazones
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18.21 Iodoform test
Methyl ketones are characterized through the iodoform test
Sodium hypoiodide
iodoform
Hypohalites can not only halogenate but also oxidize
alcohols
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(ethanol)
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18.22 Analysis of 1,2-diols. Periodic acid oxidation
Two or more C=O or –OH group attached to adjacent carbon
Periodic acid