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

18.9 Reduction

Ketone Alcohol 2O

Aldehyde

Alcohol 1O

Red

uct

ion

9-Borabicycio[3.3. 1]nonane (9-BBN) As dimer

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

18.10 Addition of cyanide

strongly basic

cyanide ion

Nitril group

hydrolysis

unsaturated acids

a-hydroxyacids

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.

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

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

SN1 route

Acetal formation thus involves

(a) nucleophilic addition to a carbonyl group,

and (b) ether formation via a carbocation.

acetals rapidly converted even at room temperature into the aldehyde and alcohol by dilute mineral acids.

“ carbonium” ion

oxonium ion

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

addition of hydroxide ion to give intermediate I

addition of a hydride ion from I to a second molecule of aldehyde.

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

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

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

18.16 Planing a Grignard synthesis

It depends upon which rectant are

more readily available

Simple alcohol

two-C

four-C

And prepare larger alcohols

And synthesis of aromatic alcohols

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

Synthesis of aromatic alcohol

Alcohol 2O Grignard reagent + aldehyde

Lithium acetylide

18.17 syntheses using alcohols

Synthesis of other compounds from alcohols

Target molecule

dehydrohalogenation

dehydration

rearrangement

dehydrohalogenation Pure form

No rearrangement

1 C more than our largest available alcohol

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)

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

18.21 Iodoform test

Methyl ketones are characterized through the iodoform test

Sodium hypoiodide

iodoform

Hypohalites can not only halogenate but also oxidize

alcohols

(ethanol)

18.22 Analysis of 1,2-diols. Periodic acid oxidation

Two or more C=O or –OH group attached to adjacent carbon

Periodic acid