18.8 Oxidation. Oxidation by silver ion requires an alkaline medium

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1 18.8 Oxidation Oxidation by silver ion requires an alkaline medium Test for detecting aldehydes Tollens reagent to prevent precipitation of the insoluble silver oxide, a complexing agent is added: ammonia does not attack carbon-carbon double bonds. occurs on either side of the carbonyl group (mixtures of carboxylic acids) Methyl ketones hypohalite

2 Reduction 18.9 Reduction Ketone Alcohol 2 O Aldehyde Alcohol 1 O 9-Borabicycio[3.3. 1]nonane (9-BBN) As dimer

3 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, NH 2 NH 2, 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, LiAlH 4 the nucleophile is hydrogen transferred with a pair of electrons as a hydride ion, H: - from the metal to carbonyl carbon

4 18.10 Addition of cyanide strongly basic cyanide ion hydrolysis Nitril group a-hydroxyacids unsaturated acids

5 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.

6 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

7 18.12 Addition of alcohols. Acetal formation Alcohols + the carbonyl group of aldehydes Anhydrous acids acetals 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 ether alcohol too unstable to be isolated

8 S N 1 route Acetal formation thus involves (a) nucleophilic addition to a carbonyl group, and (b) ether formation via a carbocation.

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

10 18.13 Cannizzaro reaction aldehydes containing no a-hydrogens concentrated alkali self-oxidationand-reduction Alcohol + salt of carboxylic acid Crossed Cannizzaro reaction If one of the aldehydes is formaldehyde yields almost exclusively sodium formate

11 addition of hydroxide ion to give intermediate I addition of a hydride ion from I to a second molecule of aldehyde.

12 18.14 Addition of Grignard reagents C-Mg (highly polar band) R= alkyl(1 O, 2 O, 3 O ); allylic; aralkyl( benzyl); aryl (ArMgCl must be made in the THF) Gelatinous material Organolithium compounds electropositivity Li Mg C-Li polarity C-Mg More negative reactivity organolithium Grignard reagents

13 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

14

15 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

16 18.16 Planing a Grignard synthesis It depends upon which rectant are more readily available Simple alcohol two-c four-c

17 And prepare larger alcohols

18 And synthesis of aromatic alcohols

19 Target molecule Let us suppose that we have available all alcohols of four carbons or fewer Alcohol 3 O Grignard reagent + ketone 4C 3C Two possibilities

20 Synthesis of aromatic alcohol Alcohol 2 O Grignard reagent + aldehyde

21 Lithium acetylide

22 18.17 syntheses using alcohols Synthesis of other compounds from alcohols Target molecule dehydrohalogenation dehydration

23 rearrangement dehydrohalogenation Pure form No rearrangement

24 1 C more than our largest available alcohol

25 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., HOCH 2 CH 2 Br)

26 18.20 Analysis of aldehyde and ketones Aldehydes or ketones 2,4-dinitrophenylhydrazine an insoluble yellow or red solid aldehydes ketones Tollens' reagent Tollens' reagent phenols and amines Schiff test aldehyde fuchsin-aldehyde reagent magenta color Aliphatic aldehydes and ketones (having a-hydrogen) Br 2 / CC1 4 too slow (- HBr) Aldehydes and ketones identified m.p. of derivatives like 2,4-dinitrophenylhydrazones, oximes, and semicarbazones

27 18.21 Iodoform test Methyl ketones are characterized through the iodoform test Sodium hypoiodide iodoform Hypohalites can not only halogenate but also oxidize alcohols

28 (ethanol)

29 18.22 Analysis of 1,2-diols. Periodic acid oxidation Two or more C=O or OH group attached to adjacent carbon Periodic acid