Reagent Tollen

At the beginning of section 2.2, you learned about Tollen s reagent. Tollen s reagent oxidizes aldehydes to produce the salt of a carboxylic acid. How does this reagent work as a test to distinguish between aldehydes and ketones ... 

Tollens Reagent Tollens A 3 g of AgN03 is dissolved in 30 cm3 of water, and put in a dark colored bottle. Then it is closed tightly. Tollens B 3 g of NaOH is dissolved in 30 cm3 of water, and put in a dark colored bottle. Then it is closed tightly. Equal volumes of Tollens A and Tollens B solutions are mixed. Diluted ammonia is then added to the mixture until the formed silver oxide dissolves. This solution should not be prepared well in advance, or heated, otherwise a black precipitate forms. The minimum amount of NH3 should be used... 

Tollens reagent An ammoniacal solution of silver oxide which is used as a lest for aldehydes, which, unlike ketones, cause the deposition of a silver mirror. 

It is preferable to use Tollen s ammoniacal silver nitrate reagent, which is prepared as follows Dissolve 3 g. of silver nitrate in 30 ml. of water (solution A) and 3 g. of sodium hydroxide in 30 ml. of water (solution B). When the reagent is requir, mix equal volumes (say, 1 ml.) of solutions A and JB in a clean test-tube, and add dilute ammonia solution drop by drop until the silver oxide is just dissolved. Great care must be taken in the preparation and use of this reagent, which must not be heated. Only a small volume should be prepared just before use, any residue washed down the sink with a large quantity of water, and the test-tubes rinsed with dilute nitric acid. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Nitro compounds and their reduction products. Tertiary nitro compounds (these are generally aromatic) are reduced by zinc and ammonium chloride solution to the corresponding hydroxylamines, which may be detected by their reducing action upon an ammoniacal solution of silver nitrate or Tollen s reagent ... 

It must be remembered, however, that nitroso, azoxy and azo compounds (which are usually more highly coloured than nitro compounds) may bo reduced by zinc powder to the corresponding hydroxylamino, hydrazo and hydrazine compounds respectively, all of which reduco Tollen s reagent in the cold. 

Color reaction Positive Ninhydrin, Nitroprusside-reagent, Br2-H20, Lemieux, and Negative Tollens, Molish, Sakaguchi, Elson-Morgan, Ferric chloride. Ultraviolet End absorption. 

I Oxidation of a primary alcohol or an aldehyde yields a carboxylic acid (Sections 17.7 and 19.3). Primary alcohols are often oxidized with C1O3 in aqueous acid, and aldehydes are oxidized with either acidic Cr03 or basic silver oxide (Tollens reagent). 

D-Mannose reacts with acetone to give a diisopropylidene derivative (Problem 25.61) that is still reducing toward Tollens reagent. Propose a likely structure tor this derivative. 

Thymine, electrostatic potential map of, 1104 structure of, 1101 Thyroxine, biosynthesis of, 551 structure of. 1020 TIme-of-flight (TOP) mass spectrometry, 417-418 Titration curve, alanine, 1023 TMS, see Tetramethylsilane see Trimethylsilyl ether Tollens reagent, 701 Tollens test, 992... 

To identify the specific aldehyde that is actually involved in the light-emitting reaction of living luminous bacteria, Shimomura et al. (1974a) extracted and purified the aldehyde from 40 g each of the bacterial cells of P. phosphoreum, Achromobacter (Vibrio or Photobacterium) fischeri, and an aldehydeless mutant of A. fischeri. The aldehyde fractions were purified, and then oxidized with Tollens reagent (silver oxide dissolved in ammonia) to convert the CHO group into the COOH group. Then the acids obtained were analyzed by mass spectrometry. The results indicated that P. phosphoreum had contained a mixture of aldehydes dodecanal (5%), tetradecanal (63%) and hexadecanal (30%), as shown in Table 2.2. Thus, tetradecanal was clearly predominant in... 

Historical. PE was first isolated by Tollens who was examining the effect of heat and reagents on formaldehyde. Apparently the crude formaldehyde he was working with contained a small % of acetaldehyde, which accounts for the PE formed. Although Tollens isolated it in 1882, it was not identified as PE until 1888 (Ref 2). Further details of the prepn and props of PE appeared in 1891 (Ref 3). The prepn is essentially a condensation betw 3 moles of formaldehyde and 1 of acetaldehyde to give an intermediate tris(hydroxymethyl)-acetaldehyde which is not isolated. An Intermolecular oxidn/redn then takes place betw this intermediate, and a 4th mole of formaldehyde, giving PE and formic ac (Ref 13, p 2). This type reaction is discussed under Cannizzaro Reaction in the Encycl (Vol 2, C25)... 

Tmta — see Acetic acid, trimethylenediaminetetra-Tollen s reagent, 5, 780 Tolman s cone angle, 2, 1015 Toluene acetoxylation... 

Dichromate oxidation of secondary alcohols produces ketones in good yield, with little additional oxidation. For example, CH,CH2CH(OH)CH3 can be oxidized to CH CH2COCH3. The difference between the ease of oxidation of aldehydes and that of ketones is used to distinguish them. Aldehydes can reduce silver ions to form a silver mirror—a coating of silver on test-tube walls—with Tollens reagent, a solution of Ag1" ions in aqueous ammonia (Fig. 19.3) ... 

Aldehydes CH3CH2CHO + Ag+ (in Tollens reagent) gives CH3CH2COOH... 

FIGURE 19.3 An aldehyde (left) produces a silver mirror with Tollens reagent, but a ketone (right) does not. 

Halogen derivatives Silver nitrate, am-moniacal (Dedonder s, Tollens or Zaffaroni s reagent) Halogen compounds yield black chromatogram zones on a pale gray background. [2]... 

See Silver nitride, also Ammonia Silver compounds See also silvering solutions, tollens reagent... 

Oxidize aldehyde to RCOOH Ag20,0H" or Ag(NH3)2+ Tollen s reagent, ketone is not oxidized... 

See FULMINATING METALS, SILVER-CONTAINING EXPLOSIVES, TOLLENS REAGENT... 

This mixture of ammoniacal silver oxide and sodium hydroxide solution is potentially dangerous, because if kept for a few horns it deposits an explosive precipitate. This danger was described by Tollens in 1882 but is not generally known now. Prepare the reagent just before use, in the tube to be used for the test, and discard... 

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